The XGo Mini Specification
=====

XGo has a recommended best practice syntax set, which we call the XGo Mini Specification. It is simple but Turing-complete and can elegantly implement any business requirements.

## Notation

The syntax is specified using a [variant](https://en.wikipedia.org/wiki/Wirth_syntax_notation) of Extended Backus-Naur Form (EBNF):

```go
Syntax      = { Production } .
Production  = production_name "=" [ Expression ] "." .
Expression  = Term { "|" Term } .
Term        = Factor { Factor } .
Factor      = production_name | token [ "…" token ] | Group | Option | Repetition .
Group       = "(" Expression ")" .
Option      = "[" Expression "]" .
Repetition  = "{" Expression "}" .
```

Productions are expressions constructed from terms and the following operators, in increasing precedence:

```go
|   alternation
()  grouping
[]  option (0 or 1 times)
{}  repetition (0 to n times)
```

Lowercase production names are used to identify lexical (terminal) tokens. Non-terminals are in CamelCase. Lexical tokens are enclosed in double quotes `""` or back quotes ``.

The form `a … b` represents the set of characters from a through b as alternatives. The horizontal ellipsis `…` is also used elsewhere in the spec to informally denote various enumerations or code snippets that are not further specified. The character … (as opposed to the three characters `...`) is not a token of the XGo language.

## Source code representation

Source code is Unicode text encoded in [UTF-8](https://en.wikipedia.org/wiki/UTF-8). The text is not canonicalized, so a single accented code point is distinct from the same character constructed from combining an accent and a letter; those are treated as two code points. For simplicity, this document will use the unqualified term character to refer to a Unicode code point in the source text.

Each code point is distinct; for instance, uppercase and lowercase letters are different characters.

Implementation restriction: For compatibility with other tools, a compiler may disallow the NUL character (U+0000) in the source text.

Implementation restriction: For compatibility with other tools, a compiler may ignore a UTF-8-encoded byte order mark (U+FEFF) if it is the first Unicode code point in the source text. A byte order mark may be disallowed anywhere else in the source.

### Characters

The following terms are used to denote specific Unicode character categories:

```go
newline        = /* the Unicode code point U+000A */ .
unicode_char   = /* an arbitrary Unicode code point except newline */ .
unicode_letter = /* a Unicode code point categorized as "Letter" */ .
unicode_digit  = /* a Unicode code point categorized as "Number, decimal digit" */ .
```

In [The Unicode Standard 8.0](https://www.unicode.org/versions/Unicode8.0.0/), Section 4.5 "General Category" defines a set of character categories. Go treats all characters in any of the Letter categories Lu, Ll, Lt, Lm, or Lo as Unicode letters, and those in the Number category Nd as Unicode digits.

### Letters and digits

The underscore character _ (U+005F) is considered a lowercase letter.

```go
letter        = unicode_letter | "_" .
decimal_digit = "0" … "9" .
binary_digit  = "0" | "1" .
octal_digit   = "0" … "7" .
hex_digit     = "0" … "9" | "A" … "F" | "a" … "f" .
```

## Lexical elements

### Comments

Comments serve as program documentation. There are three forms:

* _Line comments_ start with the character sequence `//` and stop at the end of the line.
* _Line comments_ start with the character sequence `#` and stop at the end of the line.
* _General comments_ start with the character sequence `/*` and stop with the first subsequent character sequence `*/`.

A _general comment_ containing no newlines acts like a space. Any other comment acts like a newline.

```
# this is a line comment
// this is another line comment
/* this is a general comment */
```

### Tokens

Tokens form the vocabulary of the XGo language. There are four classes: _identifiers_, _keywords_, _operators_ and _punctuation_, and _literals_. White space, formed from spaces (U+0020), horizontal tabs (U+0009), carriage returns (U+000D), and newlines (U+000A), is ignored except as it separates tokens that would otherwise combine into a single token. Also, a newline or end of file may trigger the insertion of a [semicolon](). While breaking the input into tokens, the next token is the longest sequence of characters that form a valid token.

### Semicolons

The formal syntax uses semicolons ";" as terminators in a number of productions. XGo programs may omit most of these semicolons using the following two rules:

* When the input is broken into tokens, a semicolon is automatically inserted into the token stream immediately after a line's final token if that token is
  * an [identifier](#identifiers)
  * an [integer](), [floating-point](), [imaginary](), [rune](), or [string]() literal
  * one of the [keywords]() `break`, `continue`, `fallthrough`, or `return`
  * one of the [operators and punctuation]() `++`, `--`, `)`, `]`, or `}`
* To allow complex statements to occupy a single line, a semicolon may be omitted before a closing `")"` or `"}"`.

To reflect idiomatic use, code examples in this document elide semicolons using these rules.

### Identifiers

Identifiers name program entities such as variables and types. An identifier is a sequence of one or more letters and digits. The first character in an identifier must be a letter.

```go
identifier = letter { letter | unicode_digit } .
```

```go
a
_x9
ThisVariableIsExported
αβ
```

Some identifiers are [predeclared](#predeclared-identifiers).

### Keywords

The following keywords are reserved and may not be used as identifiers (TODO: some keywords are allowed as identifiers).

```go
break        default      func         interface    select
case         defer        go           map          type
chan         else         goto         package      switch
const        fallthrough  if           range        var
continue     for          import       return
```

### Operators and punctuation

The following character sequences represent [operators](#operators) (including [assignment operators](#assignment-statements)) and punctuation:

```
+    &     +=    &=     &&    ==    !=    (    )
-    |     -=    |=     ||    <     <=    [    ]
*    ^     *=    ^=     <-    >     >=    {    }
/    <<    /=    <<=    ++    =     :=    ,    ;
%    >>    %=    >>=    --    !     ...   .    :
     &^          &^=          ~
```

### Integer literals

An integer literal is a sequence of digits representing an [integer constant](#constants). An optional prefix sets a non-decimal base: 0b or 0B for binary, 0, 0o, or 0O for octal, and 0x or 0X for hexadecimal. A single 0 is considered a decimal zero. In hexadecimal literals, letters a through f and A through F represent values 10 through 15.

For readability, an underscore character _ may appear after a base prefix or between successive digits; such underscores do not change the literal's value.

```go
int_lit        = decimal_lit | binary_lit | octal_lit | hex_lit .
decimal_lit    = "0" | ( "1" … "9" ) [ [ "_" ] decimal_digits ] .
binary_lit     = "0" ( "b" | "B" ) [ "_" ] binary_digits .
octal_lit      = "0" [ "o" | "O" ] [ "_" ] octal_digits .
hex_lit        = "0" ( "x" | "X" ) [ "_" ] hex_digits .

decimal_digits = decimal_digit { [ "_" ] decimal_digit } .
binary_digits  = binary_digit { [ "_" ] binary_digit } .
octal_digits   = octal_digit { [ "_" ] octal_digit } .
hex_digits     = hex_digit { [ "_" ] hex_digit } .
```

```go
42
4_2
0600
0_600
0o600
0O600       // second character is capital letter 'O'
0xBadFace
0xBad_Face
0x_67_7a_2f_cc_40_c6
170141183460469231731687303715884105727
170_141183_460469_231731_687303_715884_105727

_42         // an identifier, not an integer literal
42_         // invalid: _ must separate successive digits
4__2        // invalid: only one _ at a time
0_xBadFace  // invalid: _ must separate successive digits
```

### Floating-point literals

A floating-point literal is a decimal or hexadecimal representation of a [floating-point constant](#constants).

A decimal floating-point literal consists of an integer part (decimal digits), a decimal point, a fractional part (decimal digits), and an exponent part (e or E followed by an optional sign and decimal digits). One of the integer part or the fractional part may be elided; one of the decimal point or the exponent part may be elided. An exponent value exp scales the mantissa (integer and fractional part) by 10<sup>exp</sup>.

A hexadecimal floating-point literal consists of a 0x or 0X prefix, an integer part (hexadecimal digits), a radix point, a fractional part (hexadecimal digits), and an exponent part (p or P followed by an optional sign and decimal digits). One of the integer part or the fractional part may be elided; the radix point may be elided as well, but the exponent part is required. (This syntax matches the one given in IEEE 754-2008 §5.12.3.) An exponent value exp scales the mantissa (integer and fractional part) by 2<sup>exp</sup>.

For readability, an underscore character _ may appear after a base prefix or between successive digits; such underscores do not change the literal value.

```go
float_lit         = decimal_float_lit | hex_float_lit .

decimal_float_lit = decimal_digits "." [ decimal_digits ] [ decimal_exponent ] |
                    decimal_digits decimal_exponent |
                    "." decimal_digits [ decimal_exponent ] .
decimal_exponent  = ( "e" | "E" ) [ "+" | "-" ] decimal_digits .

hex_float_lit     = "0" ( "x" | "X" ) hex_mantissa hex_exponent .
hex_mantissa      = [ "_" ] hex_digits "." [ hex_digits ] |
                    [ "_" ] hex_digits |
                    "." hex_digits .
hex_exponent      = ( "p" | "P" ) [ "+" | "-" ] decimal_digits .
```

```go
0.
72.40
072.40       // == 72.40
2.71828
1.e+0
6.67428e-11
1E6
.25
.12345E+5
1_5.         // == 15.0
0.15e+0_2    // == 15.0

0x1p-2       // == 0.25
0x2.p10      // == 2048.0
0x1.Fp+0     // == 1.9375
0X.8p-0      // == 0.5
0X_1FFFP-16  // == 0.1249847412109375

0x15e-2      // == 0x15e - 2 (integer subtraction)

0x.p1        // invalid: mantissa has no digits
1p-2         // invalid: p exponent requires hexadecimal mantissa
0x1.5e-2     // invalid: hexadecimal mantissa requires p exponent
1_.5         // invalid: _ must separate successive digits
1._5         // invalid: _ must separate successive digits
1.5_e1       // invalid: _ must separate successive digits
1.5e_1       // invalid: _ must separate successive digits
1.5e1_       // invalid: _ must separate successive digits
```

### Rational literals

Rational literals in XGo come in two forms:

* **Rational integers**: An integer followed by the suffix `r`.
* **Fractions**: A numerator, division operator `/`, denominator, followed by the suffix `r`

```
rational_lit   = rational_int | rational_frac .
rational_int   = int_lit "r" .
rational_frac  = int_lit "/" int_lit "r" .
```

Examples:

```sh
1r       # Rational integer 1
2/3r     # Fraction 2/3
```

### Imaginary literals

An imaginary literal represents the imaginary part of a [complex constant](#constants). It consists of an [integer](#integer-literals) or [floating-point](#floating-point-literals) literal followed by the lowercase letter _i_. The value of an imaginary literal is the value of the respective integer or floating-point literal multiplied by the imaginary unit _i_.

```go
imaginary_lit = (decimal_digits | int_lit | float_lit) "i" .
```

For backward compatibility, an imaginary literal's integer part consisting entirely of decimal digits (and possibly underscores) is considered a decimal integer, even if it starts with a leading `0`.

```go
0i
0123i         // == 123i for backward-compatibility
0o123i        // == 0o123 * 1i == 83i
0xabci        // == 0xabc * 1i == 2748i
0.i
2.71828i
1.e+0i
6.67428e-11i
1E6i
.25i
.12345E+5i
0x1p-2i       // == 0x1p-2 * 1i == 0.25i
```

### Boolean literals

The boolean truth values are represented by the [predeclared constants](#constants) `true` and `false`.

```go
true
false
```

### Rune literals

A rune literal represents a [rune constant](#constants), an integer value identifying a Unicode code point. A rune literal is expressed as one or more characters enclosed in single quotes, as in `'x'` or `'\n'`. Within the quotes, any character may appear except newline and unescaped single quote. A single quoted character represents the Unicode value of the character itself, while multi-character sequences beginning with a backslash encode values in various formats.

The simplest form represents the single character within the quotes; since XGo source text is Unicode characters encoded in UTF-8, multiple UTF-8-encoded bytes may represent a single integer value. For instance, the literal `'a'` holds a single byte representing a literal a, Unicode `U+0061`, value 0x61, while `'ä'` holds two bytes (0xc3 0xa4) representing a literal a-dieresis, `U+00E4`, value 0xe4.

Several backslash escapes allow arbitrary values to be encoded as ASCII text. There are four ways to represent the integer value as a numeric constant: `\x` followed by exactly two hexadecimal digits; `\u` followed by exactly four hexadecimal digits; `\U` followed by exactly eight hexadecimal digits, and a plain backslash `\` followed by exactly three octal digits. In each case the value of the literal is the value represented by the digits in the corresponding base.

Although these representations all result in an integer, they have different valid ranges. Octal escapes must represent a value between 0 and 255 inclusive. Hexadecimal escapes satisfy this condition by construction. The escapes `\u` and `\U` represent Unicode code points so within them some values are illegal, in particular those above 0x10FFFF and surrogate halves.

After a backslash, certain single-character escapes represent special values:

```
\a   U+0007 alert or bell
\b   U+0008 backspace
\f   U+000C form feed
\n   U+000A line feed or newline
\r   U+000D carriage return
\t   U+0009 horizontal tab
\v   U+000B vertical tab
\\   U+005C backslash
\'   U+0027 single quote  (valid escape only within rune literals)
\"   U+0022 double quote  (valid escape only within string literals)
```

An unrecognized character following a backslash in a rune literal is illegal.

```go
rune_lit         = "'" ( unicode_value | byte_value ) "'" .
unicode_value    = unicode_char | little_u_value | big_u_value | escaped_char .
byte_value       = octal_byte_value | hex_byte_value .
octal_byte_value = `\` octal_digit octal_digit octal_digit .
hex_byte_value   = `\` "x" hex_digit hex_digit .
little_u_value   = `\` "u" hex_digit hex_digit hex_digit hex_digit .
big_u_value      = `\` "U" hex_digit hex_digit hex_digit hex_digit
                           hex_digit hex_digit hex_digit hex_digit .
escaped_char     = `\` ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | `\` | "'" | `"` ) .
```

```go
'a'
'ä'
'本'
'\t'
'\000'
'\007'
'\377'
'\x07'
'\xff'
'\u12e4'
'\U00101234'
'\''         // rune literal containing single quote character
'aa'         // illegal: too many characters
'\k'         // illegal: k is not recognized after a backslash
'\xa'        // illegal: too few hexadecimal digits
'\0'         // illegal: too few octal digits
'\400'       // illegal: octal value over 255
'\uDFFF'     // illegal: surrogate half
'\U00110000' // illegal: invalid Unicode code point
```

### String literals

A string literal represents a [string constant](#constants) obtained from concatenating a sequence of characters. There are two forms: raw string literals and interpreted string literals.

Raw string literals are character sequences between back quotes, as in \`foo\`. Within the quotes, any character may appear except back quote. The value of a raw string literal is the string composed of the uninterpreted (implicitly UTF-8-encoded) characters between the quotes; in particular, backslashes have no special meaning and the string may contain newlines. Carriage return characters (`'\r'`) inside raw string literals are discarded from the raw string value.

Interpreted string literals are character sequences between double quotes, as in `"bar"`. Within the quotes, any character may appear except newline and unescaped double quote. The text between the quotes forms the value of the literal, with backslash escapes interpreted as they are in [rune literals](#rune-literals) (except that `\'` is illegal and `\"` is legal), with the same restrictions. The three-digit octal (`\nnn`) and two-digit hexadecimal (`\xnn`) escapes represent individual bytes of the resulting string; all other escapes represent the (possibly multi-byte) UTF-8 encoding of individual characters. Thus inside a string literal `\377` and `\xFF` represent a single byte of value 0xFF=255, while `ÿ`, `\u00FF`, `\U000000FF` and `\xc3\xbf` represent the two bytes 0xc3 0xbf of the UTF-8 encoding of character `U+00FF`.

```go
string_lit             = raw_string_lit | interpreted_string_lit .
raw_string_lit         = "`" { unicode_char | newline } "`" .
interpreted_string_lit = `"` { unicode_value | byte_value } `"` .
```

```go
`abc`                // same as "abc"
`\n
\n`                  // same as "\\n\n\\n"
"\n"
"\""                 // same as `"`
"Hello, world!\n"
"日本語"
"\u65e5本\U00008a9e"
"\xff\u00FF"
"\uD800"             // illegal: surrogate half
"\U00110000"         // illegal: invalid Unicode code point
```

These examples all represent the same string:

```go
"日本語"                                 // UTF-8 input text
`日本語`                                 // UTF-8 input text as a raw literal
"\u65e5\u672c\u8a9e"                    // the explicit Unicode code points
"\U000065e5\U0000672c\U00008a9e"        // the explicit Unicode code points
"\xe6\x97\xa5\xe6\x9c\xac\xe8\xaa\x9e"  // the explicit UTF-8 bytes
```

If the source code represents a character as two code points, such as a combining form involving an accent and a letter, the result will be an error if placed in a rune literal (it is not a single code point), and will appear as two code points if placed in a string literal.

### Special literals

TODO

```go
nil
iota
```

## Constants

There are _boolean constants_, _rune constants_, _integer constants_, _floating-point constants_, _complex constants_, and _string constants_. Rune, integer, floating-point, and complex constants are collectively called _numeric constants_.

A constant value is represented by a [rune](#rune-literals), [integer](#integer-literals), [floating-point](#floating-point-literals), [imaginary](#imaginary-literals), [boolean](#boolean-literals) or [string](#string-literals) literal, an identifier denoting a constant, a [constant expression](#constant-expressions), a [conversion](#conversions) with a result that is a constant, or the result value of some built-in functions such as `min` or `max` applied to constant arguments, `unsafe.Sizeof` applied to [certain values](), `cap` or `len` applied to [some expressions](), `real` and `imag` applied to a complex constant and complex applied to numeric constants. The boolean truth values are represented by the predeclared constants `true` and `false`. The predeclared identifier `iota` denotes an integer constant.

Not all literals are constants. For example:

```sh
nil
1r
2/3r
```

In general, complex constants are a form of [constant expression](#constant-expressions) and are discussed in that section.

Numeric constants represent exact values of arbitrary precision and do not overflow. Consequently, there are no constants denoting the IEEE-754 negative zero, infinity, and not-a-number values.

Constants may be [typed](#types) or _untyped_. Literal constants (including `true`, `false`, `iota`), and certain [constant expressions]() containing only untyped constant operands are untyped.

A constant may be given a type explicitly by a [constant declaration]() or [conversion](#conversions), or implicitly when used in a [variable declaration]() or an [assignment statement]() or as an operand in an [expression](#expressions). It is an error if the constant value cannot be [represented]() as a value of the respective type.

An untyped constant has a _default type_ which is the type to which the constant is implicitly converted in contexts where a typed value is required, for instance, in a [short variable declaration]() such as `i := 0` where there is no explicit type. The default type of an untyped constant is `bool`, `rune`, `int`, `float64`, `complex128`, or `string` respectively, depending on whether it is a boolean, rune, integer, floating-point, complex, or string constant.


## Variables

A variable is a storage location for holding a value. The set of permissible values is determined by the variable's [type](#types).

A [variable declaration]() or, for function parameters and results, the signature of a [function declaration]() or [function literal]() reserves storage for a named variable. Calling the built-in function [new]() or taking the address of a [composite literal]() allocates storage for a variable at run time. Such an anonymous variable is referred to via a (possibly implicit) [pointer indirection](#address-operators).

_Structured_ variables of [array](#array-types), [slice](#slice-types), and [class](#classes) types have elements and fields that may be [addressed](#address-operators) individually. Each such element acts like a variable.

The _static type_ (or just _type_) of a variable is the type given in its declaration, the type provided in the new call or composite literal, or the type of an element of a class variable. Variables of interface type also have a distinct _dynamic type_, which is the (non-interface) type of the value assigned to the variable at run time (unless the value is the predeclared identifier `nil`, which has no type). The dynamic type may vary during execution but values stored in interface variables are always [assignable]() to the static type of the variable.

```go
var x any  // x is nil and has static type any
var v *T   // v has value nil, static type *T
x = 42     // x has value 42 and dynamic type int
x = v      // x has value (*T)(nil) and dynamic type *T
```

A variable's value is retrieved by referring to the variable in an [expression](#expressions); it is the most recent value [assigned]() to the variable. If a variable has not yet been assigned a value, its value is the [zero value]() for its type.


## Types

A type determines a set of values together with operations and methods specific to those values. A type may be denoted by a _type name_. A type may also be specified using a type literal, which composes a type from existing types.

```go
Type      = TypeName [ TypeArgs ] | TypeLit | "(" Type ")" .
TypeName  = identifier | QualifiedIdent .
TypeArgs  = "[" TypeList [ "," ] "]" .
TypeList  = Type { "," Type } .
TypeLit   = ArrayType | TupleType | PointerType | FunctionType | InterfaceType |
            SliceType | MapType . // TODO: check this
```

The language [predeclares]() certain type names. Others are introduced with [type declarations](#type-declarations). _Composite types_—array, tuple, pointer, function, interface, slice, map—may be constructed using type literals.

Predeclared types and defined types are called _named types_. An alias denotes a named type if the type given in the alias declaration is a named type.

### Boolean types

A _boolean type_ represents the set of Boolean truth values denoted by the predeclared constants true and false. The predeclared boolean type is `bool`; it is a defined type.

```go
bool
```

### Numeric types

An _integer_, _floating-point_, _rational_ or _complex_ type represents the set of integer, floating-point, or complex values, respectively. They are collectively called _numeric types_. The predeclared architecture-independent numeric types are:

```go
uint8       // the set of all unsigned  8-bit integers (0 to 255)
uint16      // the set of all unsigned 16-bit integers (0 to 65535)
uint32      // the set of all unsigned 32-bit integers (0 to 4294967295)
uint64      // the set of all unsigned 64-bit integers (0 to 18446744073709551615)

int8        // the set of all signed  8-bit integers (-128 to 127)
int16       // the set of all signed 16-bit integers (-32768 to 32767)
int32       // the set of all signed 32-bit integers (-2147483648 to 2147483647)
int64       // the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807)

float32     // the set of all IEEE-754 32-bit floating-point numbers
float64     // the set of all IEEE-754 64-bit floating-point numbers

complex64   // the set of all complex numbers with float32 real and imaginary parts
complex128  // the set of all complex numbers with float64 real and imaginary parts

byte        // alias for uint8
rune        // alias for int32
```

The value of an _n_-bit integer is n bits wide and represented using [two's complement arithmetic](https://en.wikipedia.org/wiki/Two's_complement).

There is also a set of predeclared integer types with implementation-specific sizes:

```go
uint     // either 32 or 64 bits
int      // same size as uint
uintptr  // an unsigned integer large enough to store the uninterpreted bits of a pointer value
```

To avoid portability issues all numeric types are defined types and thus distinct except _byte_, which is an [alias]() for _uint8_, and _rune_, which is an alias for _int32_. Explicit conversions are required when different numeric types are mixed in an expression or assignment. For instance, _int32_ and _int_ are not the same type even though they may have the same size on a particular architecture.

TODO:

```go
bigint  // TODO
bigrat  // TODO
```

### String types

A _string type_ represents the set of string values. A string value is a (possibly empty) sequence of bytes. The number of bytes is called the length of the string and is never negative. Strings are immutable: once created, it is impossible to change the contents of a string. The predeclared string type is `string`; it is a defined type.

```go
string
```

The length of a string `s` can be discovered using the built-in function [len](). The length is a compile-time constant if the string is a constant. A string's bytes can be accessed by integer [indices]() `0` through `len(s)-1`. It is illegal to take the address of such an element; if `s[i]` is the i'th byte of a string, `&s[i]` is invalid.


### Array types

An array is a numbered sequence of elements of a single type, called the element type. The number of elements is called the length of the array and is never negative.

```go
ArrayType   = "[" ArrayLength "]" ElementType .
ArrayLength = Expression .
ElementType = Type .
```

The length is part of the array's type; it must evaluate to a non-negative [constant](#constants) [representable]() by a value of type int. The length of array `a` can be discovered using the built-in function [len](). The elements can be addressed by integer [indices]() `0` through `len(a)-1`. Array types are always one-dimensional but may be composed to form multi-dimensional types.

```go
[32]byte
[2*N](x int32, y int32)
[1000]*float64
[3][5]int
[2][2][2]float64  // same as [2]([2]([2]float64))
```

An array type T may not have an element of type T, or of a type containing T as a component, directly or indirectly, if those containing types are only array or tuple types.

```go
// invalid array types
type (
	T1 [10]T1                 // element type of T1 is T1
	T2 [10](f T2)             // T2 contains T2 as component of a tuple
	T3 [10]T4                 // T3 contains T3 as component of a tuple in T4
	T4 (f [10]T3)             // T4 contains T4 as component of array T3 in a tuple
)

// valid array types
type (
	T5 [10]*T5                // T5 contains T5 as component of a pointer
	T6 [10]func() T6          // T6 contains T6 as component of a function type
	T7 [10](f []T7)           // T7 contains T7 as component of a slice in a tuple
)
```

### Pointer types

A _pointer_ type denotes the set of all pointers to [variables](#variables) of a given type, called the base type of the pointer. The value of an uninitialized pointer is `nil`.

```go
PointerType = "*" BaseType .
BaseType    = Type .
```

```go
*Point
*[4]int
```

### Slice types

A _slice_ is a descriptor for a contiguous segment of an _underlying array_ and provides access to a numbered sequence of elements from that array. A slice type denotes the set of all slices of arrays of its element type. The number of elements is called the length of the slice and is never negative. The value of an uninitialized slice is `nil`.

```go
SliceType = "[" "]" ElementType .
```

The length of a slice `s` can be discovered by the built-in function [len](); unlike with arrays it may change during execution. The elements can be addressed by integer [indices]() `0` through `len(s)-1`. The slice index of a given element may be less than the index of the same element in the underlying array.

A slice, once initialized, is always associated with an underlying array that holds its elements. A slice therefore shares storage with its array and with other slices of the same array; by contrast, distinct arrays always represent distinct storage.

The array underlying `a` slice may extend past the end of the slice. The capacity is a measure of that extent: it is the sum of the length of the slice and the length of the array beyond the slice; a slice of length up to that capacity can be created by [slicing]() a new one from the original slice. The capacity of a slice a can be discovered using the built-in function `cap(a)`.

A new, initialized slice value for a given element type `T` may be made using the built-in function [make](), which takes a slice type and parameters specifying the length and optionally the capacity. A slice created with make always allocates a new, hidden array to which the returned slice value refers. That is, executing

```go
make([]T, length, capacity)
```

produces the same slice as allocating an array and [slicing]() it, so these two expressions are equivalent:

```
make([]int, 50, 100)
new([100]int)[0:50]
```

Like arrays, slices are always one-dimensional but may be composed to construct higher-dimensional objects. With arrays of arrays, the inner arrays are, by construction, always the same length; however with slices of slices (or arrays of slices), the inner lengths may vary dynamically. Moreover, the inner slices must be initialized individually.

### Tuple types

A _tuple_ type is a lightweight data structure for grouping multiple values together. Tuple types are syntactic sugar for anonymous structs with ordinal field names (`X_0`, `X_1`, etc.). Named fields in tuples serve as compile-time aliases for better code readability but map to ordinal fields at runtime.

```go
TupleType = Parameters .
```

A tuple type is denoted by a parenthesized, comma-separated list of types, optionally with names for the elements:

```go
// Empty tuple (equivalent to struct{})
()

// Anonymous tuples (elements accessed by ordinal: .0, .1, .2)
(int, string)
(int, string, bool)

// Named tuples (compile-time names for readability, runtime uses ordinal fields)
type Point (x int, y int)
type Person (name string, age int)

// Type shorthand syntax (same as: x int, y int, z int)
type Point3D (x, y, z int)
```

**Single-Element Degeneracy**: A tuple with a single element `(T)` or `(value T)` degenerates to `T` itself, not a tuple.

#### Tuple Construction

Tuples are constructed using **function-style syntax**:

```go
// Positional construction
p := Point(10, 20)
result := (42, "success", true)

// Keyword argument construction (using = for clarity)
p := Point(x = 10, y = 20)
person := Person(name = "Alice", age = 30)
```

#### Field Access

Tuple fields can be accessed by:
- **Named fields**: `p.x`, `p.y` (using declared names)
- **Ordinal notation**: `p.0`, `p.1` (numeric shorthand)

```go
type Point (x int, y int)
p := Point(10, 20)

echo p.x      // 10 (using compile-time name)
echo p.0      // 10 (using ordinal index, equivalent to p.x)
echo p.y      // 20
echo p.1      // 20
```

#### Type Identity

Two tuple types are identical if they have the same number of elements and corresponding element types are identical. Named fields are not part of type identity:

```go
type Point (x int, y int)
type Coord (a int, b int)

// Point and Coord have identical underlying types (both map to struct{ X_0 int; X_1 int })
// but are different named types
```

#### Tuples in Composite Types

Tuples can be used as element types in arrays, slices, maps, and channels:

```go
// Tuple as map value type
var cache map[string](int, bool)

// Tuple as slice element type
var pairs [](string, int)

// Tuple as channel element type
var ch chan (int, error)
```

#### Recursive Type Restrictions

A tuple type `T` may not contain an element of type `T`, or of a type containing `T` as a component, directly or indirectly, if those containing types are only array or tuple types.

```go
// invalid tuple types
type (
	T1 (T1, int)              // T1 contains an element of T1
	T2 ([10]T2, string)       // T2 contains T2 as component of an array
)

// valid tuple types
type (
	T3 (*T3, int)             // T3 contains T3 as component of a pointer
	T4 (func() T4, string)    // T4 contains T4 as component of a function type
	T5 ([]T5, int)            // T5 contains T5 as component of a slice
)
```

### Map types

A _map_ is an unordered group of elements of one type, called the element type, indexed by a set of unique _keys_ of another type, called the key type. The value of an uninitialized map is `nil`.

```go
MapType     = "map" "[" KeyType "]" ElementType .
KeyType     = Type .
```

The comparison operators `==` and `!=` must be fully defined for operands of the key type; thus the key type must not be a function, map, or slice. If the key type is an interface type, these comparison operators must be defined for the dynamic key values; failure will cause a run-time panic.

```go
map[string]int
map[*T](x float64, y float64)
map[string]any
```

The number of map elements is called its length. For a map `m`, it can be discovered using the built-in function [len]() and may change during execution. Elements may be added during execution using [assignments]() and retrieved with [index expressions](); they may be removed with the [delete]() and [clear]() built-in function.

A new, empty map value is made using the built-in function [make](), which takes the map type and an optional capacity hint as arguments:

```go
make(map[string]int)
make(map[string]int, 100)
```

The initial capacity does not bound its size: maps grow to accommodate the number of items stored in them, with the exception of nil maps. A nil map is equivalent to an empty map except that no elements may be added.

### Function types

A _function_ type denotes the set of all functions with the same parameter and result types. The value of an uninitialized variable of function type is `nil`.

```go
FunctionType   = "func" Signature .
Signature      = Parameters [ Result ] .
Result         = Parameters | Type .
Parameters     = "(" [ ParameterList [ "," ] ] ")" .
ParameterList  = ParameterDecl { "," ParameterDecl } .
ParameterDecl  = [ IdentifierList ] [ "..." ] Type .
```

Within a list of parameters or results, the names (IdentifierList) must either all be present or all be absent. If present, each name stands for one item (parameter or result) of the specified type and all non-[blank]() names in the signature must be [unique](). If absent, each type stands for one item of that type. Parameter and result lists are always parenthesized except that if there is exactly one unnamed result it may be written as an unparenthesized type.

The final incoming parameter in a function signature may have a type prefixed with `...`. A function with such a parameter is called _variadic_ and may be invoked with zero or more arguments for that parameter.

```go
func()
func(x int) int
func(a, _ int, z float32) bool
func(a, b int, z float32) (bool)
func(prefix string, values ...int)
func(a, b int, z float64, opt ...any) (success bool)
func(int, int, float64) (float64, *[]int)
func(n int) func(p *T)
```

### Interface types

An interface type specifies a [method set]() called its _interface_. A variable of interface type can store a value of any type with a method set that is any superset of the interface. Such a type is said to _implement the interface_. The value of an uninitialized variable of interface type is `nil`.

```go
InterfaceType      = "interface" "{" { ( MethodSpec | InterfaceTypeName ) ";" } "}" .
MethodSpec         = MethodName Signature .
MethodName         = identifier .
InterfaceTypeName  = TypeName .
```

An interface type may specify methods _explicitly_ through method specifications, or it may _embed_ methods of other interfaces through interface type names.

```go
// A simple File interface.
interface {
	Read([]byte) (int, error)
	Write([]byte) (int, error)
	Close() error
}
```

The name of each explicitly specified method must be [unique]() and not [blank]().

```go
interface {
	String() string
	String() string  // illegal: String not unique
	_(x int)         // illegal: method must have non-blank name
}
```

More than one type may implement an interface. For instance, if two types `S1` and `S2` have the method set

```go
func Read(p []byte) (n int, err error)
func Write(p []byte) (n int, err error)
func Close() error
```

(where `T` stands for either `S1` or `S2`) then the `File` interface is implemented by both `S1` and `S2`, regardless of what other methods `S1` and `S2` may have or share.

A type implements any interface comprising any subset of its methods and may therefore implement several distinct interfaces. For instance, all types implement the _empty interface_:

```go
interface{}
```

Similarly, consider this interface specification, which appears within a [type declaration](#type-declarations) to define an interface called `Locker`:

```go
type Locker interface {
	Lock()
	Unlock()
}
```

If `S1` and `S2` also implement

```go
func Lock() { … }
func Unlock() { … }
```

they implement the `Locker` interface as well as the `File` interface.

An interface `T` may use a (possibly qualified) interface type name `E` in place of a method specification. This is called _embedding_ interface `E` in `T`. The method set of `T` is the union of the method sets of T’s explicitly declared methods and of T’s embedded interfaces.

```go
type Reader interface {
	Read(p []byte) (n int, err error)
	Close() error
}

type Writer interface {
	Write(p []byte) (n int, err error)
	Close() error
}

// ReadWriter's methods are Read, Write, and Close.
type ReadWriter interface {
	Reader  // includes methods of Reader in ReadWriter's method set
	Writer  // includes methods of Writer in ReadWriter's method set
}
```

A union of method sets contains the (exported and non-exported) methods of each method set exactly once, and methods with the [same]() names must have [identical]() signatures.

```go
type ReadCloser interface {
	Reader   // includes methods of Reader in ReadCloser's method set
	Close()  // illegal: signatures of Reader.Close and Close are different
}
```

An interface type `T` may not embed itself or any interface type that embeds `T`, recursively.

```go
// illegal: Bad cannot embed itself
type Bad interface {
	Bad
}

// illegal: Bad1 cannot embed itself using Bad2
type Bad1 interface {
	Bad2
}
type Bad2 interface {
	Bad1
}
```

#### Built-in interfaces

The following interfaces are predeclared in XGo:

```go
any    // alias for interface{}, the empty interface
error  // interface for error handling
```

##### The any interface

The predeclared type `any` is an alias for the empty interface `interface{}`. It represents the set of all types and is useful when you need to work with values of any type:

```go
var x any  // x is nil and has static type any
x = 42     // x has value 42 and dynamic type int
x = "hello" // x has value "hello" and dynamic type string
```

##### The error interface

The predeclared type `error` is defined as:

```go
type error interface {
	Error() string
}
```

It is the conventional interface for representing an error condition, with the nil value representing no error. For instance, a function to read data from a file might be defined:

```go
func Read(f *File, b []byte) (n int, err error)
```

Any type that implements an `Error()` method returning a string satisfies the `error` interface.

### Classes

TODO (classfile)


## Properties of types and values

### Underlying types

Each type T has an _underlying type_: If T is one of the predeclared boolean, numeric, or string types, or a type literal, the corresponding underlying type is T itself. Otherwise, T's underlying type is the underlying type of the type to which T refers in its declaration.

```go
type (
	A1 = string
	A2 = A1
)

type (
	B1 string
	B2 B1
	B3 []B1
	B4 B3
)
```

The underlying type of `string`, `A1`, `A2`, `B1`, and `B2` is `string`. The underlying type of `[]B1`, `B3`, and `B4` is `[]B1`.

### Type identity

Two types are either _identical_ or _different_.

A [named type](#types) is always different from any other type. Otherwise, two types are identical if their [underlying type](#underlying-types) literals are structurally equivalent; that is, they have the same literal structure and corresponding components have identical types. In detail:

* Two array types are identical if they have identical element types and the same array length.
* Two slice types are identical if they have identical element types.
* Two struct types are identical if they have the same sequence of fields, and if corresponding fields have the same names, and identical types, and identical tags. [Non-exported](#exported-identifiers) field names from different packages are always different.
* Two pointer types are identical if they have identical base types.
* Two function types are identical if they have the same number of parameters and result values, corresponding parameter and result types are identical, and either both functions are variadic or neither is. Parameter and result names are not required to match.
* Two interface types are identical if they define the same type set.
* Two map types are identical if they have identical key and element types.

Given the declarations

```go
type (
	A0 = []string
	A1 = A0
	A2 = struct{ a, b int }
	A3 = int
	A4 = func(A3, float64) *A0
	A5 = func(x int, _ float64) *[]string

	B0 A0
	B1 []string
	B2 struct{ a, b int }
	B3 struct{ a, c int }
	B4 func(int, float64) *B0
	B5 func(x int, y float64) *A1

	C0 = B0
)
```

these types are identical:

```go
A0, A1, and []string
A2 and struct{ a, b int }
A3 and int
A4, func(int, float64) *[]string, and A5

B0 and C0
[]int and []int
struct{ a, b *B5 } and struct{ a, b *B5 }
func(x int, y float64) *[]string, func(int, float64) (result *[]string), and A5
```

`B0` and `B1` are different because they are new types created by distinct [type definitions](#type-definitions); `func(int, float64) *B0` and `func(x int, y float64) *[]string` are different because `B0` is different from `[]string`.

### Assignability

A value x of type V is _assignable_ to a [variable](#variables) of type T ("x is assignable to T") if one of the following conditions applies:

* V and T are identical.
* V and T have identical [underlying types](#underlying-types) but are not type parameters and at least one of V or T is not a [named type](#types).
* T is an interface type, and x [implements]() T.
* x is the predeclared identifier nil and T is a pointer, function, slice, map, or interface type.
* x is an untyped [constant](#constants) [representable](#representability) by a value of type T.

### Representability

A [constant](#constants) x is _representable_ by a value of type T if one of the following conditions applies:

* x is in the set of values [determined](#types) by T.
* T is a floating-point type and x can be rounded to T's precision without overflow. Rounding uses IEEE 754 round-to-even rules but with an IEEE negative zero further simplified to an unsigned zero. Note that constant values never result in an IEEE negative zero, NaN, or infinity.
* T is a complex type, and x's [components](#manipulating-complex-numbers) `real(x)` and `imag(x)` are representable by values of T's component type (`float32` or `float64`).

```go
x                   T           x is representable by a value of T because

'a'                 byte        97 is in the set of byte values
97                  rune        rune is an alias for int32, and 97 is in the set of 32-bit integers
"foo"               string      "foo" is in the set of string values
1024                int16       1024 is in the set of 16-bit integers
42.0                byte        42 is in the set of unsigned 8-bit integers
1e10                uint64      10000000000 is in the set of unsigned 64-bit integers
2.718281828459045   float32     2.718281828459045 rounds to 2.7182817 which is in the set of float32 values
-1e-1000            float64     -1e-1000 rounds to IEEE -0.0 which is further simplified to 0.0
0i                  int         0 is an integer value
(42 + 0i)           float32     42.0 (with zero imaginary part) is in the set of float32 values
```

```go
x                   T           x is not representable by a value of T because

0                   bool        0 is not in the set of boolean values
'a'                 string      'a' is a rune, it is not in the set of string values
1024                byte        1024 is not in the set of unsigned 8-bit integers
-1                  uint16      -1 is not in the set of unsigned 16-bit integers
1.1                 int         1.1 is not an integer value
42i                 float32     (0 + 42i) is not in the set of float32 values
1e1000              float64     1e1000 overflows to IEEE +Inf after rounding
```

### Method sets

The _method set_ of a type determines the methods that can be [called](#commands-and-calls) on an [operand]() of that type. Every type has a (possibly empty) method set associated with it:

* The method set of a [defined type](#type-definitions) T consists of all [methods]() declared with receiver type T.
* The method set of a pointer to a defined type T (where T is neither a pointer nor an interface) is the set of all methods declared with receiver *T or T.
* The method set of an [interface type](#interface-types) is the intersection of the method sets of each type in the interface's [type set](#interface-types) (the resulting method set is usually just the set of declared methods in the interface).

Further rules apply to structs (and pointer to structs) containing embedded fields, as described in the section on [struct types](#struct-types). Any other type has an empty method set.

In a method set, each method must have a [unique](#uniqueness-of-identifiers) non-[blank](#blank-identifier) [method name]().


## Expressions

An expression specifies the computation of a value by applying operators and functions to operands.

### Operands

Operands denote the elementary values in an expression. An operand may be a literal, a (possibly [qualified]()) non-[blank](#blank-identifier) identifier denoting a [constant](#constant-declarations), [variable](#variable-declarations), or [function](#function-declarations), or a parenthesized expression.

```go
Operand     = Literal | OperandName [ TypeArgs ] .
Literal     = BasicLit | FunctionLit | TupleLit .
BasicLit    = int_lit | float_lit | imaginary_lit | rune_lit | string_lit .
OperandName = identifier | QualifiedIdent .
```

The [blank identifier](#blank-identifier) may appear as an operand only on the left-hand side of an [assignment statement](#assignment-statements).

### Qualified identifiers

A _qualified identifier_ is an identifier qualified with a package name prefix. Both the package name and the identifier must not be [blank](#blank-identifier).

```go
QualifiedIdent = PackageName "." identifier .
```

A qualified identifier accesses an identifier in a different package, which must be [imported](#import-declarations). The identifier must be [exported](#exported-identifiers) and declared in the [package block](#blocks) of that package.

```go
math.Sin // denotes the Sin function in package math
```

### Tuple literals

Tuple literals construct tuple values using function-style syntax. Both anonymous and named tuples are created using parenthesized, comma-separated expressions.

```go
TupleLit = "(" [ ExpressionList [ "," ] ] ")" .
```

#### Anonymous Tuple Literals

Anonymous tuple literals create values of anonymous tuple types:

```go
result := (42, "success", true)        // creates a (int, string, bool)
pair := (10, 20)                       // creates a (int, int)
empty := ()                            // creates an empty tuple ()
```

#### Named Tuple Construction

Named tuple types are instantiated using function-style call syntax:

**Positional Arguments**:

```go
type Point (x int, y int)
p := Point(10, 20)              // positional construction

type Person (name string, age int)
alice := Person("Alice", 30)    // positional construction
```

**Keyword Arguments**:

Keyword arguments use `=` for assignment and provide clarity when initializing tuples with many fields:

```go
p := Point(x = 10, y = 20)              // keyword argument construction
person := Person(name = "Bob", age = 25) // keyword argument construction
```

Keyword arguments can be mixed, but all positional arguments must come before keyword arguments:

```go
// This is valid
p := Point(10, y = 20)

// This is invalid
// p := Point(x = 10, 20)  // error: positional arg after keyword arg
```

#### Zero Values

Calling a tuple type with no arguments returns the zero value for that type:

```go
type Point (x int, y int)
zero := Point()   // equivalent to Point(0, 0)
```

#### Type Inference

The type of a tuple literal is inferred from context when possible:

```go
func process(p (int, string)) {
	// ...
}

process((42, "hello"))  // tuple literal type inferred as (int, string)
```

#### Tuple Literals in Composite Types

Tuple literals can be used as elements in slices, arrays, and maps:

```go
pairs := [](string, int){
	("a", 1),
	("b", 2),
	("c", 3),
}

// In a map
cache := map[string](int, bool){
	"key1": (100, true),
	"key2": (200, false),
}
```

### Function literals

A function literal represents an anonymous [function](#function-declarations).

```go
FunctionLit                 = [ AnonymousParameters ] "=>" AnonymousFunctionBody .
AnonymousParameters         = SingleAnonymousParameter | MultipleAnonymousParameters .
SingleAnonymousParameter    = identifier .
MultipleAnonymousParameters = "(" AnonymousParameterList ")" .
AnonymousParameterList      = identifier { "," identifier } .
AnonymousFunctionBody       = Expression | FunctionBody .
```

A anonymous function body can be a single expression.

```go
x => math.Sin(x)
(x, y) => x*x + y*y
```

It can also be the same as a normal function body. The above examples are equivalent to

```go
x => {
	return math.Sin(x)
}

(x, y) => {
	return x*x + y*y
}
```

Anonymous functions are typically passed in as parameters for function calls.

```go
import "sort"

a := [32, 100, 50, 6, 92]
sort.Slice a, (i, j) => {
	return a[i] < b[j]
}
```

Note: Both the parameter types and return value types of anonymous functions are omitted. XGo determines the parameter types and return value types of anonymous functions through type inference.

Function literals are _closures_: they may refer to variables defined in a surrounding function. Those variables are then shared between the surrounding function and the function literal, and they survive as long as they are accessible.


### Commands and calls

XGo supports two styles for function and method calls: the traditional function-call style with parentheses, and a command-style syntax without parentheses.

#### Function-call style

The traditional function-call syntax uses parentheses:

**Syntax:**

```go
CallOrConversion = Operand "(" [ ArgList ] [ "..." ] [ "," ] ")" .
ArgList          = (KwargExpr | LambdaExpr) { "," (KwargExpr | LambdaExpr) } .
KwargExpr        = IDENT "=" LambdaExpr .
```

Examples:

```go
echo("Hello world")
fmt.Println("Hello, world")
```

#### Command-style syntax

XGo recommends command-style code where the function name is followed by a space and then arguments, without parentheses:

```go
CommandStmt = IDENT [ "." IDENT ] [ SPACE ArgList ] [ "..." ] .
```

Examples:

```go
echo "Hello world"
fmt.Println "Hello, world"
```

Command-style calls can be qualified (with package or receiver name) or unqualified:

```go
echo "unqualified call"
fmt.Println "qualified call"
os.Exit 1
```

Variadic arguments are supported with the `...` operator:

```go
echo elements...
```

Both styles are equivalent and can be used interchangeably. XGo prefers command-style for its cleaner, more natural appearance, similar to shell commands. The built-in function `echo` is provided as an alias for `println` to emphasize this command-oriented approach.

#### Keyword arguments

XGo supports keyword arguments (kwargs) in commands and calls, allowing arguments to be specified by parameter name. When calling functions with many parameters, you can use `key=value` syntax to make your code more expressive and command-line-style.

#### Using kwargs with tuples

You can use tuples or tuple pointers as keyword parameters, which provides type safety:

```go
type Config (timeout, maxRetries int, debug bool)

func run(task int, cfg Config?) {
	if cfg.timeout == 0 {
		cfg.timeout = 30
	}
	if cfg.maxRetries == 0 {
		cfg.maxRetries = 3
	}
    echo "timeout:", cfg.timeout, "maxRetries:", cfg.maxRetries, "debug:", cfg.debug
	echo "task:", task
}

run 100, timeout = 60, maxRetries = 5
run 200
```

#### Using kwargs with maps

You also can use maps as keyword parameters:

```go
func process(opts map[string]any?, args ...any) {
    if name, ok := opts["name"]; ok {
        echo "name:", name
    }
    if age, ok := opts["age"]; ok {
        echo "age:", age
    }
    echo "args:", args
}

process name = "Ken", age = 17              // keyword parameters only
process "extra", 1, name = "Ken", age = 17  // variadic parameters first, then keyword parameters
process                                     // all parameters optional
```

**Key rules:**
- The keyword parameter must be an optional parameter.
- The keyword parameter must be the last parameter (without variadic) or second-to-last (with variadic).
- When calling a function, keyword arguments must be placed after all normal parameters (including variadic parameters). This might seem inconsistent with the order of keyword and variadic parameters in a function declaration, but that's the rule.


### Built-in functions

TODO

### Operators

Operators combine operands into expressions.

Binary operators:

```go
|| && == != < <= > >=
+ - * / %
| & ^ &^ << >>
```

Unary operators:

```go
+ - ! ^ * &
```

#### Operator precedence

_Unary operators_ have the highest precedence. As the ++ and -- operators form statements, not expressions, they fall outside the operator hierarchy. As a consequence, statement *p++ is the same as (*p)++.

There are five precedence levels for _binary operators_. Multiplication operators bind strongest, followed by addition operators, comparison operators, && (logical AND), and finally || (logical OR):

```
Precedence    Operator
    5             *  /  %  <<  >>  &  &^
    4             +  -  |  ^
    3             ==  !=  <  <=  >  >=
    2             &&
    1             ||
```

Binary operators of the same precedence associate from left to right. For instance, `x / y * z` is the same as `(x / y) * z`.

```go
+x                         // x
42 + a - b                 // (42 + a) - b
23 + 3*x[i]                // 23 + (3 * x[i])
x <= f()                   // x <= f()
^a >> b                    // (^a) >> b
f() || g()                 // f() || g()
x == y+1 && <-chanInt > 0  // (x == (y+1)) && ((<-chanInt) > 0)
```

#### Arithmetic operators

_Arithmetic operators_ apply to numeric values and yield a result of the same type as the first operand. The four standard arithmetic operators (+, -, *, /) apply to [integer](), [floating-point](), [rational]() and [complex]() types; + also applies to [strings](). The bitwise logical and shift operators apply to integers only.

```
+    sum                    integers (including bigint), floats, bigrat, complex values, strings
-    difference             integers (including bigint), floats, bigrat, complex values
*    product                integers (including bigint), floats, bigrat, complex values
/    quotient               integers (including bigint), floats, bigrat, complex values
%    remainder              integers (including bigint)

&    bitwise AND            integers (including bigint)
|    bitwise OR             integers (including bigint)
^    bitwise XOR            integers (including bigint)
&^   bit clear (AND NOT)    integers (including bigint)

<<   left shift             integer << integer >= 0
>>   right shift            integer >> integer >= 0
```

TODO

#### Comparison operators

_Comparison operators_ compare two operands and yield an untyped boolean value.

```go
==    equal
!=    not equal
<     less
<=    less or equal
>     greater
>=    greater or equal
```

In any comparison, the first operand must be [assignable]() to the type of the second operand, or vice versa.

The equality operators == and != apply to operands of comparable types. The ordering operators <, <=, >, and >= apply to operands of ordered types. These terms and the result of the comparisons are defined as follows:

* Boolean types are comparable. Two boolean values are equal if they are either both true or both false.
* Integer types are comparable and ordered. Two integer values are compared in the usual way.
* Floating-point types are comparable and ordered. Two floating-point values are compared as defined by the IEEE-754 standard.
* Complex types are comparable. Two complex values u and v are equal if both real(u) == real(v) and imag(u) == imag(v).
* String types are comparable and ordered. Two string values are compared lexically byte-wise.
* Pointer types are comparable. Two pointer values are equal if they point to the same variable or if both have value `nil`. Pointers to distinct [zero-size]() variables may or may not be equal.
* Interface types are comparable. Two interface values are equal if they have [identical]() dynamic types and equal dynamic values or if both have value `nil`.
* A value x of non-interface type X and a value t of interface type T can be compared if type X is comparable and X [implements]() T. They are equal if t's dynamic type is identical to X and t's dynamic value is equal to x.
* Array types are comparable if their array element types are comparable. Two array values are equal if their corresponding element values are equal. The elements are compared in ascending index order, and comparison stops as soon as two element values differ (or all elements have been compared).

A comparison of two interface values with identical dynamic types causes a [run-time panic]() if that type is not comparable. This behavior applies not only to direct interface value comparisons but also when comparing arrays of interface values or structs with interface-valued fields.

Slice, map, and function types are not comparable. However, as a special case, a slice, map, or function value may be compared to the predeclared identifier `nil`. Comparison of pointer, and interface values to `nil` is also allowed and follows from the general rules above.

#### Logical operators

Logical operators apply to [boolean]() values and yield a result of the same type as the operands. The left operand is evaluated, and then the right if the condition requires it.

```
&&    conditional AND    p && q  is  "if p then q else false"
||    conditional OR     p || q  is  "if p then true else q"
!     NOT                !p      is  "not p"
```

#### Address operators

For an operand x of type T, the address operation &x generates a pointer of type *T to x. The operand must be addressable, that is, either a variable, pointer indirection, or slice indexing operation; or a field selector of an addressable struct operand; or an array indexing operation of an addressable array. As an exception to the addressability requirement, x may also be a (possibly parenthesized) [composite literal](). If the evaluation of x would cause a [run-time panic](), then the evaluation of &x does too.

For an operand x of pointer type *T, the pointer indirection *x denotes the [variable]() of type T pointed to by x. If x is nil, an attempt to evaluate *x will cause a [run-time panic]().

```go
&x
&a[f(2)]
&Point{2, 3}
*p
*pf(x)

var x *int = nil
*x   // causes a run-time panic
&*x  // causes a run-time panic
```

#### Conversions

A _conversion_ changes the [type](#types) of an expression to the type specified by the conversion. A conversion may appear literally in the source, or it may be _implied_ by the context in which an expression appears.

An _explicit conversion_ is an expression of the form `T(x)` where `T` is a type and `x` is an expression that can be converted to type `T`.

```go
T(x)
```

If the type starts with the operator * or <-, or if the type starts with the keyword func and has no result list, it must be parenthesized when necessary to avoid ambiguity:

```go
*Point(p)        // same as *(Point(p))
(*Point)(p)      // p is converted to *Point
func()(x)        // function signature func() x
(func())(x)      // x is converted to func()
(func() int)(x)  // x is converted to func() int
func() int(x)    // x is converted to func() int (unambiguous)
```

A [constant]() value `x` can be converted to type `T` if `x` is [representable]() by a value of `T`. As a special case, an integer constant `x` can be explicitly converted to a [string type]() using the [same rule]() as for non-constant `x`.

Converting a constant to a type yields a typed constant.

```go
uint(iota)               // iota value of type uint
float32(2.718281828)     // 2.718281828 of type float32
complex128(1)            // 1.0 + 0.0i of type complex128
float32(0.49999999)      // 0.5 of type float32
float64(-1e-1000)        // 0.0 of type float64
string('x')              // "x" of type string
string(0x266c)           // "♬" of type string
myString("foo" + "bar")  // "foobar" of type myString
string([]byte{'a'})      // not a constant: []byte{'a'} is not a constant
(*int)(nil)              // not a constant: nil is not a constant, *int is not a boolean, numeric, or string type
int(1.2)                 // illegal: 1.2 cannot be represented as an int
string(65.0)             // illegal: 65.0 is not an integer constant
```

##### Conversions between numeric types

For the conversion of non-constant numeric values, the following rules apply:

* When converting between [integer types](#numeric-types), if the value is a signed integer, it is sign extended to implicit infinite precision; otherwise it is zero extended. It is then truncated to fit in the result type's size. For example, if v := uint16(0x10F0), then uint32(int8(v)) == 0xFFFFFFF0. The conversion always yields a valid value; there is no indication of overflow.
* When converting a [floating-point number](#numeric-types) to an integer, the fraction is discarded (truncation towards zero).
* When converting an integer or floating-point number to a floating-point type, or a [complex number](#numeric-types) to another complex type, the result value is rounded to the precision specified by the destination type. For instance, the value of a variable x of type float32 may be stored using additional precision beyond that of an IEEE-754 32-bit number, but float32(x) represents the result of rounding x's value to 32-bit precision. Similarly, x + 0.1 may use more than 32 bits of precision, but float32(x + 0.1) does not.

In all non-constant conversions involving floating-point or complex values, if the result type cannot represent the value the conversion succeeds but the result value is implementation-dependent.

##### Conversions to and from a string type

TODO

##### Conversions from slice to array or array pointer

TODO

### Constant expressions

Constant expressions may contain only [constant](#constants) operands and are evaluated at compile time.

Untyped boolean, numeric, and string constants may be used as operands wherever it is legal to use an operand of boolean, numeric, or string type, respectively.

A constant [comparison](#comparison-operators) always yields an untyped boolean constant. If the left operand of a constant [shift expression](#operators) is an untyped constant, the result is an integer constant; otherwise it is a constant of the same type as the left operand, which must be of [integer type](#numeric-types).

Any other operation on untyped constants results in an untyped constant of the same kind; that is, a boolean, integer, floating-point, complex, or string constant. If the untyped operands of a binary operation (other than a shift) are of different kinds, the result is of the operand's kind that appears later in this list: integer, rune, floating-point, complex. For example, an untyped integer constant divided by an untyped complex constant yields an untyped complex constant.

```go
const a = 2 + 3.0          // a == 5.0   (untyped floating-point constant)
const b = 15 / 4           // b == 3     (untyped integer constant)
const c = 15 / 4.0         // c == 3.75  (untyped floating-point constant)
const Θ float64 = 3/2      // Θ == 1.0   (type float64, 3/2 is integer division)
const Π float64 = 3/2.     // Π == 1.5   (type float64, 3/2. is float division)
const d = 1 << 3.0         // d == 8     (untyped integer constant)
const e = 1.0 << 3         // e == 8     (untyped integer constant)
const f = int32(1) << 33   // illegal    (constant 8589934592 overflows int32)
const g = float64(2) >> 1  // illegal    (float64(2) is a typed floating-point constant)
const h = "foo" > "bar"    // h == true  (untyped boolean constant)
const j = true             // j == true  (untyped boolean constant)
const k = 'w' + 1          // k == 'x'   (untyped rune constant)
const l = "hi"             // l == "hi"  (untyped string constant)
const m = string(k)        // m == "x"   (type string)
const Σ = 1 - 0.707i       //            (untyped complex constant)
const Δ = Σ + 2.0e-4       //            (untyped complex constant)
const Φ = iota*1i - 1/1i   //            (untyped complex constant)
```

Applying the built-in function `complex` to untyped integer, rune, or floating-point constants yields an untyped complex constant.

```go
const ic = complex(0, c)   // ic == 3.75i  (untyped complex constant)
const iΘ = complex(0, Θ)   // iΘ == 1i     (type complex128)
```

Constant expressions are always evaluated exactly; intermediate values and the constants themselves may require precision significantly larger than supported by any predeclared type in the language. The following are legal declarations:

```go
const Huge = 1 << 100         // Huge == 1267650600228229401496703205376  (untyped integer constant)
const Four int8 = Huge >> 98  // Four == 4                                (type int8)
```

The divisor of a constant division or remainder operation must not be zero:

```go
3.14 / 0.0   // illegal: division by zero
```

The values of typed constants must always be accurately [representable]() by values of the constant type. The following constant expressions are illegal:

```go
uint(-1)     // -1 cannot be represented as a uint
int(3.14)    // 3.14 cannot be represented as an int
int64(Huge)  // 1267650600228229401496703205376 cannot be represented as an int64
Four * 300   // operand 300 cannot be represented as an int8 (type of Four)
Four * 100   // product 400 cannot be represented as an int8 (type of Four)
```

The mask used by the unary bitwise complement operator ^ matches the rule for non-constants: the mask is all 1s for unsigned constants and -1 for signed and untyped constants.

```go
^1         // untyped integer constant, equal to -2
uint8(^1)  // illegal: same as uint8(-2), -2 cannot be represented as a uint8
^uint8(1)  // typed uint8 constant, same as 0xFF ^ uint8(1) = uint8(0xFE)
int8(^1)   // same as int8(-2)
^int8(1)   // same as -1 ^ int8(1) = -2
```

### Short variable declarations

A short variable declaration uses the syntax:

```go
ShortVarDecl = IdentifierList ":=" ExpressionList .
```

It is shorthand for a regular [variable declaration](#variable-declarations) with initializer expressions but no types:

```go
"var" IdentifierList "=" ExpressionList .
```

```go
i, j := 0, 10
f := func() int { return 7 }
ints := make([]int)
r, w, _ := os.Pipe()  // os.Pipe() returns a connected pair of Files and an error, if any
_, y, _ := coord(p)   // coord() returns three values; only interested in y coordinate
```

Unlike regular variable declarations, a short variable declaration may redeclare variables provided they were originally declared earlier in the same block (or the parameter lists if the block is the function body) with the same type, and at least one of the non-[blank]() variables is new. As a consequence, redeclaration can only appear in a multi-variable short declaration. Redeclaration does not introduce a new variable; it just assigns a new value to the original. The non-blank variable names on the left side of := must be [unique]().

```go
field1, offset := nextField(str, 0)
field2, offset := nextField(str, offset)  // redeclares offset
x, y, x := 1, 2, 3                        // illegal: x repeated on left side of :=
```

Short variable declarations may appear only inside functions. In some contexts such as the initializers for "[if]()", "[for]()", or "[switch]()" statements, they can be used to declare local temporary variables.


### Slice literals

TODO

```go
[expression1, ...]
```

For example:

```go
[]                   // []any
[1, 2, 3]            // []int
[10, 3.14, 200]      // []float64
["Hello", "world"]   // []string
["Hello", 100, true] // []any
```

The type of slice literals can be inferred from the context:

```go
func echoF32s(vals []float32) {
	echo vals
}

echo [10, 3.14, 200]           // []float64
echoF32s [10, 3.14, 200]       // []float32

var a []any = [10, 3.14, 200]  // []any
echo a
```

### Map literals

TODO

```go
{key1: value1, ...}
```

For example:

```go
{}                           // map[string]any
{"Monday": 1, "Sunday": 7}   // map[string]int
{1: 100, 3: 3.14, 5: 10}     // map[int]float64
```

The type of map literals can be inferred from the context:

```go
func echoS2f32(vals map[string]float32) {
	echo vals
}

echo {"Monday": 1, "Sunday": 7}
echoS2f32 {"Monday": 1, "Sunday": 7}

var a map[string]any = {"Monday": 1, "Sunday": 7}
echo a
```

### Order of evaluation

At package level, [initialization dependencies]() determine the evaluation order of individual initialization expressions in [variable declarations](). Otherwise, when evaluating the [operands]() of an expression, assignment, or [return statement](#return-statements), all function calls, method calls, [receive operations](), and [binary logical operations](#logical-operators) are evaluated in lexical left-to-right order.

For example, in the (function-local) assignment

```go
y[f()], ok = g(z || h(), i()+x[j()], <-c), k()
```

the function calls and communication happen in the order `f()`, `h()` (if z evaluates to false), `i()`, `j()`, `<-c`, `g()`, and `k()`. However, the order of those events compared to the evaluation and indexing of `x` and the evaluation of `y` and `z` is not specified, except as required lexically. For instance, `g` cannot be called before its arguments are evaluated.

```go
a := 1
f := func() int { a++; return a }
x := [a, f()]      // x may be [1, 2] or [2, 2]: evaluation order between a and f() is not specified
m := {a: 1, a: 2}  // m may be {2: 1} or {2: 2}: evaluation order between the two map assignments is not specified
n := {a: f()}      // n may be {2: 3} or {3: 3}: evaluation order between the key and the value is not specified
```

At package level, initialization dependencies override the left-to-right rule for individual initialization expressions, but not for operands within each expression:

```go
var a, b, c = f() + v(), g(), sqr(u()) + v()

func f() int        { return c }
func g() int        { return a }
func sqr(x int) int { return x*x }

// functions u and v are independent of all other variables and functions
```

The function calls happen in the order `u()`, `sqr()`, `v()`, `f()`, `v()`, and `g()`.

Floating-point operations within a single expression are evaluated according to the associativity of the operators. Explicit parentheses affect the evaluation by overriding the default associativity. In the expression `x + (y + z)` the addition `y + z` is performed before adding `x`.


## Statements

Statements control execution.

```go
Statement =
	Declaration | SimpleStmt | IfStmt | ForStmt | SwitchStmt |
    LabeledStmt | BreakStmt | ContinueStmt | FallthroughStmt | GotoStmt |
	ReturnStmt | DeferStmt | Block .

SimpleStmt = EmptyStmt | ExpressionStmt | IncDecStmt | Assignment | ShortVarDecl .
```

### Empty statements

The empty statement does nothing.

```go
EmptyStmt = .
```

### Expression statements

With the exception of specific built-in functions, function and method [calls](#commands-and-calls) and [receive operations]() can appear in statement context. Such statements may be parenthesized.

```go
ExpressionStmt = Expression .
```

The following built-in functions are not permitted in statement context:

```go
append cap complex imag len make new real
unsafe.Add unsafe.Alignof unsafe.Offsetof unsafe.Sizeof unsafe.Slice unsafe.SliceData unsafe.String unsafe.StringData
```

```go
h(x+y)
f.Close()
<-ch
(<-ch)
len("foo")  // illegal if len is the built-in function
```

### IncDec statements

The "++" and "--" statements increment or decrement their operands by the untyped constant 1. As with an assignment, the operand must be addressable or a map index expression.

```go
IncDecStmt = Expression ( "++" | "--" ) .
```

The following [assignment statements]() are semantically equivalent:

```go
IncDec statement    Assignment
x++                 x += 1
x--                 x -= 1
```

### Assignment statements

An assignment replaces the current value stored in a [variable](#variables) with a new value specified by an [expression](#expressions). An assignment statement may assign a single value to a single variable, or multiple values to a matching number of variables.

```go
Assignment = ExpressionList assign_op ExpressionList .
ExpressionList = Expression { "," Expression } .
```

Here `assign_op` can be:

```go
= += -= |= ^= *= /= %= <<= >>= &= &^=
```

Each left-hand side operand must be [addressable](#address-operators), a map index expression, or (for = assignments only) the [blank identifier](). Operands may be parenthesized.

```go
x = 1
*p = f()
a[i] = 23
(k) = <-ch  // same as: k = <-ch
```

An _assignment operation_ x _op=_ y where op is a binary [arithmetic operator](#arithmetic-operators) is equivalent to x = x op (y) but evaluates x only once. The op= construct is a single token. In assignment operations, both the left- and right-hand expression lists must contain exactly one single-valued expression, and the left-hand expression must not be the blank identifier.

```go
a[i] <<= 2
i &^= 1<<n
```

A tuple assignment assigns the individual elements of a multi-valued operation to a list of variables. There are two forms. In the first, the right hand operand is a single multi-valued expression such as a function call or [map](#map-types) operation, or a [type assertion](). The number of operands on the left hand side must match the number of values. For instance, if f is a function returning two values,

```go
x, y = f()
```

assigns the first value to x and the second to y. In the second form, the number of operands on the left must equal the number of expressions on the right, each of which must be single-valued, and the nth expression on the right is assigned to the nth operand on the left:

```go
one, two, three = '一', '二', '三'
```

The [blank identifier]() provides a way to ignore right-hand side values in an assignment:

```go
_ = x       // evaluate x but ignore it
x, _ = f()  // evaluate f() but ignore second result value
```

The assignment proceeds in two phases. First, the operands of [index expressions]() and [pointer indirections]() (including implicit pointer indirections in selectors) on the left and the expressions on the right are all [evaluated in the usual order](). Second, the assignments are carried out in left-to-right order.

```go
a, b = b, a  // exchange a and b

x := [1, 2, 3]
i := 0
i, x[i] = 1, 2  // set i = 1, x[0] = 2

i = 0
x[i], i = 2, 1  // set x[0] = 2, i = 1

x[0], x[0] = 1, 2  // set x[0] = 1, then x[0] = 2 (so x[0] == 2 at end)

x[1], x[3] = 4, 5  // set x[1] = 4, then panic setting x[3] = 5.

i = 2
x = [3, 5, 7]
for i, x[i] <- x {  // set i, x[2] = 0, x[0]
	break
}
// after this loop, i == 0 and x is [3, 5, 3]
```

In assignments, each value must be [assignable]() to the type of the operand to which it is assigned, with the following special cases:

* Any typed value may be assigned to the blank identifier.
* If an untyped constant is assigned to a variable of interface type or the blank identifier, the constant is first implicitly [converted](#conversions) to its [default type](#constants).
* If an untyped boolean value is assigned to a variable of interface type or the blank identifier, it is first implicitly converted to type bool.


### If statements

"If" statements specify the conditional execution of two branches according to the value of a boolean expression. If the expression evaluates to true, the "if" branch is executed, otherwise, if present, the "else" branch is executed.

```go
IfStmt = "if" [ SimpleStmt ";" ] Expression Block [ "else" ( IfStmt | Block ) ] .
```

```go
if x > 1 {
	x = 1
}
```

The expression may be preceded by a simple statement, which executes before the expression is evaluated.

```go
if x := f(); x < y {
	return x
} else if x > z {
	return z
} else {
	return y
}
```

### For statements

A "for" statement specifies repeated execution of a block. There are three forms: The iteration may be controlled by a single condition, a "for" clause, or a "range" clause.

```go
ForStmt = "for" [ Condition | ForClause | RangeClause ] Block .
Condition = Expression .
```

#### For statements with single condition

In its simplest form, a "for" statement specifies the repeated execution of a block as long as a boolean condition evaluates to true. The condition is evaluated before each iteration. If the condition is absent, it is equivalent to the boolean value true.

```go
for a < b {
	a *= 2
}
```

#### For statements with for clause

A "for" statement with a ForClause is also controlled by its condition, but additionally it may specify an init and a post statement, such as an assignment, an increment or decrement statement. The init statement may be a [short variable declaration](#short-variable-declarations), but the post statement must not.

```go
ForClause = [ InitStmt ] ";" [ Condition ] ";" [ PostStmt ] .
InitStmt = SimpleStmt .
PostStmt = SimpleStmt .
```

```go
for i := 0; i < 10; i++ {
	f(i)
}
```

If non-empty, the init statement is executed once before evaluating the condition for the first iteration; the post statement is executed after each execution of the block (and only if the block was executed). Any element of the ForClause may be empty but the [semicolons]() are required unless there is only a condition. If the condition is absent, it is equivalent to the boolean value true.

```go
for cond { S() }    is the same as    for ; cond ; { S() }
for      { S() }    is the same as    for true     { S() }
```

Each iteration has its own separate declared variable (or variables) [Go 1.22](). The variable used by the first iteration is declared by the init statement. The variable used by each subsequent iteration is declared implicitly before executing the post statement and initialized to the value of the previous iteration's variable at that moment.

```go
var prints []func()
for i := 0; i < 5; i++ {
	prints = append(prints, func() { println(i) })
	i++
}
for _, p := range prints {
	p()
}
```

prints

```
1
3
5
```

Prior to [Go 1.22], iterations share one set of variables instead of having their own separate variables. In that case, the example above prints

```
6
6
6
```

#### For statements with range clause

TODO

### Switch statements

"Switch" statements provide multi-way execution. An expression or type is compared to the "cases" inside the "switch" to determine which branch to execute.

```go
SwitchStmt = ExprSwitchStmt | TypeSwitchStmt .
```

There are two forms: expression switches and type switches. In an expression switch, the cases contain expressions that are compared against the value of the switch expression. In a type switch, the cases contain types that are compared against the type of a specially annotated switch expression. The switch expression is evaluated exactly once in a switch statement.

#### Expression switches

In an expression switch, the switch expression is evaluated and the case expressions, which need not be constants, are evaluated left-to-right and top-to-bottom; the first one that equals the switch expression triggers execution of the statements of the associated case; the other cases are skipped. If no case matches and there is a "default" case, its statements are executed. There can be at most one default case and it may appear anywhere in the "switch" statement. A missing switch expression is equivalent to the boolean value true.

```go
ExprSwitchStmt = "switch" [ SimpleStmt ";" ] [ Expression ] "{" { ExprCaseClause } "}" .
ExprCaseClause = ExprSwitchCase ":" StatementList .
ExprSwitchCase = "case" ExpressionList | "default" .
```

If the switch expression evaluates to an untyped constant, it is first implicitly [converted](#conversions) to its [default type](#constants). The predeclared untyped value nil cannot be used as a switch expression. The switch expression type must be [comparable](#comparison-operators).

If a case expression is untyped, it is first implicitly [converted](#conversions) to the type of the switch expression. For each (possibly converted) case expression x and the value t of the switch expression, x == t must be a valid [comparison](#comparison-operators).

In other words, the switch expression is treated as if it were used to declare and initialize a temporary variable t without explicit type; it is that value of t against which each case expression x is tested for equality.

In a case or default clause, the last non-empty statement may be a (possibly [labeled](#labeled-statements)) ["fallthrough" statement]() to indicate that control should flow from the end of this clause to the first statement of the next clause. Otherwise control flows to the end of the "switch" statement. A "fallthrough" statement may appear as the last statement of all but the last clause of an expression switch.

The switch expression may be preceded by a simple statement, which executes before the expression is evaluated.

```go
switch tag {
default: s3()
case 0, 1, 2, 3: s1()
case 4, 5, 6, 7: s2()
}

switch x := f(); {  // missing switch expression means "true"
case x < 0: return -x
default: return x
}

switch {
case x < y: f1()
case x < z: f2()
case x == 4: f3()
}
```

Implementation restriction: A compiler may disallow multiple case expressions evaluating to the same constant. For instance, the current compilers disallow duplicate integer, floating point, or string constants in case expressions.

#### Type switches

A type switch compares types rather than values. It is otherwise similar to an expression switch. It is marked by a special switch expression that has the form of a [type assertion]() using the keyword type rather than an actual type:

```go
switch x.(type) {
// cases
}
```

Cases then match actual types T against the dynamic type of the expression x. As with type assertions, x must be of interface type, but not a type parameter, and each non-interface type T listed in a case must implement the type of x. The types listed in the cases of a type switch must all be different.

```go
TypeSwitchStmt  = "switch" [ SimpleStmt ";" ] TypeSwitchGuard "{" { TypeCaseClause } "}" .
TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" .
TypeCaseClause  = TypeSwitchCase ":" StatementList .
TypeSwitchCase  = "case" TypeList | "default" .
```

The TypeSwitchGuard may include a [short variable declaration](#short-variable-declarations). When that form is used, the variable is declared at the end of the TypeSwitchCase in the [implicit block]() of each clause. In clauses with a case listing exactly one type, the variable has that type; otherwise, the variable has the type of the expression in the TypeSwitchGuard.

Instead of a type, a case may use the predeclared identifier [nil](); that case is selected when the expression in the TypeSwitchGuard is a `nil` interface value. There may be at most one `nil` case.

Given an expression x of type `any`, the following type switch:

```
switch i := x.(type) {
case nil:
	printString("x is nil")                // type of i is type of x (any)
case int:
	printInt(i)                            // type of i is int
case float64:
	printFloat64(i)                        // type of i is float64
case func(int) float64:
	printFunction(i)                       // type of i is func(int) float64
case bool, string:
	printString("type is bool or string")  // type of i is type of x (any)
default:
	printString("don't know the type")     // type of i is type of x (any)
}
```

could be rewritten:

```go
v := x  // x is evaluated exactly once
if v == nil {
	i := v                                 // type of i is type of x (any)
	printString("x is nil")
} else if i, isInt := v.(int); isInt {
	printInt(i)                            // type of i is int
} else if i, isFloat64 := v.(float64); isFloat64 {
	printFloat64(i)                        // type of i is float64
} else if i, isFunc := v.(func(int) float64); isFunc {
	printFunction(i)                       // type of i is func(int) float64
} else {
	_, isBool := v.(bool)
	_, isString := v.(string)
	if isBool || isString {
		i := v                         // type of i is type of x (any)
		printString("type is bool or string")
	} else {
		i := v                         // type of i is type of x (any)
		printString("don't know the type")
	}
}
```

The type switch guard may be preceded by a simple statement, which executes before the guard is evaluated.

The "fallthrough" statement is not permitted in a type switch.


### Labeled statements

A labeled statement may be the target of a goto, break or continue statement.

```go
LabeledStmt = Label ":" Statement .
Label       = identifier .
```

```go
Error:
	log.Panic("error encountered")
```

### Break statements

A "break" statement terminates execution of the innermost "[for](#for-statements)" or "[switch](#switch-statements)" statement within the same function.

```go
BreakStmt = "break" [ Label ] .
```

If there is a label, it must be that of an enclosing "for" or "switch" statement, and that is the one whose execution terminates.

```go
OuterLoop:
	for i = 0; i < n; i++ {
		for j = 0; j < m; j++ {
			switch a[i][j] {
			case nil:
				state = Error
				break OuterLoop
			case item:
				state = Found
				break OuterLoop
			}
		}
	}
```

### Continue statements

A "continue" statement begins the next iteration of the innermost enclosing "[for](#for-statements)" loop by advancing control to the end of the loop block. The "for" loop must be within the same function.

```go
ContinueStmt = "continue" [ Label ] .
```

If there is a label, it must be that of an enclosing "for" statement, and that is the one whose execution advances.

```go
RowLoop:
	for y, row := range rows {
		for x, data := range row {
			if data == endOfRow {
				continue RowLoop
			}
			row[x] = data + bias(x, y)
		}
	}
```

### Fallthrough statements

A "fallthrough" statement transfers control to the first statement of the next case clause in an expression "[switch](#switch-statements)" statement. It may be used only as the final non-empty statement in such a clause.

```go
FallthroughStmt = "fallthrough" .
```

### Goto statements

A "goto" statement transfers control to the statement with the corresponding label within the same function.

```go
GotoStmt = "goto" Label .
```

```go
goto Error
```

Executing the "goto" statement must not cause any variables to come into [scope]() that were not already in scope at the point of the goto. For instance, this example:

```go
	goto L  // BAD
	v := 3
L:
```

is erroneous because the jump to label L skips the creation of v.

A "goto" statement outside a [block]() cannot jump to a label inside that block. For instance, this example:

```go
if n%2 == 1 {
	goto L1  // BAD
}
for n > 0 {
	f()
	n--
L1:
	f()
	n--
}
```

is erroneous because the label L1 is inside the "for" statement's block but the goto is not.

### Return statements

A "return" statement in a function F terminates the execution of F, and optionally provides one or more result values. Any functions [deferred](#defer-statements) by F are executed before F returns to its caller.

```go
ReturnStmt = "return" [ ExpressionList ] .
```

In a function without a result type, a "return" statement must not specify any result values.

```go
func noResult() {
	return
}
```

There are three ways to return values from a function with a result type:

* The return value or values may be explicitly listed in the "return" statement. Each expression must be single-valued and [assignable]() to the corresponding element of the function's result type.

```go
func simpleF() int {
	return 2
}

func complexF1() (re float64, im float64) {
	return -7.0, -4.0
}
```

* The expression list in the "return" statement may be a single call to a multi-valued function. The effect is as if each value returned from that function were assigned to a temporary variable with the type of the respective value, followed by a "return" statement listing these variables, at which point the rules of the previous case apply.

```go
func complexF2() (re float64, im float64) {
	return complexF1()
}
```

* The expression list may be empty if the function's result type specifies names for its [result parameters](). The result parameters act as ordinary local variables and the function may assign values to them as necessary. The "return" statement returns the values of these variables.

```go
func complexF3() (re float64, im float64) {
	re = 7.0
	im = 4.0
	return
}
```

Regardless of how they are declared, all the result values are initialized to the [zero values]() for their type upon entry to the function. A "return" statement that specifies results sets the result parameters before any deferred functions are executed.

Implementation restriction: A compiler may disallow an empty expression list in a "return" statement if a different entity (constant, type, or variable) with the same name as a result parameter is in [scope]() at the place of the return.

```go
func f(n int) (res int, err error) {
	if _, err := f(n-1); err != nil {
		return  // invalid return statement: err is shadowed
	}
	return
}
```

### Defer statements

A "defer" statement invokes a function whose execution is deferred to the moment the surrounding function returns, either because the surrounding function executed a [return statement](#return-statements), reached the end of its [function body](), or because the corresponding goroutine is [panicking]().

```go
DeferStmt = "defer" Expression .
```

The expression must be a function or method call; it cannot be parenthesized. Calls of built-in functions are restricted as for [expression statements](#expression-statements).

Each time a "defer" statement executes, the function value and parameters to the call are [evaluated as usual]() and saved anew but the actual function is not invoked. Instead, deferred functions are invoked immediately before the surrounding function returns, in the reverse order they were deferred. That is, if the surrounding function returns through an explicit [return statement](#return-statements), deferred functions are executed after any result parameters are set by that return statement but before the function returns to its caller. If a deferred function value evaluates to nil, execution [panics]() when the function is invoked, not when the "defer" statement is executed.

For instance, if the deferred function is a [function literal]() and the surrounding function has [named result parameters]() that are in scope within the literal, the deferred function may access and modify the result parameters before they are returned. If the deferred function has any return values, they are discarded when the function completes. (See also the section on [handling panics]().)

```go
lock(l)
defer unlock(l)  // unlocking happens before surrounding function returns

// f returns 42
func f() (result int) {
	defer func() {
		// result is accessed after it was set to 6 by the return statement
		result *= 7
	}()
	return 6
}
```

### Terminating statements

TODO


## Built-in functions

Built-in functions are [predeclared](). They are called like any other function but some of them accept a type instead of an expression as the first argument.

The built-in functions do not have standard Go types, so they can only appear in [call expressions](#commands-and-calls); they cannot be used as function values.

### Appending to and copying slices

The built-in functions `append` and `copy` assist in common slice operations. For both functions, the result is independent of whether the memory referenced by the arguments overlaps.

The [variadic](#function-types) function append appends zero or more values x to a slice s and returns the resulting slice of the same type as s. The [underlying type](#underlying-types) of s must be a slice of type `[]E`. The values x are passed to a parameter of type `...E` and the respective [parameter passing rules]() apply. As a special case, if the underlying type of s is []byte, append also accepts a second argument with underlying type [bytestring]() followed by `...`. This form appends the bytes of the byte slice or string.

```go
append(s S, x ...E) S  // underlying type of S is []E
```

If the capacity of s is not large enough to fit the additional values, append [allocates]() a new, sufficiently large underlying array that fits both the existing slice elements and the additional values. Otherwise, append re-uses the underlying array.

```go
s0 := [0, 0]
s1 := append(s0, 2)                // append a single element     s1 is [0, 0, 2]
s2 := append(s1, 3, 5, 7)          // append multiple elements    s2 is [0, 0, 2, 3, 5, 7]
s3 := append(s2, s0...)            // append a slice              s3 is [0, 0, 2, 3, 5, 7, 0, 0]
s4 := append(s3[3:6], s3[2:]...)   // append overlapping slice    s4 is [3, 5, 7, 2, 3, 5, 7, 0, 0]

var t []any
t = append(t, 42, 3.1415, "foo")   //                             t is [42, 3.1415, "foo"]

var b []byte
b = append(b, "bar"...)            // append string contents      b is []byte("bar")
```

The function copy copies slice elements from a source src to a destination dst and returns the number of elements copied. The [underlying types](#underlying-types) of both arguments must be slices with [identical]() element type. The number of elements copied is the minimum of `len(src)` and `len(dst)`. As a special case, if the destination's underlying type is `[]byte`, copy also accepts a source argument with underlying type [bytestring](). This form copies the bytes from the byte slice or string into the byte slice.

```go
copy(dst, src []T) int
copy(dst []byte, src string) int
```

Examples:

```go
a := [0, 1, 2, 3, 4, 5, 6, 7]
s := make([]int, 6)
b := make([]byte, 5)
n1 := copy(s, a)                // n1 == 6, s is []int{0, 1, 2, 3, 4, 5}
n2 := copy(s, s[2:])            // n2 == 4, s is []int{2, 3, 4, 5, 4, 5}
n3 := copy(b, "Hello, World!")  // n3 == 5, b is []byte("Hello")
```

### Clear

The built-in function `clear` takes an argument of `map` or `slice` and deletes or zeroes out all elements.

```
Call        Argument type     Result

clear(m)    map[K]T           deletes all entries, resulting in an
                              empty map (len(m) == 0)

clear(s)    []T               sets all elements up to the length of
                              s to the zero value of T
```

If the map or slice is `nil`, `clear` is a no-op.

### Manipulating complex numbers

Three functions assemble and disassemble complex numbers. The built-in function complex constructs a complex value from a floating-point real and imaginary part, while real and imag extract the real and imaginary parts of a complex value.

```go
complex(realPart, imaginaryPart floatT) complexT
real(complexT) floatT
imag(complexT) floatT
```

The type of the arguments and return value correspond. For `complex`, the two arguments must be of the same [floating-point type](#numeric-types) and the return type is the [complex type](#numeric-types) with the corresponding floating-point constituents: `complex64` for `float32` arguments, and `complex128` for `float64` arguments. If one of the arguments evaluates to an untyped constant, it is first implicitly [converted](#conversions) to the type of the other argument. If both arguments evaluate to untyped constants, they must be non-complex numbers or their imaginary parts must be zero, and the return value of the function is an untyped complex constant.

For `real` and `imag`, the argument must be of complex type, and the return type is the corresponding floating-point type: `float32` for a `complex64` argument, and float64 for a complex128 argument. If the argument evaluates to an untyped constant, it must be a number, and the return value of the function is an untyped floating-point constant.

The `real` and `imag` functions together form the inverse of `complex`, so for a value `z` of a complex type `Z`, `z == Z(complex(real(z), imag(z)))`.

If the operands of these functions are all constants, the return value is a constant.

```go
var a = complex(2, -2)             // complex128
const b = complex(1.0, -1.4)       // untyped complex constant 1 - 1.4i
x := float32(math.Cos(math.Pi/2))  // float32
var c64 = complex(5, -x)           // complex64
var s int = complex(1, 0)          // untyped complex constant 1 + 0i can be converted to int
_ = complex(1, 2<<s)               // illegal: 2 assumes floating-point type, cannot shift
var rl = real(c64)                 // float32
var im = imag(a)                   // float64
const c = imag(b)                  // untyped constant -1.4
_ = imag(3 << s)                   // illegal: 3 assumes complex type, cannot shift
```

### Deletion of map elements

The built-in function `delete` removes the element with key `k` from a [map](#map-types) `m`. The value `k` must be [assignable]() to the key type of `m`.

```go
delete(m, k)  // remove element m[k] from map m
```

If the map m is nil or the element m[k] does not exist, delete is a no-op.

### Length and capacity

The built-in functions `len` and `cap` take arguments of various types and return a result of type `int`. The implementation guarantees that the result always fits into an `int`.

```go
Call      Argument type    Result

len(s)    string type      string length in bytes
          [n]T, *[n]T      array length (== n)
          []T              slice length
          map[K]T          map length (number of defined keys)
          type parameter   see below

cap(s)    [n]T, *[n]T      array length (== n)
          []T              slice capacity
          type parameter   see below
```

The capacity of a slice is the number of elements for which there is space allocated in the underlying array. At any time the following relationship holds:

```go
0 <= len(s) <= cap(s)
```

The length of a `nil` slice, map is `0`. The capacity of a `nil` slice is `0`.

The expression `len(s)` is [constant](#constants) if s is a string constant. The expressions `len(s)` and `cap(s)` are constants if the type of `s` is an array or pointer to an array and the expression `s` does not contain (non-constant) [function calls](#commands-and-calls); in this case `s` is not evaluated. Otherwise, invocations of `len` and `cap` are not constant and `s` is evaluated.

```go
const (
	c1 = imag(2i)                    // imag(2i) = 2.0 is a constant
	c2 = len([10]float64{2})         // [10]float64{2} contains no function calls
	c3 = len([10]float64{c1})        // [10]float64{c1} contains no function calls
	c4 = len([10]float64{imag(2i)})  // imag(2i) is a constant and no function call is issued
	c5 = len([10]float64{imag(z)})   // invalid: imag(z) is a (non-constant) function call
)
var z complex128
```

Making slices and maps

The built-in function `make` takes a type `T`, optionally followed by a type-specific list of expressions. The [underlying type]() of `T` must be a slice or map. It returns a value of type `T` (not `*T`). The memory is initialized as described in the section on [initial values]().

```go
Call             Underlying type    Result

make(T, n)       slice        slice of type T with length n and capacity n
make(T, n, m)    slice        slice of type T with length n and capacity m

make(T)          map          map of type T
make(T, n)       map          map of type T with initial space for approximately n elements
```

Each of the size arguments `n` and `m` must be of [integer type](#numeric-types), have a [type set](#interface-types) containing only integer types, or be an untyped constant. A constant size argument must be non-negative and [representable]() by a value of type `int`; if it is an untyped constant it is given type `int`. If both `n` and `m` are provided and are constant, then `n` must be no larger than `m`. For slices, if `n` is negative or larger than `m` at run time, a [run-time panic]() occurs.

```go
s := make([]int, 10, 100)       // slice with len(s) == 10, cap(s) == 100
s := make([]int, 1e3)           // slice with len(s) == cap(s) == 1000
s := make([]int, 1<<63)         // illegal: len(s) is not representable by a value of type int
s := make([]int, 10, 0)         // illegal: len(s) > cap(s)
m := make(map[string]int, 100)  // map with initial space for approximately 100 elements
```

Calling make with a map type and size hint `n` will create a map with initial space to hold `n` map elements. The precise behavior is implementation-dependent.


### Allocation

The built-in function `new` takes a type `T`, allocates storage for a [variable](#variables) of that type at run time, and returns a value of type `*T` [pointing](#pointer-types) to it. The variable is initialized as described in the section on [initial values]().

```go
new(T)
```

For instance

```go
new(int)
```

allocates storage for a variable of type `int`, initializes it `0`, and returns a value of type `*int` containing the address of the location.


### Min and max

The built-in functions `min` and `max` compute the smallest—or largest, respectively—value of a fixed number of arguments of [ordered types](). There must be at least one argument.

The same type rules as for [operators](#operators) apply: for [ordered]() arguments `x` and `y`, `min(x, y)` is valid if `x + y` is valid, and the type of `min(x, y)` is the type of `x + y` (and similarly for `max`). If all arguments are constant, the result is constant.

```go
var x, y int
m := min(x)                 // m == x
m := min(x, y)              // m is the smaller of x and y
m := max(x, y, 10)          // m is the larger of x and y but at least 10
c := max(1, 2.0, 10)        // c == 10.0 (floating-point kind)
f := max(0, float32(x))     // type of f is float32
var s []string
_ = min(s...)               // invalid: slice arguments are not permitted
t := max("", "foo", "bar")  // t == "foo" (string kind)
```

For numeric arguments, assuming all `NaN`s are equal, `min` and `max` are commutative and associative:

```
min(x, y)    == min(y, x)
min(x, y, z) == min(min(x, y), z) == min(x, min(y, z))
```

For floating-point arguments negative zero, `NaN`, and infinity the following rules apply:

```go
x        y    min(x, y)    max(x, y)

-0.0    0.0         -0.0          0.0    // negative zero is smaller than (non-negative) zero
-Inf      y         -Inf            y    // negative infinity is smaller than any other number
+Inf      y            y         +Inf    // positive infinity is larger than any other number
 NaN      y          NaN          NaN    // if any argument is a NaN, the result is a NaN
```

For string arguments the result for min is the first argument with the smallest (or for max, largest) value, compared lexically byte-wise:

```go
min(x, y)    == if x <= y then x else y
min(x, y, z) == min(min(x, y), z)
```

### Handling panics

Two built-in functions, `panic` and `recover`, assist in reporting and handling [run-time panics]() and program-defined error conditions.

```go
func panic(any)
func recover() any
```

While executing a function `F`, an explicit call to `panic` or a [run-time panic]() terminates the execution of `F`. Any functions [deferred]() by `F` are then executed as usual. Next, any deferred functions run by `F`'s caller are run, and so on up to any deferred by the top-level function in the executing goroutine. At that point, the program is terminated and the error condition is reported, including the value of the argument to panic. This termination sequence is called _panicking_.

```go
panic(42)
panic("unreachable")
panic(Error("cannot parse"))
```

The `recover` function allows a program to manage behavior of a panicking goroutine. Suppose a function `G` defers a function `D` that calls `recover` and a panic occurs in a function on the same goroutine in which `G` is executing. When the running of deferred functions reaches `D`, the return value of `D`'s call to recover will be the value passed to the call of panic. If `D` returns normally, without starting a new panic, the panicking sequence stops. In that case, the state of functions called between `G` and the call to panic is discarded, and normal execution resumes. Any functions deferred by `G` before `D` are then run and `G`'s execution terminates by returning to its caller.

The return value of `recover` is `nil` when the goroutine is not panicking or `recover` was not called directly by a deferred function. Conversely, if a goroutine is panicking and recover was called directly by a deferred function, the return value of recover is guaranteed not to be `nil`. To ensure this, calling panic with a `nil` interface value (or an untyped nil) causes a [run-time panic]().

The `protect` function in the example below invokes the function argument `g` and protects callers from run-time panics raised by `g`.

```go
func protect(g func()) {
	defer func() {
		log.Println("done")  // Println executes normally even if there is a panic
		if x := recover(); x != nil {
			log.Printf("run time panic: %v", x)
		}
	}()
	log.Println("start")
	g()
}
```

### TODO

```go
print
printf
println
...
```

## Blocks

A _block_ is a possibly empty sequence of declarations and statements within matching brace brackets.

```go
Block = "{" StatementList "}" .
StatementList = { Statement ";" } .
```

In addition to explicit blocks in the source code, there are implicit blocks:

* The _universe block_ encompasses all XGo source text.
* Each [package](#packages) has a _package block_ containing all XGo source text for that package.
* Each file has a _file block_ containing all XGo source text in that file.
* Each "[if](#if-statements)", "[for](#for-statements)", and "[switch](#switch-statements)" statement is considered to be in its own implicit block.
* Each clause in a "[switch](#switch-statements)" statement acts as an implicit block.

Blocks nest and influence [scoping]().


## Declarations and scope

A _declaration_ binds a non-[blank]() identifier to a [constant](), [type](), [variable](), [function](), [label](), or [package](). Every identifier in a program must be declared. No identifier may be declared twice in the same block, and no identifier may be declared in both the file and package block.

The [blank identifier]() may be used like any other identifier in a declaration, but it does not introduce a binding and thus is not declared. In the package block, the identifier `init` may only be used for [init function]() declarations, and like the blank identifier it does not introduce a new binding.

```go
Declaration   = ConstDecl | TypeDecl | VarDecl .
TopLevelDecl  = Declaration | FunctionDecl .
```

The scope of a declared identifier is the extent of source text in which the identifier denotes the specified constant, type, variable, function, label, or package.

XGo is lexically scoped using blocks:

* The scope of a [predeclared identifier]() is the universe block.
* The scope of an identifier denoting a constant, type, variable, or function declared at top level (outside any function) is the package block.
* The scope of the package name of an imported package is the file block of the file containing the import declaration.
* The scope of an identifier denoting function parameter, or result variable is the function body.
* The scope of a constant or variable identifier declared inside a function begins at the end of the `ConstSpec` or `VarSpec` (`ShortVarDecl` for short variable declarations) and ends at the end of the innermost containing block.

An identifier declared in a block may be redeclared in an inner block. While the identifier of the inner declaration is in scope, it denotes the entity declared by the inner declaration.

The [package clause]() is not a declaration; the package name does not appear in any scope. Its purpose is to identify the files belonging to the same [package](#packages) and to specify the default package name for import declarations.

### Label scopes

Labels are declared by [labeled statements](#labeled-statements) and are used in the "[break](#break-statements)", "[continue](#continue-statements)", and "[goto](#goto-statements)" statements. It is illegal to define a label that is never used. In contrast to other identifiers, labels are not block scoped and do not conflict with identifiers that are not labels. The scope of a label is the body of the function in which it is declared and excludes the body of any nested function.


### Blank identifier

The _blank identifier_ is represented by the underscore character `_`. It serves as an anonymous placeholder instead of a regular (non-blank) identifier and has special meaning in [declarations](#declarations-and-scope), as an [operand](), and in [assignment statements](#assignment-statements).

### Predeclared identifiers

The following identifiers are implicitly declared in the [universe block]():

```go
Types:
	any bigint bigrat bool byte comparable
	complex64 complex128 error float32 float64
	int int8 int16 int32 int64 rune string
	uint uint8 uint16 uint32 uint64 uintptr

Constants:
	true false iota

Zero value:
	nil

Functions:
	append cap clear close complex copy delete echo imag len
	make max min new panic print printf println real recover // TODO(xsw): more
```

### Exported identifiers

An identifier may be _exported_ to permit access to it from another package. An identifier is exported if both:

* the first character of the identifier's name is a Unicode uppercase letter (Unicode character category Lu); and
* the identifier is declared in the [package block]() or it is a [field name]() or [method name]().

All other identifiers are not exported.

### Uniqueness of identifiers

Given a set of identifiers, an identifier is called _unique_ if it is _different_ from every other in the set. Two identifiers are different if they are spelled differently, or if they appear in different [packages](#packages) and are not [exported](#exported-identifiers). Otherwise, they are the same.

### Constant declarations

A constant declaration binds a list of identifiers (the names of the constants) to the values of a list of [constant expressions](#constant-expressions). The number of identifiers must be equal to the number of expressions, and the nth identifier on the left is bound to the value of the nth expression on the right.

```go
ConstDecl      = "const" ( ConstSpec | "(" { ConstSpec ";" } ")" ) .
ConstSpec      = IdentifierList [ [ Type ] "=" ExpressionList ] .

IdentifierList = identifier { "," identifier } .
ExpressionList = Expression { "," Expression } .
```

If the type is present, all constants take the type specified, and the expressions must be [assignable]() to that type, which must not be a type parameter. If the type is omitted, the constants take the individual types of the corresponding expressions. If the expression values are untyped [constants](#constants), the declared constants remain untyped and the constant identifiers denote the constant values. For instance, if the expression is a floating-point literal, the constant identifier denotes a floating-point constant, even if the literal's fractional part is zero.

```go
const Pi float64 = 3.14159265358979323846
const zero = 0.0         // untyped floating-point constant
const (
	size int64 = 1024
	eof        = -1  // untyped integer constant
)
const a, b, c = 3, 4, "foo"  // a = 3, b = 4, c = "foo", untyped integer and string constants
const u, v float32 = 0, 3    // u = 0.0, v = 3.0
```

Within a parenthesized const declaration list the expression list may be omitted from any but the first ConstSpec. Such an empty list is equivalent to the textual substitution of the first preceding non-empty expression list and its type if any. Omitting the list of expressions is therefore equivalent to repeating the previous list. The number of identifiers must be equal to the number of expressions in the previous list. Together with the [iota constant generator](#iota) this mechanism permits light-weight declaration of sequential values:

```go
const (
	Sunday = iota
	Monday
	Tuesday
	Wednesday
	Thursday
	Friday
	Partyday
	numberOfDays  // this constant is not exported
)
```

### Iota

Within a [constant declaration](#constant-declarations), the predeclared identifier iota represents successive untyped integer [constants](#constants). Its value is the index of the respective [ConstSpec]() in that constant declaration, starting at zero. It can be used to construct a set of related constants:

```go
const (
	c0 = iota  // c0 == 0
	c1 = iota  // c1 == 1
	c2 = iota  // c2 == 2
)

const (
	a = 1 << iota  // a == 1  (iota == 0)
	b = 1 << iota  // b == 2  (iota == 1)
	c = 3          // c == 3  (iota == 2, unused)
	d = 1 << iota  // d == 8  (iota == 3)
)

const (
	u         = iota * 42  // u == 0     (untyped integer constant)
	v float64 = iota * 42  // v == 42.0  (float64 constant)
	w         = iota * 42  // w == 84    (untyped integer constant)
)

const x = iota  // x == 0
const y = iota  // y == 0
```

By definition, multiple uses of iota in the same ConstSpec all have the same value:

```go
const (
	bit0, mask0 = 1 << iota, 1<<iota - 1  // bit0 == 1, mask0 == 0  (iota == 0)
	bit1, mask1                           // bit1 == 2, mask1 == 1  (iota == 1)
	_, _                                  //                        (iota == 2, unused)
	bit3, mask3                           // bit3 == 8, mask3 == 7  (iota == 3)
)
```

This last example exploits the [implicit repetition](#constant-declarations) of the last non-empty expression list.

### Type declarations

A type declaration binds an identifier, the _type name_, to a [type](#types). Type declarations come in two forms: alias declarations and type definitions.

```go
TypeDecl = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
TypeSpec = AliasDecl | TypeDef .
```

#### Alias declarations

An alias declaration binds an identifier to the given type.

```go
AliasDecl = identifier "=" Type .
```

Within the [scope]() of the identifier, it serves as an alias for the type.

```go
type (
	nodeList = []*Node  // nodeList and []*Node are identical types
	Polar    = polar    // Polar and polar denote identical types
)
```

#### Type definitions

A type definition creates a new, distinct type with the same [underlying type](#underlying-types) and operations as the given type and binds an identifier, the _type name_, to it.

```go
TypeDef = identifier Type .
```

The new type is called a _defined type_. It is [different]() from any other type, including the type it is created from.

```go
type (
	Point struct{ x, y float64 }  // Point and struct{ x, y float64 } are different types
	polar Point                   // polar and Point denote different types
)

type TreeNode struct {
	left, right *TreeNode
	value any
}

type Block interface {
	BlockSize() int
	Encrypt(src, dst []byte)
	Decrypt(src, dst []byte)
}
```

Type definitions may be used to define different boolean, numeric, or string types:

```go
type TimeZone int

const (
	EST TimeZone = -(5 + iota)
	CST
	MST
	PST
)
```

### Variable declarations

A variable declaration creates one or more [variables](#variables), binds corresponding identifiers to them, and gives each a type and an initial value.

```go
VarDecl     = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) .
VarSpec     = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .
```

```go
var i int
var U, V, W float64
var k = 0
var x, y float32 = -1, -2
var (
	i       int
	u, v, s = 2.0, 3.0, "bar"
)
var re, im = complexSqrt(-1)
var _, found = entries[name]  // map lookup; only interested in "found"
```

If a list of expressions is given, the variables are initialized with the expressions following the rules for [assignment statements](#assignment-statements). Otherwise, each variable is initialized to its [zero value](#the-zero-value).

If a type is present, each variable is given that type. Otherwise, each variable is given the type of the corresponding initialization value in the assignment. If that value is an untyped constant, it is first implicitly [converted](#conversions) to its [default type](#constants); if it is an untyped boolean value, it is first implicitly converted to type `bool`. The predeclared value `nil` cannot be used to initialize a variable with no explicit type.

```go
var d = math.Sin(0.5)  // d is float64
var i = 42             // i is int
var t, ok = x.(T)      // t is T, ok is bool
var n = nil            // illegal
```

Implementation restriction: A compiler may make it illegal to declare a variable inside a [function body](#function-declarations) if the variable is never used.

In a function body, variables do not need to be explicitly defined.

```go
d := math.Sin(0.5)  // d is float64
i := 42             // i is int
t, ok := x.(T)      // t is T, ok is bool
```

See [short variable declarations](#short-variable-declarations).

### Function declarations

A function declaration binds an identifier, the function name, to a function.

```go
FunctionDecl = "func" FunctionName Signature [ FunctionBody ] .
FunctionName = identifier .
FunctionBody = Block .
```

If the function's [signature](#function-types) declares result parameters, the function body's statement list must end in a [terminating statement](#terminating-statements).

```go
func IndexRune(s string, r rune) int {
	for i, c := range s {
		if c == r {
			return i
		}
	}
	// invalid: missing return statement
}
```


## Packages

XGo programs are constructed by linking together packages. A package in turn is constructed from one or more source files that together declare constants, types, variables and functions belonging to the package and which are accessible in all files of the same package. Those elements may be [exported]() and used in another package.

### Source file organization

Each source file consists of a package clause defining the package to which it belongs, followed by a possibly empty set of import declarations that declare packages whose contents it wishes to use, followed by a possibly empty set of declarations of functions, types, variables, and constants.

```go
SourceFile       = [ PackageClause ";" ] { ImportDecl ";" } { TopLevelDecl ";" } .
```

### Package clause

A package clause begins each source file and defines the package to which the file belongs.

```go
PackageClause  = "package" PackageName .
PackageName    = identifier .
```

The PackageName must not be the [blank identifier]().

```go
package math
```

### Import declarations

An import declaration states that the source file containing the declaration depends on functionality of the `imported` package ([Program initialization and execution]()) and enables access to [exported]() identifiers of that package. The import names an identifier (PackageName) to be used for access and an ImportPath that specifies the package to be imported.

```go
ImportDecl       = "import" ( ImportSpec | "(" { ImportSpec ";" } ")" ) .
ImportSpec       = [ "." | PackageName ] ImportPath .
ImportPath       = string_lit .
```

The PackageName is used in [qualified identifiers]() to access exported identifiers of the package within the importing source file. It is declared in the [file block](). If the PackageName is omitted, it defaults to the identifier specified in the [package clause](#package-clause) of the imported package. If an explicit period (.) appears instead of a name, all the package's exported identifiers declared in that package's [package block]() will be declared in the importing source file's file block and must be accessed without a qualifier.

The interpretation of the ImportPath is implementation-dependent but it is typically a substring of the full file name of the compiled package and may be relative to a repository of installed packages.

Implementation restriction: A compiler may restrict ImportPaths to non-empty strings using only characters belonging to [Unicode's]() L, M, N, P, and S general categories (the Graphic characters without spaces) and may also exclude the characters !"#$%&'()*,:;<=>?[\]^`{|} and the Unicode replacement character U+FFFD.

Consider a compiled a package containing the package clause package math, which exports function `Sin`, and installed the compiled package in the file identified by "lib/math". This table illustrates how `Sin` is accessed in files that import the package after the various types of import declaration.

```go
Import declaration          Local name of Sin

import   "lib/math"         math.Sin
import m "lib/math"         m.Sin
import . "lib/math"         Sin
```

An import declaration declares a dependency relation between the importing and imported package. It is illegal for a package to import itself, directly or indirectly, or to directly import a package without referring to any of its exported identifiers. To import a package solely for its side-effects (initialization), use the [blank]() identifier as explicit package name:

```go
import _ "lib/math"
```

### An example package

Here is a complete XGo package that implements XXX.

```go
TODO
```

## Program initialization and execution

### The zero value

When storage is allocated for a [variable](#variables), either through a declaration or a call of `new`, or when a new value is created, either through a composite literal or a call of `make`, and no explicit initialization is provided, the variable or value is given a default value. Each element of such a variable or value is set to the _zero_ value for its type: `false` for booleans, `0` for numeric types, `""` for strings, and `nil` for pointers, functions, interfaces, slices, and maps. This initialization is done recursively, so for instance each element of an array of structs will have its fields zeroed if no value is specified.

These two simple declarations are equivalent:

```go
var i int
var i int = 0
```

TODO

### Package initialization

Within a package, package-level variable initialization proceeds stepwise, with each step selecting the variable earliest in `declaration order` which has no dependencies on uninitialized variables.

More precisely, a package-level variable is considered ready for `initialization` if it is not yet initialized and either has no [initialization expression]() or its initialization expression has no dependencies on uninitialized variables. Initialization proceeds by repeatedly initializing the next package-level variable that is earliest in declaration order and ready for initialization, until there are no variables ready for initialization.

If any variables are still uninitialized when this process ends, those variables are part of one or more initialization cycles, and the program is not valid.

Multiple variables on the left-hand side of a variable declaration initialized by single (multi-valued) expression on the right-hand side are initialized together: If any of the variables on the left-hand side is initialized, all those variables are initialized in the same step.

```go
var x = a
var a, b = f() // a and b are initialized together, before x is initialized
```

For the purpose of package initialization, [blank]() variables are treated like any other variables in declarations.

The declaration order of variables declared in multiple files is determined by the order in which the files are presented to the compiler: Variables declared in the first file are declared before any of the variables declared in the second file, and so on. To ensure reproducible initialization behavior, build systems are encouraged to present multiple files belonging to the same package in lexical file name order to a compiler.

Dependency analysis does not rely on the actual values of the variables, only on lexical `references` to them in the source, analyzed transitively. For instance, if a variable x's initialization expression refers to a function whose body refers to variable y then x depends on y. Specifically:

* A reference to a variable or function is an identifier denoting that variable or function.
* A reference to a method m is a [method value]() or [method expression]() of the form `t.m`, where the (static) type of t is not an interface type, and the method `m` is in the method set of `t`. It is immaterial whether the resulting function value `t.m` is invoked.
* A variable, function, or method x depends on a variable y if x's initialization expression or body (for functions and methods) contains a reference to y or to a function or method that depends on y.

For example, given the declarations

```go
var (
	a = c + b  // == 9
	b = f()    // == 4
	c = f()    // == 5
	d = 3      // == 5 after initialization has finished
)

func f() int {
	d++
	return d
}
```

the initialization order is `d`, `b`, `c`, `a`. Note that the order of subexpressions in initialization expressions is irrelevant: `a = c + b` and `a = b + c` result in the same initialization order in this example.

Dependency analysis is performed per package; only references referring to variables, functions, and (non-interface) methods declared in the current package are considered. If other, hidden, data dependencies exists between variables, the initialization order between those variables is unspecified.

For instance, given the declarations (TODO: use classfile instead of method)

```go
var x = I(T{}).ab()   // x has an undetected, hidden dependency on a and b
var _ = sideEffect()  // unrelated to x, a, or b
var a = b
var b = 42

type I interface      { ab() []int }
type T struct{}
func (T) ab() []int   { return []int{a, b} }
```

the variable `a` will be initialized after `b` but whether `x` is initialized before `b`, between `b` and `a`, or after `a`, and thus also the moment at which `sideEffect()` is called (before or after `x` is initialized) is not specified.

Variables may also be initialized using functions named init declared in the package block, with no arguments and no result parameters.

```go
func init() { … }
```

Multiple such functions may be defined per package, even within a single source file. In the package block, the `init` identifier can be used only to declare `init` functions, yet the identifier itself is not [declared](). Thus init functions cannot be referred to from anywhere in a program.

The entire package is initialized by assigning initial values to all its package-level variables followed by calling all `init` functions in the order they appear in the source, possibly in multiple files, as presented to the compiler.

### Program initialization

The packages of a complete program are initialized stepwise, one package at a time. If a package has imports, the imported packages are initialized before initializing the package itself. If multiple packages import a package, the imported package will be initialized only once. The importing of packages, by construction, guarantees that there can be no cyclic initialization dependencies. More precisely:

Given the list of all packages, sorted by import path, in each step the first uninitialized package in the list for which all imported packages (if any) are already initialized is [initialized](#package-initialization). This step is repeated until all packages are initialized.

Package initialization—variable initialization and the invocation of `init` functions—happens in a single goroutine, sequentially, one package at a time. An `init` function may launch other goroutines, which can run concurrently with the initialization code. However, initialization always sequences the `init` functions: it will not invoke the next one until the previous one has returned.

### Program execution

A complete program is created by linking a single, unimported package called the main package with all the packages it imports, transitively. The main package must have package name main and declare a function main that takes no arguments and returns no value.

```go
func main() { … }
```

Program execution begins by [initializing the program](#program-initialization) and then invoking the function `main` in package `main`. When that function invocation returns, the program exits. It does not wait for other (non-main) goroutines to complete.


### Run-time panics

Execution errors such as attempting to index an array out of bounds trigger a _run-time panic_ equivalent to a call of the built-in function [panic]() with a value of the implementation-defined interface type `runtime.Error`. That type satisfies the predeclared interface type [error](#errors). The exact error values that represent distinct run-time error conditions are unspecified.

```go
package runtime

type Error interface {
	error
	// and perhaps other methods
}
```


## System considerations

### Package unsafe

The built-in package `unsafe`, known to the compiler and accessible through the [import path]() `"unsafe"`, provides facilities for low-level programming including operations that violate the type system. A package using unsafe must be vetted manually for type safety and may not be portable. The package provides the following interface:

```go
package unsafe

type ArbitraryType int  // shorthand for an arbitrary Go type; it is not a real type
type Pointer *ArbitraryType

func Alignof(variable ArbitraryType) uintptr
func Offsetof(selector ArbitraryType) uintptr
func Sizeof(variable ArbitraryType) uintptr

type IntegerType int  // shorthand for an integer type; it is not a real type
func Add(ptr Pointer, len IntegerType) Pointer
func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType
func SliceData(slice []ArbitraryType) *ArbitraryType
func String(ptr *byte, len IntegerType) string
func StringData(str string) *byte
```

A Pointer is a [pointer type](#pointer-types) but a Pointer value may not be [dereferenced](#address-operators). Any pointer or value of [underlying type](#underlying-types) `uintptr` can be [converted](#conversions) to a type of underlying type Pointer and vice versa. The effect of converting between Pointer and `uintptr` is implementation-defined.

```go
var f float64
bits = *(*uint64)(unsafe.Pointer(&f))

type ptr unsafe.Pointer
bits = *(*uint64)(ptr(&f))

func f[P ~*B, B any](p P) uintptr {
	return uintptr(unsafe.Pointer(p))
}

var p ptr = nil
```

The functions `Alignof` and `Sizeof` take an expression `x` of any type and return the alignment or size, respectively, of a hypothetical variable `v` as if `v` was declared via `var v = x`.

The function `Offsetof` takes a (possibly parenthesized) [selector]() `s.f`, denoting a field `f` of the struct denoted by `s` or `*s`, and returns the field offset in bytes relative to the struct's address. If `f` is an [embedded field](), it must be reachable without pointer indirections through fields of the struct. For a struct `s` with field `f`:

```go
uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f) == uintptr(unsafe.Pointer(&s.f))
```

Computer architectures may require memory addresses to be _aligned_; that is, for addresses of a variable to be a multiple of a factor, the variable's type's _alignment_. The function `Alignof` takes an expression denoting a variable of any type and returns the alignment of the (type of the) variable in bytes. For a variable `x`:

```go
uintptr(unsafe.Pointer(&x)) % unsafe.Alignof(x) == 0
```

A (variable of) type T has _variable size_ if T is a [type parameter](), or if it is an array or struct type containing elements or fields of variable size. Otherwise the size is constant. Calls to `Alignof`, `Offsetof`, and `Sizeof` are compile-time [constant expressions](#constant-expressions) of type `uintptr` if their arguments (or the struct `s` in the selector expression `s.f` for `Offsetof`) are types of constant size.

The function `Add` adds `len` to `ptr` and returns the updated pointer `unsafe.Pointer(uintptr(ptr) + uintptr(len))`. The `len` argument must be of [integer type](#numeric-types) or an untyped [constant](#constants). A constant `len` argument must be [representable]() by a value of type `int`; if it is an untyped constant it is given type `int`. The rules for [valid uses]() of Pointer still apply.

The function `Slice` returns a slice whose underlying array starts at `ptr` and whose length and capacity are `len`. `Slice(ptr, len)` is equivalent to

```go
(*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]
```

except that, as a special case, if `ptr` is `nil` and `len` is zero, `Slice` returns `nil`.

The `len` argument must be of [integer type](#numeric-types) or an untyped [constant](#constants). A constant `len` argument must be non-negative and [representable]() by a value of type `int`; if it is an untyped constant it is given type `int`. At run time, if `len` is negative, or if `ptr` is nil and `len` is not zero, a [run-time panic](#run-time-panics) occurs.

The function `SliceData` returns a pointer to the underlying array of the `slice` argument. If the slice's capacity `cap(slice)` is not zero, that pointer is `&slice[:1][0]`. If slice is `nil`, the result is `nil`. Otherwise it is a non-nil pointer to an unspecified memory address.

The function `String` returns a `string` value whose underlying bytes start at `ptr` and whose length is `len`. The same requirements apply to the `ptr` and `len` argument as in the function `Slice`. If `len` is zero, the result is the empty string `""`. Since XGo strings are immutable, the bytes passed to `String` must not be modified afterwards.

The function `StringData` returns a pointer to the underlying bytes of the `str` argument. For an empty string the return value is unspecified, and may be `nil`. Since XGo strings are immutable, the bytes returned by `StringData` must not be modified.

### Size and alignment guarantees

For the [numeric types](#numeric-types), the following sizes are guaranteed:

```go
type                                 size in bytes

byte, uint8, int8                     1
uint16, int16                         2
uint32, int32, float32                4
uint64, int64, float64, complex64     8
complex128                           16
```

The following minimal alignment properties are guaranteed:

* For a variable `x` of any type: `unsafe.Alignof(x)` is at least 1.
* For a variable `x` of struct type: `unsafe.Alignof(x)` is the largest of all the values `unsafe.Alignof(x.f)` for each field `f` of `x`, but at least 1.
* For a variable `x` of array type: `unsafe.Alignof(x)` is the same as the alignment of a variable of the array's element type.

A struct or array type has size zero if it contains no fields (or elements, respectively) that have a size greater than zero. Two distinct zero-size variables may have the same address in memory.

## Appendix

### Language versions

TODO

### Type unification rules

The type unification rules describe if and how two types unify. The precise details are relevant for XGo implementations, affect the specifics of error messages (such as whether a compiler reports a type inference or other error), and may explain why type inference fails in unusual code situations. But by and large these rules can be ignored when writing Go code: type inference is designed to mostly "work as expected", and the unification rules are fine-tuned accordingly.

Type unification is controlled by a matching mode, which may be `exact` or `loose`. As unification recursively descends a composite type structure, the matching mode used for elements of the type, the element matching mode, remains the same as the matching mode except when two types are unified for [assignability]() (≡A): in this case, the matching mode is loose at the top level but then changes to exact for element types, reflecting the fact that types don't have to be identical to be assignable.

Two types that are not bound type parameters unify exactly if any of following conditions is true:

* Both types are [identical]().
* Both types have identical structure and their element types unify exactly.
* Exactly one type is an [unbound]() type parameter with a [underlying type](#underlying-types), and that underlying type unifies with the other type per the unification rules for ≡A (loose unification at the top level and exact unification for element types).

If both types are bound type parameters, they unify per the given matching modes if:

* Both type parameters are identical.
* At most one of the type parameters has a known type argument. In this case, the type parameters are `joined`: they both stand for the same type argument. If neither type parameter has a known type argument yet, a future type argument inferred for one the type parameters is simultaneously inferred for both of them.
* Both type parameters have a known type argument and the type arguments unify per the given matching modes.

A single bound type parameter P and another type T unify per the given matching modes if:

* P doesn't have a known type argument. In this case, T is inferred as the type argument for P.
* P does have a known type argument A, A and T unify per the given matching modes, and one of the following conditions is true:
  * Both A and T are interface types: In this case, if both A and T are also [defined]() types, they must be [identical](). Otherwise, if neither of them is a defined type, they must have the same number of methods (unification of A and T already established that the methods match).
  * Neither A nor T are interface types: In this case, if T is a defined type, T replaces A as the inferred type argument for P.

Finally, two types that are not bound type parameters unify loosely (and per the element matching mode) if:

* Both types unify exactly.
* One type is a [defined type](), the other type is a type literal, but not an interface, and their underlying types unify per the element matching mode.
* Both types are interfaces (but not type parameters) with identical [type terms](), both or neither embed the predeclared type [comparable](), corresponding method types unify exactly, and the method set of one of the interfaces is a subset of the method set of the other interface.
* Only one type is an interface (but not a type parameter), corresponding methods of the two types unify per the element matching mode, and the method set of the interface is a subset of the method set of the other type.
* Both types have the same structure and their element types unify per the element matching mode.
