go/doc/go_lang.txt

The Go Programming Language (DRAFT)
----

Robert Griesemer, Rob Pike, Ken Thompson

----
(July 1, 2008)

This document is a semi-formal specification/proposal for a new
systems programming language.  The document is under active
development; any piece may change substantially as design progresses;
also there remain a number of unresolved issues.


Guiding principles
----

Go is a new systems programming language intended as an alternative to C++ at
Google. Its main purpose is to provide a productive and efficient programming
environment for compiled programs such as servers and distributed systems.

The design is motivated by the following guidelines:

- very fast compilation (1MLOC/s stretch goal); instantaneous incremental compilation
- procedural
- strongly typed
- concise syntax avoiding repetition
- few, orthogonal, and general concepts
- support for threading and interprocess communication
- garbage collection
- container library written in Go
- reasonably efficient (C ballpark)

The language should be strong enough that the compiler and run time can be
written in itself.


Program structure
----

A Go program consists of a number of ``packages''.

A package is built from one or more source files, each of which consists
of a package specifier followed by import declarations followed by other
declarations.  There are no statements at the top level of a file.

By convention, one package, by default called main, is the starting point for
execution. It contains a function, also called main, that is the first function
invoked by the run time system.

If any package within the program
contains a function init(), that function will be executed
before main.main() is called.  The details of initialization are
still under development.

Source files can be compiled separately (without the source
code of packages they depend on), but not independently (the compiler does
check dependencies by consulting the symbol information in compiled packages).


Modularity, identifiers and scopes
----

A package is a collection of import, constant, type, variable, and function
declarations. Each declaration associates an ``identifier'' with a program
entity (such as a type).

In particular, all identifiers in a package are either
declared explicitly within the package, arise from an import statement,
or belong to a small set of predefined identifiers (such as "int32").

A package may make explicitly declared identifiers visible to other
packages by marking them as exported; there is no ``header file''.
Imported identifiers cannot be re-exported.

Scoping is essentially the same as in C: The scope of an identifier declared
within a ``block'' extends from the declaration of the identifier (that is, the
position immediately after the identifier) to the end of the block. An identifier
shadows identifiers with the same name declared in outer scopes. Within a
block, a particular identifier must be declared at most once.


Typing, polymorphism, and object-orientation
----

Go programs are strongly typed.  Certain values can also be
polymorphic.  The language provides mechanisms to make use of such
polymorphic values type-safe.

Interface types provide the mechanisms to support object-oriented
programming. Different interface types are independent of each
other and no explicit hierarchy is required (such as single or
multiple inheritance explicitly specified through respective type
declarations).  Interface types only define a set of methods that a
corresponding implementation must provide.  Thus interface and
implementation are strictly separated.

An interface is implemented by associating methods with types.
If a type defines all methods of an interface, it
implements that interface and thus can be used where that interface is
required.  Unless used through a variable of interface type, methods
can always be statically bound (they are not ``virtual''), and incur no
runtime overhead compared to an ordinary function.

[OLD
Interface types, building on structures with methods, provide
the mechanisms to support object-oriented programming.
Different interface types are independent of each
other and no explicit hierarchy is required (such as single or
multiple inheritance explicitly specified through respective type
declarations).  Interface types only define a set of methods that a
corresponding implementation must provide.  Thus interface and
implementation are strictly separated.

An interface is implemented by associating methods with
structures.  If a structure implements all methods of an interface, it
implements that interface and thus can be used where that interface is
required.  Unless used through a variable of interface type, methods
can always be statically bound (they are not ``virtual''), and incur no
runtime overhead compared to an ordinary function.
END]

Go has no explicit notion of classes, sub-classes, or inheritance.
These concepts are trivially modeled in Go through the use of
functions, structures, associated methods, and interfaces.

Go has no explicit notion of type parameters or templates.  Instead,
containers (such as stacks, lists, etc.) are implemented through the
use of abstract operations on interface types or polymorphic values.


Pointers and garbage collection
----

Variables may be allocated automatically (when entering the scope of
the variable) or explicitly on the heap.  Pointers are used to refer
to heap-allocated variables.  Pointers may also be used to point to
any other variable; such a pointer is obtained by "taking the
address" of that variable.  Variables are automatically reclaimed when
they are no longer accessible.  There is no pointer arithmetic in Go.


Functions
----

Functions contain declarations and statements.  They may be
recursive.  Functions may be anonymous and appear as
literals in expressions.


Multithreading and channels
----

Go supports multithreaded programming directly. A function may
be invoked as a parallel thread of execution.  Communication and
synchronization is provided through channels and their associated
language support.


Values and references
----

All objects have value semantics, but their contents may be accessed
through different pointers referring to the same object.
For example, when calling a function with an array, the array is
passed by value, possibly by making a copy.   To pass a reference,
one must explicitly pass a pointer to the array.  For arrays in
particular, this is different from C.

There is also a built-in string type, which represents immutable
strings of bytes.


Syntax
----

The syntax of statements and expressions in Go borrows from the C tradition;
declarations are loosely derived from the Pascal tradition to allow more
comprehensible composability of types.

Here is a complete example Go program that implements a concurrent prime sieve:


  package main
  
  // Send the sequence 2, 3, 4, ... to channel 'ch'.
  func Generate(ch *chan> int) {
    for i := 2; ; i++ {
      >ch = i  // Send 'i' to channel 'ch'.
    }
  }
  
  // Copy the values from channel 'in' to channel 'out',
  // removing those divisible by 'prime'.
  func Filter(in *chan< int, out *chan> int, prime int) {
    for {
      i := <in;  // Receive value of new variable 'i' from 'in'.
      if i % prime != 0 {
        >out = i  // Send 'i' to channel 'out'.
      }
    }
  }
  
  // The prime sieve: Daisy-chain Filter processes together.
  func Sieve() {
    ch := new(chan int);  // Create a new channel.
    go Generate(ch);  // Start Generate() as a subprocess.
    for {
      prime := <ch;
      printf("%d\n",  prime);
      ch1 := new(chan int);
      go Filter(ch, ch1, prime);
      ch = ch1
    }
  }
  
  func main() {
    Sieve()
  }


Notation
----

The syntax is specified using Extended Backus-Naur Form (EBNF). 
In particular:

- |   separates alternatives (least binding strength)
- ()  groups
- []  specifies an option (0 or 1 times)
- {}  specifies repetition (0 to n times)

Lexical symbols are enclosed in double quotes '''' (the
double quote symbol is written as ''"'').

A production may be referenced from various places in this document
but is usually defined close to its first use.  Productions and code
examples are indented.

Lower-case production names are used to identify productions that cannot
be broken by white space or comments; they are usually tokens. Other
productions are in CamelCase.


Common productions
----

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

  QualifiedIdent = [ PackageName "." ] identifier .
  PackageName = identifier .


Source code representation
----

Source code is Unicode text encoded in UTF-8.

Tokenization follows the usual rules.  Source text is case-sensitive.

White space is blanks, newlines, carriage returns, or tabs.

Comments are // to end of line or /* */ without nesting and are treated as white space.

Some Unicode characters (e.g., the character U+00E4) may be representable in
two forms, as a single code point or as two code points.  For simplicity of
implementation, Go treats these as distinct characters.


Characters
----

In the grammar we use the notation

  utf8_char

to refer to an arbitrary Unicode code point encoded in UTF-8. We use

  non_ascii

to refer to the subset of "utf8_char" code points with values >= 128.


Digits and Letters
----

  oct_digit = { "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" } .
  dec_digit = { "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" } .
  hex_digit = { "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" | "a" |
                "A" | "b" | "B" | "c" | "C" | "d" | "D" | "e" | "E" | "f" | "F" } .
  letter = "A" | "a" | ... "Z" | "z" | "_" | non_ascii .

All non-ASCII code points are considered letters; digits are always ASCII.


Identifiers
----

An identifier is a name for a program entity such as a variable, a
type, a function, etc. An identifier must not be a reserved word.

  identifier = letter { letter | dec_digit } .

  a
  _x
  ThisIsVariable9
  αβ


Reserved words
----

  break         fallthrough       import            return
  case          false             interface         select
  const         for               map               struct
  continue      func              new               switch
  default       go                nil               true
  else          goto              package           type
  export        if                range             var


TODO: "len" is currently also a reserved word - it shouldn't be.


Types
----

A type specifies the set of values that variables of that type may
assume, and the operators that are applicable.

There are basic types and composite types.


Basic types
----

Go defines a number of basic types, referred to by their predeclared
type names.  These include traditional arithmetic types, booleans,
strings, and a special polymorphic type.

The arithmetic types are:

  uint8    the set of all unsigned 8-bit integers
  uint16   the set of all unsigned 16-bit integers
  uint32   the set of all unsigned 32-bit integers
  uint64   the set of all unsigned 64-bit integers

  int8     the set of all signed 8-bit integers, in 2's complement
  int16    the set of all signed 16-bit integers, in 2's complement
  int32    the set of all signed 32-bit integers, in 2's complement
  int64    the set of all signed 64-bit integers, in 2's complement

  float32  the set of all valid IEEE-754 32-bit floating point numbers
  float64  the set of all valid IEEE-754 64-bit floating point numbers
  float80  the set of all valid IEEE-754 80-bit floating point numbers

Additionally, Go declares several platform-specific type aliases:
ushort, short, uint, int, ulong, long, float, and double.  The bit
width of these types is ``natural'' for the respective types for the
given platform.  For instance, int is usually the same as int32 on a
32-bit architecture, or int64 on a 64-bit architecture.

The integer sizes are defined such that short is at least 16 bits, int
is at least 32 bits, and long is at least 64 bits (and ditto for the
unsigned equivalents).  Also, the sizes are such that short <= int <=
long.  Similarly, float is at least 32 bits, double is at least 64
bits, and the sizes have float <= double.

Also, ``byte'' is an alias for uint8.

An arithmetic type ``ptrint'' is also defined.  It is an unsigned
integer type that is the smallest natural integer type of the machine
large enough to store the uninterpreted bits of a pointer value.

Generally, programmers should use these types rather than the explicitly
sized types to maximize portability.

Other basic types include:

  bool     the truth values true and false
  string   immutable strings of bytes
  any      polymorphic type

Two reserved words, ``true'' and ``false'', represent the
corresponding boolean constant values.

Strings are described in a later section.

[OLD
The polymorphic ``any'' type can represent a value of any type.
TODO: we need a section about any
END]


Numeric literals
----

Integer literals take the usual C form, except for the absence of the
'U', 'L', etc. suffixes, and represent integer constants.  Character
literals are also integer constants.  Similarly, floating point
literals are also C-like, without suffixes and in decimal representation
only.

An integer constant represents an abstract integer value of arbitrary
precision.  Only when an integer constant (or arithmetic expression
formed from integer constants) is bound to a typed variable
or constant is it required to fit into a particular size - that of the type
of the variable.  In other words, integer constants and arithmetic
upon them is not subject to overflow; only finalization of integer
constants (and constant expressions) can cause overflow.
It is an error if the value of the constant or expression cannot be
represented correctly in the range of the type of the receiving
variable or constant.  By extension, it is also possible to use
an integer as a floating constant (such as 1 instead of 1.0) if
it can be represented accurately, and vice versa (such as 1e9
instead of 1000000000).

Floating point literals also represent an abstract, ideal floating
point value that is constrained only upon assignment.

  sign = "+" | "-" .
  int_lit = [ sign ] unsigned_int_lit .
  unsigned_int_lit = decimal_int_lit | octal_int_lit | hex_int_lit .
  decimal_int_lit = ( "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" )
                    { dec_digit } .
  octal_int_lit = "0" { oct_digit } .
  hex_int_lit = "0" ( "x" | "X" ) hex_digit { hex_digit } .
  float_lit =  [ sign ] ( fractional_lit | exponential_lit ) .
  fractional_lit = { dec_digit } ( dec_digit "." | "." dec_digit )
                   { dec_digit } [ exponent ] .
  exponential_lit = dec_digit { dec_digit } exponent .
  exponent = ( "e" | "E" ) [ sign ] dec_digit { dec_digit } .

  07
  0xFF
  -44
  +3.24e-7


The string type
----

The string type represents the set of string values (strings).
Strings behave like arrays of bytes, with the following properties:

- They are immutable: after creation, it is not possible to change the
  contents of a string.
- No internal pointers: it is illegal to create a pointer to an inner
  element of a string.
- They can be indexed: given string "s1", "s1[i]" is a byte value.
- They can be concatenated: given strings "s1" and "s2", "s1 + s2" is a value
  combining the elements of "s1" and "s2" in sequence.
- Known length: the length of a string "s1" can be obtained by the function/
  operator "len(s1)".  The length of a string is the number of bytes within.
  Unlike in C, there is no terminal NUL byte.
- Creation 1: a string can be created from an integer value by a conversion;
  the result is a string containing the UTF-8 encoding of that code point.
    "string('x')" yields "x"; "string(0x1234)" yields the equivalent of "\u1234"
- Creation 2: a string can by created from an array of integer values (maybe
  just array of bytes) by a conversion
    a [3]byte; a[0] = 'a'; a[1] = 'b'; a[2] = 'c';  string(a) == "abc";


Character and string literals
----

Character and string literals are almost the same as in C, with the
following differences:

  - The encoding is UTF-8
  - `` strings exist; they do not interpret backslashes
  - Octal character escapes are always 3 digits ("\077" not "\77")
  - Hexadecimal character escapes are always 2 digits ("\x07" not "\x7")

This section is precise but can be skipped on first reading. The rules are:

  char_lit = "'" ( unicode_value | byte_value ) "'" .
  unicode_value = utf8_char | little_u_value | big_u_value | escaped_char .
  byte_value = octal_byte_value | hex_byte_value .
  octal_byte_value = "\" oct_digit oct_digit oct_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" | "\" | "'" | """ ) .

A unicode_value takes one of four forms:

* The UTF-8 encoding of a Unicode code point.  Since Go source
text is in UTF-8, this is the obvious translation from input
text into Unicode characters.
* The usual list of C backslash escapes: "\n", "\t", etc.
* A `little u' value, such as "\u12AB".  This represents the Unicode
code point with the corresponding hexadecimal value.  It always
has exactly 4 hexadecimal digits.
* A `big U' value, such as "\U00101234".  This represents the
Unicode code point with the corresponding hexadecimal value.
It always has exactly 8 hexadecimal digits.

Some values that can be represented this way are illegal because they
are not valid Unicode code points.  These include values above
0x10FFFF and surrogate halves.

An octal_byte_value contains three octal digits.  A hex_byte_value
contains two hexadecimal digits.  (Note: This differs from C but is
simpler.)

It is erroneous for an octal_byte_value to represent a value larger than 255. 
(By construction, a hex_byte_value cannot.)

A character literal is a form of unsigned integer constant.  Its value
is that of the Unicode code point represented by the text between the
quotes.

  'a'
  'ä'
  '本'
  '\t'
  '\000'
  '\007'
  '\377'
  '\x07'
  '\xff'
  '\u12e4'
  '\U00101234'

String literals come in two forms: double-quoted and back-quoted.
Double-quoted strings have the usual properties; back-quoted strings
do not interpret backslashes at all.

  string_lit = raw_string_lit | interpreted_string_lit .
  raw_string_lit = "`" { utf8_char } "`" .
  interpreted_string_lit = """ { unicode_value | byte_value } """ .

A string literal has type 'string'.  Its value is constructed by
taking the byte values formed by the successive elements of the
literal.  For byte_values, these are the literal bytes; for
unicode_values, these are the bytes of the UTF-8 encoding of the
corresponding Unicode code points.  Note that
  "\u00FF"
and
  "\xFF"
are
different strings: the first contains the two-byte UTF-8 expansion of
the value 255, while the second contains a single byte of value 255.
The same rules apply to raw string literals, except the contents are
uninterpreted UTF-8.

  `abc`
  `\n`
  "hello, world\n"
  "\n"
  ""
  "Hello, world!\n"
  "日本語"
  "\u65e5本\U00008a9e"
  "\xff\u00FF"

These examples all represent the same string:

  "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

The language does not canonicalize Unicode text or evaluate combining
forms.  The text of source code is passed uninterpreted.

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 character literal (it is not a single code
point), and will appear as two code points if placed in a string
literal.


More about types
----

The static type of a variable is the type defined by the variable's
declaration. The dynamic type of a variable is the actual type of the
value stored in a variable at runtime. Except for variables of interface
type, the static and dynamic type of variables is always the same.

Variables of interface type may hold values of different types during
execution. However, the dynamic type of the variable is always compatible
with the static type of the variable.

Types may be composed from other types by assembling arrays, maps,
channels, structures, and functions. They are called composite types.

  Type = TypeName | ArrayType | ChannelType | InterfaceType |
         FunctionType | MapType | StructType | PointerType .
  TypeName = QualifiedIdent.


Array types
----

[TODO: this section needs work regarding the precise difference between
static, open and dynamic arrays]

An array is a composite type consisting of a number of elements
all of the same type, called the element type.  The number of
elements of an array is called its length.  The elements of an array
are designated by indices which are integers between 0 and the length - 1.

An array type specifies arrays with a given element type and
an optional array length. If the length is present, it is part of the type.
Arrays without a length specification are called open arrays.
Any array may be assigned to an open array variable with the
same element type. Typically, open arrays are used as
formal parameters for functions.

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

  [] uint8
  [2*n] int
  [64] struct { x, y: int32; }
  [1000][1000] float64

The length of an array can be discovered at run time (or compile time, if
its length is a constant) using the built-in special function len():

  len(a)


Map types
----

A map is a composite type consisting of a variable number of entries
called (key, value) pairs. For a given map,
the keys and values must each be of a specific type.
Upon creation, a map is empty and values may be added and removed
during execution.  The number of entries in a map is called its length.
[OLD
A map whose value type is 'any' can store values of all types.
END]

  MapType = "map" "[" KeyType "]" ValueType .
  KeyType = Type .
  ValueType = Type | "any" .

  map [string] int
  map [struct { pid int; name string }] *chan Buffer
  map [string] any

Implementation restriction: Currently, only pointers to maps are supported.


Struct types
----

Struct types are similar to C structs.

Each field of a struct represents a variable within the data
structure.

  StructType = "struct" "{" [ FieldDeclList [ ";" ] ] "}" .
  FieldDeclList = FieldDecl { ";" FieldDecl } .
  FieldDecl = IdentifierList Type .

  // An empty struct.
  struct {}

  // A struct with 5 fields.
  struct {
    x, y int;
    u float;
    a []int;
    f func();
  }


Compound Literals
----

Literals for compound data structures consist of the type of the constant
followed by a parenthesized expression list.  In effect, they are a
conversion from expression list to compound value.

TODO: Needs to be updated.


Pointer types
----

Pointer types are similar to those in C.

  PointerType = "*" Type.

Pointer arithmetic of any kind is not permitted.

  *int
  *map[string] *chan

It is legal to write a pointer type (only) such as *T even if T is
an incomplete type (i.e., either not yet fully defined or forward
declared). This allows the construction of recursive types such as:

  type S struct { s *S }

Together with a type forward declaration, mutually recursive types
can be constructed such as:

  type S2  // forward declaration of S2
  type S1 struct { s2 *S2 }
  type S2 struct { s1 *S1 }

By the end of the package source, all forward-declared types must be
fully declared if they are used.


Channel types
----

A channel provides a mechanism for two concurrently executing functions
to synchronize execution and exchange values of a specified type.

Upon creation, a channel can be used both to send and to receive.
By conversion or assignment, it may be restricted only to send or
to receive; such a restricted channel
is called a 'send channel' or a 'receive channel'.

  ChannelType = "chan" [ "<" | ">" ] ValueType .

  chan any    // a generic channel
  chan int    // a channel that can exchange only ints
  chan> float // a channel that can only be used to send floats
  chan< any   // a channel that can receive (only) values of any type

Channel variables always have type pointer to channel.
It is an error to attempt to use a channel value and in
particular to dereference a channel pointer.

  var ch *chan int;
  ch = new(chan int);  // new returns type *chan int

Function types
----

A function type denotes the set of all functions with the same signature.

A method is a function with a receiver declaration.
[OLD
, which is of type pointer to struct.
END]

Functions can return multiple values simultaneously.

  FunctionType = "func" AnonymousSignature .
  AnonymousSignature = [ Receiver "." ] Parameters [ Result ] .
  Receiver = "(" identifier Type ")" .
  Parameters = "(" [ ParameterList ] ")" .
  ParameterList = ParameterSection { "," ParameterSection } .
  ParameterSection = IdentifierList Type .
  Result = Type | "(" ParameterList ")" .

  // Function types
  func ()
  func (a, b int, z float) bool
  func (a, b int, z float) (success bool)
  func (a, b int, z float) (success bool, result float)

  // Method types
  func (p *T) . ()
  func (p *T) . (a, b int, z float) bool
  func (p *T) . (a, b int, z float) (success bool)
  func (p *T) . (a, b int, z float) (success bool, result float)

A variable can hold only a pointer to a function, not a function value.
In particular, v := func() {} creates a variable of type *func(). To call the
function referenced by v, one writes v(). It is illegal to dereference a
function pointer.

TODO: For consistency, we should require the use of & to get the pointer to
a function: &func() {}.


Function Literals
----

Function literals represent anonymous functions.

  FunctionLit = FunctionType Block .
  Block = "{" [ StatementList [ ";" ] ] "}" .

A function literal can be invoked
or assigned to a variable of the corresponding function pointer type.
For now, a function literal can reference only its parameters, global
variables, and variables declared within the function literal.

  // Function literal
  func (a, b int, z float) bool { return a*b < int(z); }

  // Method literal
  func (p *T) . (a, b int, z float) bool { return a*b < int(z) + p.x; }

Unresolved issues: Are there method literals? How do you use them?


Methods
----

A method is a function bound to a particular type T, where T is the
type of the receiver. For instance, given type Point

  type Point struct { x, y float }

the declaration

  func (p *Point) distance(scale float) float {
    return scale * (p.x*p.x + p.y*p.y);
  }

creates a method of type *Point.  Note that methods may appear anywhere
after the declaration of the receiver type and may be forward-declared.

When invoked, a method behaves like a function whose first argument
is the receiver, but at the call site the receiver is bound to the method
using the notation

  receiver.method()

For instance, given a *Point variable pt, one may call

  pt.distance(3.5)


Interface of a type
----

The interface of a type is defined to be the unordered set of methods
associated with that type.


Interface types
----

An interface type denotes a set of methods.

  InterfaceType = "interface" "{" [ MethodDeclList [ ";" ] ] "}" .
  MethodDeclList = MethodDecl { ";" MethodDecl } .
  MethodDecl = identifier Parameters [ Result ] .

  // A basic file interface.
  type File interface {
    Read(b Buffer) bool;
    Write(b Buffer) bool;
    Close();
  }

Any type whose interface has, possibly as a subset, the complete
set of methods of an interface I is said to implement interface I.
For instance, if two types S1 and S2 have the methods

  func (p T) Read(b Buffer) bool { return ... }
  func (p T) Write(b Buffer) bool { return ... }
  func (p T) Close() { ... }

(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.

All types implement the empty interface:

  interface {}

In general, a type implements an arbitrary number of interfaces.
For instance, if we have

  type Lock interface {
    lock();
    unlock();
  }

and S1 and S2 also implement

  func (p T) lock() { ... }
  func (p T) unlock() { ... }

they implement the Lock interface as well as the File interface.

[OLD
It is legal to assign a pointer to a struct to a variable of
compatible interface type.  It is legal to assign an interface
variable to any struct pointer variable but if the struct type is
incompatible the result will be nil.
END]


[OLD
The polymorphic "any" type
----

Given a variable of type "any", one can store any value into it by
plain assignment or implicitly, such as through a function parameter
or channel operation.  Given an "any" variable v storing an underlying
value of type T, one may:

  - copy v's value to another variable of type "any"
  - extract the stored value by an explicit conversion operation T(v)
  - copy v's value to a variable of type T
  
Attempts to convert/extract to an incompatible type will yield nil.

No other operations are defined (yet).

Note that type
  interface {}
is a special case that can match any struct type, while type
  any
can match any type at all, including basic types, arrays, etc.

TODO: details about reflection
END]


Equivalence of types
---

TODO: We may need to rethink this because of the new ways interfaces work.

Types are structurally equivalent: Two types are equivalent (``equal'') if they
are constructed the same way from equivalent types.

For instance, all variables declared as "*int" have equivalent type,
as do all variables declared as "map [string] chan int".

More precisely, two struct types are equivalent if they have exactly the same fields
in the same order, with equal field names and types. For all other composite types,
the types of the components must be equivalent. Additionally, for equivalent arrays,
the lengths must be equal (or absent), and for channel types the mode must be equal
(">", "<", or none). The names of receivers, parameters, or result values of functions
are ignored for the purpose of type equivalence.

For instance, the struct type

  struct {
    a int;
    b int;
    f *func (m *[32] float, x int, y int) bool
  }
  
is equivalent to

  struct {
    a, b int;
    f *F
  }
  
where "F" is declared as "func (a *[30 + 2] float, b, c int) (ok bool)".

Finally, two interface types are equivalent if they both declare the same set of
methods: For each method in the first interface type there is a method in the
second interface type with the same method name and equivalent signature, and
vice versa. Note that the declaration order of the methods is not relevant.


Literals
----

  Literal = char_lit | string_lit | int_lit | float_lit | FunctionLit | "nil" .


Declarations
----

A declaration associates a name with a language entity such as a type,
constant, variable, or function.

  Declaration = ConstDecl | TypeDecl | VarDecl | FunctionDecl | ExportDecl .

TODO: specify range of visibility, scope rules.


Const declarations
----

A constant declaration gives a name to the value of a constant expression.

  ConstDecl = "const" ( ConstSpec | "(" ConstSpecList [ ";" ] ")" ).
  ConstSpec = identifier [ Type ] "=" Expression .
  ConstSpecList = ConstSpec { ";" ConstSpec }.

  const pi float = 3.14159265
  const e = 2.718281828
  const (
    one int = 1;
    two = 3
  )


Type declarations
----

A type declaration introduces a name as a shorthand for a type. The name refers
to an incomplete type until the type specification is complete. If no type is
provided at all, the declaration effectively serves as a forward declaration.
Incomplete types can be used together (and only) with pointer types.

  TypeDecl = "type" ( TypeSpec | "(" TypeSpecList [ ";" ] ")" ).
  TypeSpec = identifier [ Type ] .
  TypeSpecList = TypeSpec { ";" TypeSpec }.


  type List  // forward declaration
  type IntArray [16] int
  type (
    Point struct { x, y float };
    Polar Point
  )

Since incomplete types can only be used with pointer types, in a type
declaration a type may not refer to itself unless it does so with a
pointer type.


Variable declarations
----

A variable declaration creates a variable and gives it a type and a name.
It may optionally give the variable an initial value; in some forms of
declaration the type of the initial value defines the type of the variable.

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

  var i int
  var u, v, w float
  var k = 0
  var x, y float = -1.0, -2.0
  var (
    i int;
    u, v = 2.0, 3.0
  )

If the expression list is present, it must have the same number of elements
as there are variables in the variable specification.

If the variable type is omitted, an initialization expression (or expression
list) must be present, and the variable type is the type of the expression
value (in case of a list of variables, the variables assume the types of the
corresponding expression values).

If the variable type is omitted, and the corresponding initialization expression
is a constant expression of abstract int or floating point type, the type
of the variable is "int" or "float" respectively:

  var i = 0  // i has int type
  var f = 3.1415  // f has float type

The syntax

  SimpleVarDecl = identifier ":=" Expression .

is shorthand for

  var identifier = Expression.

  i := 0
  f := func() int { return 7; }
  ch := new(chan int);
  
Also, in some contexts such as "if", "for", or "switch" statements,
this construct can be used to declare local temporary variables.


Function and method declarations
----

Functions and methods have a special declaration syntax, slightly
different from the type syntax because an identifier must be present
in the signature.

Implementation restriction: Functions and methods can only be declared
at the global level.

  FunctionDecl = "func" NamedSignature  ( ";" | Block ) .
  NamedSignature = [ Receiver ] identifier Parameters [ Result ] .

  func min(x int, y int) int {
    if x < y {
      return x;
    }
    return y;
  }

  func foo(a, b int, z float) bool {
    return a*b < int(z);
  }


A method is a function that also declares a receiver.

  func (p *T) foo(a, b int, z float) bool {
    return a*b < int(z) + p.x; 
  }

  func (p *Point) Length() float {
    return Math.sqrt(p.x * p.x + p.y * p.y);
  }

  func (p *Point) Scale(factor float) {
    p.x = p.x * factor;
    p.y = p.y * factor;
  }

Functions and methods can be forward declared by omitting the body:

  func foo(a, b int, z float) bool;
  func (p *T) foo(a, b int, z float) bool;


Initial values
----

When memory is allocated to store a value, either through a declaration
or new(), and no explicit initialization is provided, the memory is
given a default initialization.  Each element of such a value is
set to the ``zero'' for that type: "false" for booleans, "0" for integers,
"0.0" for floats, '''' for strings, and nil for pointers.  This intialization
is done recursively, so for instance each element of an array of integers will
be set to 0 if no other value is specified.

These two simple declarations are equivalent:

  var i int;
  var i int = 0;

After

  type T struct { i int; f float; next *T };
  t := new(T);

the following holds:

  t.i == 0
  t.f == 0.0
  t.next == nil


Export declarations
----

Global identifiers may be exported, thus making the
exported identifer visible outside the package.  Another package may
then import the identifier to use it.

Export declarations must only appear at the global level of a
source file and can name only globally-visible identifiers.
That is, one can export global functions, types, and so on but not
local variables or structure fields.

Exporting an identifier makes the identifier visible externally to the
package.  If the identifier represents a type, the type structure is
exported as well.  The exported identifiers may appear later in the
source than the export directive itself, but it is an error to specify
an identifier not declared anywhere in the source file containing the
export directive.

  ExportDecl = "export" ExportIdentifier { "," ExportIdentifier } .
  ExportIdentifier = QualifiedIdent .

  export sin, cos
  export math.abs

TODO: complete this section

TODO: export as a mechanism for public and private struct fields?


Expressions
----

Expression syntax is based on that of C but with fewer precedence levels.

  Expression = BinaryExpr | UnaryExpr | PrimaryExpr .
  BinaryExpr = Expression binary_op Expression .
  UnaryExpr = unary_op Expression .

  PrimaryExpr =
    identifier | Literal | "(" Expression ")" | "iota" |
    Call | Conversion | Allocation |
    Expression "[" Expression [ ":" Expression ] "]" | Expression "." identifier |
    Expression "." "(" Type ")" .
  
  Call = Expression "(" [ ExpressionList ] ")" .
  Conversion = "convert" "(" Type [ "," ExpressionList ] ")" |
               ConversionType "(" [ ExpressionList ] ")" .
  ConversionType = TypeName | ArrayType | MapType | StructType | InterfaceType .
  Allocation = "new" "(" Type [ "," ExpressionList ] ")" .

  binary_op = log_op | rel_op | add_op | mul_op .
  log_op = "||" | "&&" .
  rel_op = "==" | "!=" | "<" | "<=" | ">" | ">=".
  add_op = "+" | "-" | "|" | "^".
  mul_op = "*" | "/" | "%" | "<<" | ">>" | "&".

  unary_op = "+" | "-" | "!" | "^" | "<" | ">" | "*" | "&" .

Field selection and type assertions ('.') bind tightest, followed by indexing ('[]')
and then calls and conversions. The remaining precedence levels are as follows
(in increasing precedence order):

  Precedence    Operator
      1            ||
      2            &&
      3            ==  !=  <  <=  >  >=
      4            +  -  |  ^
      5            *  /  %  <<  >>  &
      6            +  -  !  ^  <  >  *  &  (unary)
    
For integer values, / and % satisfy the following relationship:

  (a / b) * b + a % b == a

and

  (a / b) is "truncated towards zero".

There are no implicit type conversions: Except for the shift operators
"<<" and ">>", both operands of a binary operator must have the same type.
In particular, unsigned and signed integer values cannot be mixed in an
expression without explicit conversion.

The shift operators shift the left operand by the shift count specified by the
right operand. They implement arithmetic shifts if the left operand is a signed
integer, and logical shifts if it is an unsigned integer. The shift count must
be an unsigned integer. There is no upper limit on the shift count. It is
as if the left operand is shifted "n" times by 1 for a shift count of "n".

Unary "^" corresponds to C "~" (bitwise complement). There is no "~" operator
in Go.

There is no "->" operator. Given a pointer p to a struct, one writes
  p.f
to access field f of the struct. Similarly, given an array or map
pointer, one writes
  p[i]
to access an element.  Given a function pointer, one writes
  p()
to call the function.

Other operators behave as in C.

The "iota" keyword is discussed in a later section.
  
Examples of primary expressions

  x
  2
  (s + ".txt")
  f(3.1415, true)
  Point(1, 2)
  new([]int, 100)
  m["foo"]
  s[i : j + 1]
  obj.color
  Math.sin
  f.p[i].x()

Examples of general expressions

  +x
  23 + 3*x[i]
  x <= f()
  ^a >> b
  f() || g()
  x == y + 1 && <chan_ptr > 0


The nil value
----

The keyword
  nil
represents the ``zero'' value for a pointer type or interface type.

The only operations allowed for nil are to assign it to a pointer or
interface variable and to compare it for equality or inequality with a
pointer or interface value.

  var p *int;
  if p != nil {
    print p
  } else {
    print "p points nowhere"
  }

By default, pointers are initialized to nil.

TODO: This needs to be revisited.

[OLD
TODO: how does this definition jibe with using nil to specify
conversion failure if the result is not of pointer type, such
as an any variable holding an int?

TODO: if interfaces were explicitly pointers, this gets simpler.
END]


Allocation
----

The builtin-function new() allocates storage.  The function takes a
parenthesized operand list comprising the type of the value to
allocate, optionally followed by type-specific expressions that
influence the allocation.  The invocation returns a pointer to the
memory.  The memory is initialized as described in the section on
initial values.

For instance,

  type S struct { a int; b float }
  new(S)

allocates storage for an S, initializes it (a=0, b=0.0), and returns a
value of type *S pointing to that storage.

The only defined parameters affect sizes for allocating arrays,
buffered channels, and maps.

  ap := new([]int, 10); # a pointer to an array of 10 ints
  aap := new([][]int, 5, 10); # a pointer to an array of 5 arrays of 10 ints
  c := new(chan int, 10); # a pointer to a channel with a buffer size of 10
  m := new(map[string] int, 100); # a pointer to a map with space for 100 elements preallocated

TODO: argument order for dimensions in multidimensional arrays


Conversions
----

TODO: gri believes this section is too complicated. Instead we should
replace this with: 1) proper conversions of basic types, 2) compound
literals, and 3) type assertions.

Conversions create new values of a specified type derived from the
elements of a list of expressions of a different type.

The most general conversion takes the form of a call to "convert",
with the result type and a list of expressions as arguments:

	convert(int, PI * 1000.0);
	convert([]int, 1, 2, 3, 4);

If the result type is a basic type, pointer type, or
interface type, there must be exactly one expression and there is a
specific set of permitted conversions, detailed later in the section.
These conversions are called ``simple conversions''.
TODO: if interfaces were explicitly pointers, this gets simpler.

	convert(int, 3.14159);
	convert(uint32, ^0);
	convert(interface{}, new(S))
	convert(*AStructType, interface_value)

For other result types - arrays, maps, structs - the expressions
form a list of values to be assigned to successive elements of the
resulting value.  If the type is an array or map, the list may even be
empty.  Unlike in a simple conversion, the types of the expressions
must be equivalent to the types of the elements of the result type;
the individual values are not converted.  For instance, if result
type is []int, the expressions must be all of type int, not float or
uint.  (For maps, the successive elements must be key-value pairs).
For arrays and struct types, if fewer elements are provided than
specified by the result type, the missing elements are
initialized to the respective ``zero'' value for that element type.

These conversions are called ``compound conversions''.

	convert([]int)  // empty array of ints
	convert([]int, 1, 2, 3)
	convert([5]int, 1, 2);  // == convert([5]int, 1, 2, 0, 0, 0)
	convert(map[string]int, "1", 1, "2", 2)
	convert(struct{ x int; y float }, 3, sqrt(2.0))

TODO: are interface/struct and 'any' conversions legal? they're not
equivalent, just compatible. convert([]any, 1, "hi", nil);

There is syntactic help to make conversion expressions simpler to write.

If the result type is of ConversionType (a type name, array type,
map type, struct type, or interface type, essentially anything
except a pointer), the conversion can be rewritten to look
syntactically like a call to a function whose name is the type:

	int(PI * 1000.0);
	AStructType(an_interface_variable);
	struct{ x int, y float }(3, sqrt(2.0))
	[]int(1, 2, 3, 4);
	map[string]int("1", 1, "2", 2);
	
This notation is convenient for declaring and initializing
variables of composite type:

	primes := []int(2, 3, 5, 7, 9, 11, 13);

Simple conversions can also be written as a parenthesized type after
an expression and a period.  Although intended for ease of conversion
within a method call chain, this form works in any expression context.
TODO: should it?

	var s *AStructType = vec.index(2).(*AStructType);
	fld := vec.index(2).(*AStructType).field;
	a := foo[i].(string);

As said, for compound conversions the element types must be equivalent.
For simple conversions, the types can differ but only some combinations
are permitted:

1) Between integer types.  If the value is a signed quantity, it is
sign extended to implicit infinite precision; otherwise it is zero
extended.  It is then truncated to fit in the result type size.
For example, uint32(int8(0xFF)) is 0xFFFFFFFF.  The conversion always
yields a valid value; there is no signal for overflow.

2) Between integer and floating point types, or between floating point
types.  To avoid overdefining the properties of the conversion, for
now we define it as a ``best effort'' conversion.  The conversion
always succeeds but the value may be a NaN or other problematic
result.  TODO: clarify?

3) Conversions between interfaces and compatible interfaces and struct
pointers.  Invalid conversions (that is, conversions between
incompatible types) yield nil values. TODO: is nil right here? Or
should incompatible conversions fail immediately?

4) Conversions between ``any'' values and arbitrary types.  Invalid
conversions yield nil values. TODO: is nil right here? Or should
incompatible conversions fail immediately?

5) Strings permit two special conversions.

5a) Converting an integer value yields a string containing the UTF-8
representation of the integer.

	string(0x65e5)  // "\u65e5"

5b) Converting an array of uint8s yields a string whose successive
bytes are those of the array.  (Recall byte is a synonym for uint8.)

	string([]byte('h', 'e', 'l', 'l', 'o')) // "hello"

Note that there is no linguistic mechanism to convert between pointers
and integers. A library may be provided under restricted circumstances
to acccess this conversion in low-level code but it will not be available
in general.

The constant generator 'iota'
----

Within a declaration, the keyword 'iota' represents successive
elements of an integer sequence.
It is reset to zero whenever the keyword 'const'
introduces a new declaration and increments as each identifier
is declared. For instance, 'iota' can be used to construct
a set of related constants:

  const (
    enum0 = iota;  // sets enum0 to 0, etc.
    enum1 = iota;
    enum2 = iota
  )

  const (
    a = 1 << iota;  // sets a to 1 (iota has been reset)
    b = 1 << iota;  // sets b to 2
    c = 1 << iota;  // sets c to 4
  )
  
  const x = iota;  // sets x to 0
  const y = iota;  // sets y to 0

TODO: should iota work in var, type, func decls too?


Statements
----

Statements control execution.

  Statement =
    Declaration |
    SimpleStat | GoStat | ReturnStat | BreakStat | ContinueStat | GotoStat |
    Block | IfStat | SwitchStat | SelectStat | ForStat | RangeStat |
  
  SimpleStat =
    ExpressionStat | IncDecStat | Assignment | SimpleVarDecl .
    
    
Statement lists
----

Semicolons are used to separate individual statements of a statement list.
They are optional immediately before or after a closing curly brace "}",
immediately after "++" or "--", and immediately before a reserved word.

  StatementList = Statement { [ ";" ] Statement } .


TODO: This still seems to be more complicated then necessary.


Expression statements
----

  ExpressionStat = Expression .

  f(x+y)


IncDec statements
----

  IncDecStat = Expression ( "++" | "--" ) .

  a[i]++

Note that ++ and -- are not operators for expressions.


Assignments
----

  Assignment = SingleAssignment | TupleAssignment | Send .
  SingleAssignment = PrimaryExpr assign_op Expression .
  TupleAssignment = PrimaryExprList assign_op ExpressionList .
  PrimaryExprList = PrimaryExpr { "," PrimaryExpr } .
  Send = ">" Expression "=" Expression .
  
  assign_op = [ add_op | mul_op ] "=" .

The left-hand side must be an l-value such as a variable, pointer indirection,
or an array index.

  x = 1
  *p = f()
  a[i] = 23
  
As in C, arithmetic binary operators can be combined with assignments:

  j <<= 2

A tuple assignment assigns the individual elements of a multi-valued operation,
such as function evaluation or some channel and map operations, into individual
variables. For instance, a tuple assignment such as

  v1, v2, v3 = e1, e2, e3
  
assigns the expressions e1, e2, e3 to temporaries and then assigns the temporaries
to the variables v1, v2, v3. Thus

  a, b = b, a

exchanges the values of a and b. The tuple assignment

  x, y = f()

calls the function f, which must return two values, and assigns them to x and y.
As a special case, retrieving a value from a map, when written as a two-element
tuple assignment, assign a value and a boolean. If the value is present in the map,
the value is assigned and the second, boolean variable is set to true. Otherwise,
the variable is unchanged, and the boolean value is set to false.
  
  value, present = map_var[key]
  
Analogously, receiving a value from a channel can be written as a tuple assignment.

  value, success = <chan_var
  
If the receive operation would block, the boolean is set to false.
This provides a mechanism to avoid blocking on a receive operation.

Sending on a channel is a form of assignment. The left hand side expression
must denote a channel pointer value.

  >chan_ptr = value
  
In assignments, the type of the expression must match the type of the left-hand side.


Go statements
----

A go statement starts the execution of a function as an independent
concurrent thread of control within the same address space.  Unlike
with a function, the next line of the program does not wait for the
function to complete.

  GoStat = "go" Call .


  go Server()
  go func(ch chan> bool) { for { sleep(10); >ch = true; }} (c)


Return statements
----

A return statement terminates execution of the containing function
and optionally provides a result value or values to the caller.

  ReturnStat = "return" [ ExpressionList ] .


There are two ways to return values from a function.  The first is to
explicitly list the return value or values in the return statement:

  func simple_f() int {
    return 2;
  }

A function may return multiple values.
The syntax of the return clause in that case is the same as
that of a parameter list; in particular, names must be provided for
the elements of the return value.

  func complex_f1() (re float, im float) {
    return -7.0, -4.0;
  }

The second method to return values
is to use those names within the function as variables
to be assigned explicitly; the return statement will then provide no
values:

  func complex_f2() (re float, im float) {
    re = 7.0;
    im = 4.0;
    return;
  }

If statements
----

If statements have the traditional form except that the
condition need not be parenthesized and the "then" statement
must be in brace brackets. The condition may be omitted, in which
case it is assumed to have the value "true".

  IfStat = "if" [ [ Simplestat ] ";" ] [ Condition ] Block [ "else" Statement ] .

  if x > 0 {
    return true;
  }

An "if" statement may include the declaration of a single temporary variable.
The scope of the declared variable extends to the end of the if statement, and
the variable is initialized once before the statement is entered.

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


TODO: We should fix this and move to:

  IfStat =
    "if" [ [ Simplestat ] ";" ] [ Condition ] Block
    { "else" "if" Condition Block }
    [ "else" Block ] .


Switch statements
----

Switches provide multi-way execution.

  SwitchStat = "switch" [ [ Simplestat ] ";" ] [ Expression ] "{" { CaseClause } "}" .
  CaseClause = CaseList [ StatementList [ ";" ] ] [ "fallthrough" [ ";" ] ] .
  CaseList = Case { Case } .
  Case = ( "case" ExpressionList | "default" ) ":" .

There can be at most one default case in a switch statement.

The "fallthrough" keyword indicates that the control should flow from
the end of this case clause to the first statement of the next clause.

The expressions do not need to be constants. They will
be evaluated top to bottom until the first successful non-default case is reached.
If none matches and there is a default case, the statements of the default
case are executed.

  switch tag {
  default: s3()
  case 0, 1: s1()
  case 2: s2()
  }

A switch statement may include the declaration of a single temporary variable.
The scope of the declared variable extends to the end of the switch statement, and
the variable is initialized once before the switch is entered. 

  switch x := f(); true {
  case x < 0: return -x
  default: return x
  }
  
Cases do not fall through unless explicitly marked with a "fallthrough" statement.

  switch a {
  case 1:
    b();
    fallthrough
  case 2:
    c();
  }

If the expression is omitted, it is equivalent to "true".

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


Select statements
----

A select statement chooses which of a set of possible communications
will proceed.  It looks similar to a switch statement but with the
cases all referring to communication operations.

  SelectStat = "select" "{" { CommClause } "}" .
  CommClause = CommCase [ StatementList [ ";" ] ] .
  CommCase = ( "default" | ( "case" ( SendCase | RecvCase) ) ) ":" .
  SendCase = Send .
  RecvCase = [ identifier "=" ] RecvExpr .
  RecvExpr = "<" Expression .

The select statement evaluates all the channel (pointers) involved.
If any of the channels can proceed, the corresponding communication
and statements are evaluated.  Otherwise, if there is a default case,
that executes; if not, the statement blocks until one of the
communications can complete.  A channel pointer may be nil, which is
equivalent to that case not being present in the select statement.

If the channel sends or receives "any" or an interface type, its
communication can proceed only if the type of the communication
clause matches that of the dynamic value to be exchanged.

If multiple cases can proceed, a uniform fair choice is made regarding
which single communication will execute.

  var c, c1, c2 *chan int;
  select {
  case i1 = <c1:
    printf("received %d from c1\n", i1);
  case >c2 = i2:
    printf("sent %d to c2\n", i2);
  default:
    printf("no communication\n");
  }

  for {  // send random sequence of bits to c
    select {
    case >c = 0:  // note: no statement, no fallthrough, no folding of cases
    case >c = 1:
    }
  }

  var ca *chan any;
  var i int;
  var f float;
  select {
  case i = <ca:
    printf("received int %d from ca\n", i);
  case f = <ca:
    printf("received float %f from ca\n", f);
  }

TODO: do we allow case i := <c: ?
TODO: need to precise about all the details but this is not the right doc for that


For statements
----

For statements are a combination of the "for" and "while" loops of C.

  ForStat = "for" [ Condition | ForClause ] Block .
  ForClause = [ InitStat ] ";" [ Condition ] ";" [ PostStat ] .
  
  InitStat = SimpleStat .
  Condition = Expression .
  PostStat = SimpleStat .

A SimpleStat is a simple statement such as an assignment, a SimpleVarDecl,
or an increment or decrement statement. Therefore one may declare a loop
variable in the init statement.

  for i := 0; i < 10; i++ {
    printf("%d\n", i)
  }

A for statement with just a condition executes until the condition becomes
false. Thus it is the same as C's while statement.

  for a < b {
    a *= 2
  }

If the condition is absent, it is equivalent to "true".

  for {
    f()
  }


Range statements
----

Range statements are a special control structure for iterating over
the contents of arrays and maps.

  RangeStat = "range" IdentifierList ":=" RangeExpression Block .
  RangeExpression = Expression .

A range expression must evaluate to an array, map or string. The identifier list must contain
either one or two identifiers. If the range expression is a map, a single identifier is declared
to range over the keys of the map; two identifiers range over the keys and corresponding
values. For arrays and strings, the behavior is analogous for integer indices (the keys) and
array elements (the values).

  a := []int(1, 2, 3);
  m := [string]map int("fo",2, "foo",3, "fooo",4)

  range i := a {
    f(a[i]);
  }

  range v, i := a {
    f(v);
  }

  range k, v := m {
    assert(len(k) == v);
  }

TODO: is this right?


Break statements
----

Within a for or switch statement, a break statement terminates execution of
the innermost for or switch statement.

  BreakStat = "break" [ identifier ].

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

  L: for i < n {
    switch i {
    case 5: break L
    }
  }


Continue statements
----

Within a for loop a continue statement begins the next iteration of the
loop at the post statement.

  ContinueStat = "continue" [ identifier ].

The optional identifier is analogous to that of a break statement.


Label declaration
----

A label declaration serves as the target of a goto, break or continue statement.

  LabelDecl = identifier ":" .

  Error:


Goto statements
----

A goto statement transfers control to the corresponding label statement.

  GotoStat = "goto" identifier .

  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:

  goto L;  // BAD
  v := 3;
  L:

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

Packages
----

Every source file identifies the package to which it belongs.
The file must begin with a package clause.

  PackageClause = "package" PackageName .

  package Math


Import declarations
----

A program can gain access to exported items from another package
through an import declaration:

  ImportDecl = "import" ( ImportSpec | "(" ImportSpecList [ ";" ] ")" ) .
  ImportSpec = [ "." | PackageName ] PackageFileName .
  ImportSpecList = ImportSpec { ";" ImportSpec } .

An import statement makes the exported contents of the named
package file accessible in this package.

In the following discussion, assume we have a package in the
file "/lib/math", called package Math, which exports functions sin
and cos.

In the general form, with an explicit package name, the import
statement declares that package name as an identifier whose
contents are the exported elements of the imported package.
For instance, after

  import M "/lib/math"

the contents of the package /lib/math can be accessed by
M.cos, M.sin, etc.

In its simplest form, with no package name, the import statement
implicitly uses the imported package name itself as the local
package name.  After

  import "/lib/math"

the contents are accessible by Math.sin, Math.cos.

Finally, if instead of a package name the import statement uses
an explicit period, the contents of the imported package are added
to the current package. After

  import . "/lib/math"

the contents are accessible by sin and cos.  In this instance, it is
an error if the import introduces name conflicts.


Program
----

A program is a package clause, optionally followed by import declarations,
followed by a series of declarations.

  Program = PackageClause { ImportDecl [ ";" ] } { Declaration [ ";" ] } .


TODO
----

- TODO: type switch?
- TODO: words about slices
- TODO: really lock down semicolons