266b9d49bfa3d2d16b4111378b1f9794373ee141
The Go Annotated Specification
This document supersedes all previous Go spec attempts. The intent is to make this a reference for syntax and semantics. It is annotated with additional information not strictly belonging into a language spec.
Open questions
- how to delete from a map
- how to test for map membership (we may want an 'atomic install'? m[i] ?= x; )
- compound struct literals? StructTypeName { a, b, c }
- array literals should be easy/natural to write [ 1, 2, 3 ] ArrayTypeName [ 1, 2, 3 ]
- map literals [ "a" : 1, "d" : 2, "z" : 3 ] MapTypeName [ "a" : 1, "d" : 2, "z" : 3 ]
- are basic types interfaces / do they define interfaces?
- package initialization?
Design decisions
A list of decisions made but for which we haven't incorporated proper language into this spec. Keep this section small and the spec up-to-date instead.
- multi-dimensional arrays: implementation restriction for now
- no '->', always '.' - (*a)[i] can be sugared into: a[i] - '.' to select package elements
- arrays are not automatically pointers, we must always say explicitly: "*array T" if we mean a pointer to that array - there is no pointer arithmetic in the language - there are no unions
- packages: need to pin it all down
- tuple notation: (a, b) = (b, a); generally: need to make this clear
- for now: no (C) 'static' variables inside functions
- exports: we write: 'export a, b, c;' (with a, b, c, etc. a list of exported names, possibly also: structure.field) - the ordering of methods in interfaces is not relevant - structs must be identical (same decl) to be the same (Ken has different implementation: equivalent declaration is the same; what about methods?)
- new methods can be added to a struct outside the package where the struct is declared (need to think through all implications) - array assignment by value - do we need a type switch?
- write down scoping rules for statements
- semicolons: where are they needed and where are they not needed. need a simple and consistent rule - we have: postfix and -- as statements
Guiding principles
Go is an attempt at a new systems programming language. [gri: this needs to be expanded. some keywords below]
- small, concise, crisp - procedural - strongly typed - few, orthogonal, and general concepts - avoid repetition of declarations - multi-threading support in the language - garbage collected - containers w/o templates - compiler can be written in Go and so can it's GC - very fast compilation possible (1MLOC/s stretch goal) - reasonably efficient (C ballpark) - compact, predictable code (local program changes generally have local effects) - no macros
Syntax
The syntax of Go borrows from the C tradition with respect to statements and from the Pascal tradition with respect to declarations. Go programs are written using a lean notation with a small set of keywords, without filler keywords (such as 'of', 'to', etc.) or other gratuitous syntax, and with a slight preference for expressive keywords (e.g. 'function') over operators or other syntactic mechanisms. Generally, "light" language features (variables, simple control flow, etc.) are expressed using a light-weight notation (short keywords, little syntax), while "heavy" language features use a more heavy-weight notation (longer keywords, more syntax).
[gri: should say something about syntactic alternatives: if a syntactic form foreseeably will lead to a style recommendation, try to make that the syntactic form instead. For instance, Go structured statements always require the {} braces even if there is only a single sub-statement. Similar ideas apply elsewhere.]
Modularity, identifiers and scopes
A Go program consists of one or more files compiled separately, though not independently. A single file or compilation unit may make individual identifiers visible to other files by marking them as exported; there is no "header file". The exported interface of a file may be exposed in condensed form (without the corresponding implementation) through tools.
A package collects types, constants, functions, and so on into a named entity that may be imported to enable its constituents be used in another compilation unit. Each source file is part of exactly one package; each package is constructed from one source file.
Within a file, all identifiers are declared explicitly (expect for general predeclared identifiers such as true and false) and thus for each identifier in a file the corresponding declaration can be found in that same file (usually before its use, except for the rare case of forward declarations). Identifiers may denote program entities that are implemented in other files. Nevertheless, such identifiers are still declared via an import declaration in the file that is referring to them. This explicit declaration requirement ensures that every compilation unit can be read by itself.
The scoping of identifiers is uniform: An identifier is visible from the point of its declaration to the end of the immediately surrounding block, and nested identifiers shadow outer identifiers with the same name. All identifiers are in the same namespace; i.e., no two identifiers in the same scope may have the same name even if they denote different language concepts (for instance, such as variable vs a function). Uniform scoping rules make Go programs easier to read and to understand.
Program structure
A compilation unit consists of a package specifier followed by import declarations followed by other declarations. There are no statements at the top level of a file. [gri: do we have a main function? or do we treat all functions uniformly and instead permit a program to be started by providing a package name and a "start" function? I like the latter because if gives a lot of flexibility and should be not hard to implement]. [r: i suggest that we define a symbol, main or Main or start or Start, and begin execution in the single exported function of that name in the program. the flexibility of having a choice of name is unimportant and the corresponding need to define the name in order to link or execute adds complexity. by default it should be trivial; we could allow a run-time flag to override the default for gri's flexibility.]
Typing, polymorphism, and object-orientation
Go programs are strongly typed; i.e., each program entity has a static type known at compile time. Variables also have a dynamic type, which is the type of the value they hold at run-time. Generally, the dynamic and the static type of a variable are identical, except for variables of interface type. In that case the dynamic type of the variable is a pointer to a structure that implements the variable's (static) interface type. There may be many different structures implementing an interface and thus the dynamic type of such variables is generally not known at compile time. Such variables are called polymorphic.
Interface types are the mechanism to support an object-oriented programming style. 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 functions that a corresponding implementation must provide. Thus interface and implementation are strictly separated.
An interface is implemented by associating functions (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.
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 data types operating on interface types. [gri: there is some automatic boxing, semi-automatic unboxing support for basic types].
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 "getting the address" of that variable. In particular, pointers may point "inside" other variables, or to automatic variables (which are usually allocated on the stack). 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 invoked recursively. Functions may declare nested functions, and nested functions have access to the variables in the surrounding functions, they are in fact closures. Functions may be anonymous and appear as literals in expressions.
Multithreading and channels
[Rob: We need something here]
Notation
The syntax is specified in green productions using Extended Backus-Naur Form (EBNF). In particular:
'' encloses lexical symbols | separates alternatives () used for grouping [] specifies option (0 or 1 times) {} specifies repetition (0 to n times)
A production may be referred to from various places in this document but is usually defined close to its first use. Code examples are written in gray. Annotations are in blue, and open issues are in red. One goal is to get rid of all red text in this document. [r: done!]
Vocabulary and representation
REWRITE THIS: BADLY EXPRESSED
Go program source is a sequence of characters. Each character is a Unicode code point encoded in UTF-8.
A Go program is a sequence of symbols satisfying the Go syntax. A symbol is a non-empty sequence of characters. Symbols are identifiers, numbers, strings, operators, delimiters, and comments. White space must not occur within symbols (except in comments, and in the case of blanks and tabs in strings). They are ignored unless they are essential to separate two consecutive symbols.
White space is composed of blanks, newlines, carriage returns, and tabs only.
A character is a Unicode code point. In particular, capital and lower-case letters are considered as being distinct. Note that some Unicode characters (e.g., the character ä), may be representable in two forms, as a single code point, or as two code points. For the Unicode standard these two encodings represent the same character, but for Go, these two encodings correspond to two different characters).
Source encoding
The input is encoded in UTF-8. In the grammar we use the notation
utf8_char
to refer to an arbitrary Unicode code point encoded in UTF-8.
Digits and Letters
octal_digit = { '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' } . decimal_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' | '_' .
For now, letters and digits are ASCII. We may expand this to allow Unicode definitions of letters and digits.
Identifiers
An identifier is a name for a program entity such as a variable, a type, a function, etc.
identifier = letter { letter | decimal_digit } .
- need to explain scopes, visibility (elsewhere) - need to say something about predeclared identifiers, and their (universe) scope (elsewhere)
Character and string literals
A RawStringLit is a string literal delimited by back quotes ``; the first back quote encountered after the opening back quote terminates the string.
RawStringLit = '`' { utf8_char } '`' .
`abc` `n`
Character and string literals are very similar to C except: - Octal character escapes are always 3 digits ( 77 not 77) - Hexadecimal character escapes are always 2 digits (x07 not x7) - Strings are UTF-8 and represent Unicode - `` strings exist; they do not interpret backslashes
CharLit = ''' ( UnicodeValue | ByteValue ) ''' . StringLit = RawStringLit | InterpretedStringLit . InterpretedStringLit = '"' { UnicodeValue | ByteValue } '"' . ByteValue = OctalByteValue | HexByteValue . OctalByteValue = '' octal_digit octal_digit octal_digit . HexByteValue = '' 'x' hex_digit hex_digit . UnicodeValue = utf8_char | EscapedCharacter | LittleUValue | BigUValue . LittleUValue = '' 'u' hex_digit hex_digit hex_digit hex_digit . BigUValue = '' 'U' hex_digit hex_digit hex_digit hex_digit hex_digit hex_digit hex_digit hex_digit . EscapedCharacter = '' ( 'a' | 'b' | 'f' | 'n' | 'r' | 't' | 'v' ) .
An OctalByteValue contains three octal digits. A HexByteValue contains two hexadecimal digits. (Note: This differs from C but is simpler.)
It is erroneous for an OctalByteValue to represent a value larger than 255. (By construction, a HexByteValue cannot.)
A UnicodeValue takes one of four forms:
1. 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. 2. The usual list of C backslash escapes: n t etc. 3. 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. 4. 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.
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' ' ' ' 7' ' 377' 'x7' 'xff' 'u12e4' 'U00101234'
A string literal has type 'string'. Its value is constructed by taking the byte values formed by the successive elements of the literal. For ByteValues, these are the literal bytes; for UnicodeValues, 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.
"" "Hello, world!n" "日本語" "u65e5本U00008a9e" "xffu00FF"
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 "u65e5u672cu8a9e" // The explicit Unicode code points "U000065e5U0000672cU00008a9e" // The explicit Unicode code points "xe6x97xa5xe6x9cxacxe8xaax9e" // 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. [This simple strategy may be insufficient in the long run but is surely fine for now.]
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 decimal 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 assigned to a variable (or other l-value) is it required to fit into a particular size - that of type of the variable. In other words, integer constants and arithmetic upon them is not subject to overflow; only assignment of integer constants (and constant expressions) to an l-value 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 l-value.
Floating point literals also represent an abstract, ideal floating point value that is constrained only upon assignment. [r: what do we need to say here? trickier because of truncation of fractions.]
IntLit = [ ' ' | '-' ] UnsignedIntLit . UnsignedIntLit = DecimalIntLit | OctalIntLit | HexIntLit . DecimalIntLit = ( '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9' ) { decimal_digit } . OctalIntLit = '0' { octal_digit } . HexIntLit = '0' ( 'x' | 'X' ) hex_digit { hex_digit } . FloatLit = [ ' ' | '-' ] UnsignedFloatLit . UnsignedFloatLit = "the usual decimal-only floating point representation".
Compound Literals
THIS SECTION IS WRONG Compound literals require some fine tuning. I think we did ok in Sawzall but there are some loose ends. I don't like that one cannot easily distinguish between an array and a struct. We may need to specify a type if these literals appear in expressions, but we don't want to specify a type if these literals appear as intializer expressions where the variable is already typed. And we don't want to do any implicit conversions.
CompoundLit = ArrayLit | FunctionLit | StructureLit | MapLit. ArrayLit = '{' [ ExpressionList ] ']'. // all elems must have "the same" type StructureLit = '{' [ ExpressionList ] '}'. MapLit = '{' [ PairList ] '}'. PairList = Pair { ',' Pair }. Pair = Expression ':' Expression.
Literals
Literal = BasicLit | CompoundLit . BasicLit = CharLit | StringLit | IntLit | FloatLit .
Function Literals
The type of a function literal
FunctionLit = FunctionType Block.
A function literal represents a function. A function literal can be invoked or assigned to a variable of the corresponding function pointer type.
// 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; }
Operators
- incomplete
Delimiters
- incomplete
Comments
There are two forms of comments.
The first starts '//' and ends at a newline.
The second starts at '/*' and ends at the first '*/'. It may cross newlines. It does not nest.
Comments are treated like white space.
Common productions
IdentifierList = identifier { ',' identifier }. ExpressionList = Expression { ',' Expression }.
QualifiedIdent = [ PackageName '.' ] identifier. PackageName = identifier.
Types
A type specifies the set of values which variables of that type may assume, and the operators that are applicable.
Except for variables of interface types, the static type of a variable (i.e. the type the variable is declared with) is the same as the dynamic type of the variable (i.e. the type of the variable at run-time). Variables of interface types may hold variables of different dynamic types, but their dynamic types must be compatible with the static interface type. At any given instant during run-time, a variable has exactly one dynamic type. A type declaration associates an identifier with a type.
Array and struct types are called structured types, all other types are called unstructured. A structured type cannot contain itself. [gri: this needs to be formulated much more precisely].
Type = TypeName | ArrayType | ChannelType | InterfaceType | FunctionType | MapType | StructType | PointerType . TypeName = QualifiedIdent.
[gri: To make the types specifications more precise we need to introduce some general concepts such as what it means to 'contain' another type, to be 'equal' to another type, etc. Furthermore, we are imprecise as we sometimes use the word type, sometimes just the type name (int), or the structure (array) to denote different things (types and variables). We should explain more precisely. Finally, there is a difference between equality of types and assignment compatibility - or isn't there?]
Basic types
Go defines a number of basic types which are referred to by their predeclared type names. There are signed and unsigned integer types, and floating point types:
bool the truth values true and false
uint8 the set of all unsigned 8bit integers uint16 the set of all unsigned 16bit integers uint32 the set of all unsigned 32bit integers unit64 the set of all unsigned 64bit integers
byte same as uint8
int8 the set of all signed 8bit integers, in 2's complement int16 the set of all signed 16bit integers, in 2's complement int32 the set of all signed 32bit integers, in 2's complement int64 the set of all signed 64bit integers, in 2's complement
float32 the set of all valid IEEE-754 32bit floating point numbers float64 the set of all valid IEEE-754 64bit floating point numbers float80 the set of all valid IEEE-754 80bit floating point numbers double same as float64
Additionally, Go declares 3 basic types, uint, int, and float, which are platform-specific. The bit width of these types corresponds to the "natural bit width" for the respective types for the given platform (e.g. int is usally the same as int32 on a 32bit architecture, or int64 on a 64bit architecture). These types are by definition platform-specific and should be used with the appropriate caution.
[gri: do we specify minimal sizes for uint, int, float? e.g. int is at least int32?] [gri: do we say something about the correspondence of sizeof(*T) and sizeof(int)? Are they the same?] [r: do we want int128 and uint128?.]
Built-in types
Besides the basic types there is a set of built-in types: string, and chan, with maybe more to follow.
Type string
The string type represents the set of string values (strings). A string behaves like an array 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). [r: is it a bulitin? do we make it a method? etc. this is a placeholder]. 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 string('x') yields "x" - 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";
The language has string literals as dicussed above. The type of a string literal is 'string'.
Array types
An array is a structured type consisting of a number of elements which are 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.
THIS SECTION NEEDS WORK REGARDING STATIC AND DYNAMIC ARRAYS
An array type specifies a set of arrays with a given element type and an optional array length. The array length must be (compile-time) constant expression, if present. Arrays without length specification are called open arrays. An open array must not contain other open arrays, and open arrays can only be used as parameter types or in a pointer type (for instance, a struct may not contain an open array field, but only a pointer to an open array).
[gri: Need to define when array types are the same! Also need to define assignment compatibility] [gri: Need to define a mechanism to get to the length of an array at run-time. This could be a predeclared function 'length' (which may be problematic due to the name). Alternatively, we could define an interface for array types and say that there is a 'length()' method. So we would write a.length() which I think is pretty clean.]. [r: if array types have an interface and a string is an array, some stuff (but not enough) falls out nicely.]
ArrayType = 'array' { '[' ArrayLength ']' } ElementType. ArrayLength = Expression. ElementType = Type.
The notation
array [n][m] T
is a syntactic shortcut for
array [n] array [m] T.
(the shortcut may be applied recursively).
array uint8 array [64] struct { x, y: int32; } array [1000][1000] float64
Channel types
A channel provides a mechanism for two concurrently executing functions to exchange values and synchronize execution. A channel type can be 'generic', permitting values of any type to be exchanged, or it may be 'specific', permitting only values of an explicitly specified type.
Upon creation, a channel can be used both to send and to receive; 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' [ '<' | '>' ] [ Type ] .
chan // 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< // a channel that can receive (only) values of any type
Channel values are created using new(chan) (etc.). Since new() returns a pointer, channel variables are always pointers to channels:
var c *chan int = new(chan int);
It is an error to attempt to dereference a channel pointer.
Pointer types
- TODO: Need some intro here.
Two pointer types are the same if they are pointing to variables of the same type.
PointerType = '*' Type.
- We do not allow pointer arithmetic of any kind.
Interface types
- TBD: This needs to be much more precise. For now we understand what it means.
An interface type specifies a set of methods, the "method interface" of structs. No two methods in one interface can have the same name.
Two interfaces are the same if their set of functions is the same, i.e., if all methods exist in both interfaces and if the function names and signatures are the same. The order of declaration of methods in an interface is irrelevant.
A set of interface types implicitly creates an unconnected, ordered lattice of types. An interface type T1 is said to be smaller than or equalt to an interface type T2 (T1 <= T2) if the entire interface of T1 "is part" of T2. Thus, two interface types T1, T2 are the same if T1 <= T2, and T2 <= T1, and thus we can write T1 == T2.
InterfaceType = 'interface' '{' { MethodDecl } '}' . MethodDecl = identifier Signature ';',
// An empty interface. interface {};
// A basic file interface. interface { Read(Buffer) bool; Write(Buffer) bool; Close(); }
Interface pointers can be implemented as "fat pointers"; namely a pair (ptr, tdesc) where ptr is simply the pointer to a struct instance implementing the interface, and tdesc is the structs type descriptor. Only when crossing the boundary from statically typed structs to interfaces and vice versa, does the type descriptor come into play. In those places, the compiler statically knows the value of the type descriptor.
Function types
FunctionType = 'func' Signature . Signature = [ 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 only hold a pointer to a function, but 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.
Map types
A map is a structured 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.
MapType = 'map' '[' KeyType ']' ValueType . KeyType = Type . ValueType = Type .
map [string] int map [struct { pid int; name string }] *chan Buffer
Struct types
Struct types are similar to C structs.
NEED TO DEFINE STRUCT EQUIVALENCE Two struct types are the same if and only if they are declared by the same struct type; i.e., struct types are compared via equivalence, and *not* structurally. For that reason, struct types are usually given a type name so that it is possible to refer to the same struct in different places in a program. What about equivalence of structs w/ respect to methods? What if methods can be added in another package? TBD.
Each field of a struct represents a variable within the data structure. In particular, a function field represents a function variable, not a method.
StructType = 'struct' '{' { FieldDecl } '}' . FieldDecl = IdentifierList Type ';' .
// An empty struct. struct {}
// A struct with 5 fields. struct { x, y int; u float; a []int; f func(); }
Note that a program which never uses interface types can be fully statically typed. That is, the "usual" implementation of structs (or classes as they are called in other languages) having an extra type descriptor prepended in front of every single struct is not required. Only when a pointer to a struct is assigned to an interface variable, the type descriptor comes into play, and at that point it is statically known at compile-time!
Package specifiers
Every source file is an element of a package, and defines which package by the first element of every source file, which must be a package specifier:
PackageSpecifier = 'package' PackageName .
package Math
Package import declarations
A program can access exported items from another package. It does so by in effect declaring a local name providing access to the package, and then using the local name as a namespace with which to address the elements of the package.
ImportDecl = 'import' PackageName FileName . FileName = DoubleQuotedString . DoubleQuotedString = '"' TEXT '"' .
(DoubleQuotedString should be replaced by the correct string literal production!) Package import declarations must be the first statements in a file after the package specifier.
A package import associates an identifier with a package, named by a file. In effect, it is a declaration:
import Math "lib/Math"; import library "my/library";
After such an import, one can use the Math (e.g) identifier to access elements within it
x float = Math.sin(y);
Note that this process derives nothing explicit about the type of the `imported' function (here Math.sin()). The import must execute to provide this information to the compiler (or the programmer, for that matter).
An angled-string refers to official stuff in a public place, in effect the run-time library. A double-quoted-string refers to arbitrary code; it is probably a local file name that needs to be discovered using rules outside the scope of the language spec.
The file name in a package must be complete except for a suffix. Moreover, the package name must correspond to the (basename of) the source file name. For instance, the implementation of package Bar must be in file Bar.go, and if it lives in directory foo we write
import Bar "foo/bar";
to import it.
[This is a little redundant but if we allow multiple files per package it will seem less so, and in any case the redundancy is useful and protective.]
We assume Unix syntax for file names: / separators, no suffix for directories. If the language is ported to other systems, the environment must simulate these properties to avoid changing the source code.
Declarations
- This needs to be expanded. - We need to think about enums (or some alternative mechanism).
Declaration = (ConstDecl | VarDecl | TypeDecl | FunctionDecl | ForwardDecl | AliasDecl) .
Const declarations
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 )
Variable declarations
VarDecl = 'var' ( VarSpec | '(' VarSpecList [ ';' ] ')' ) | ShortVarDecl . VarSpec = IdentifierList ( Type [ '=' ExpressionList ] | '=' ExpressionList ) . VarSpecList = VarSpec { ';' VarSpec } . ShortVarDecl = identifier ':=' Expression .
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.
[ TODO: why is x := 0 not legal at the global level? ]
Type declarations
TypeDecl = 'type' ( TypeSpec | '(' TypeSpecList [ ';' ] ')' ). TypeSpec = identifier Type . TypeSpecList = TypeSpec { ';' TypeSpec }.
type IntArray [16] int type ( Point struct { x, y float }; Polar Point )
Function and method declarations
FunctionDecl = 'func' [ Receiver ] identifier Parameters [ Result ] ( ';' | Block ) . Block = '{' { Statement } '}' .
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. The receiver is a struct with which the function is associated. The receiver type must denote a pointer to a struct.
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; }
The last two examples are methods of struct type Point. The variable p is the receiver; within the body of the method it represents the value of the receiving struct.
Note that methods are declared outside the body of the corresponding struct.
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;
Statements
Statement = EmptyStat | Assignment | CompoundStat | Declaration | ExpressionStat | IncDecStat | IfStat | WhileStat | ForStat | RangeStat | ReturnStat .
Empty statements
EmptyStat = ';' .
Assignments
Assignment = Designator '=' Expression .
- no automatic conversions - values can be assigned to variables if they are of the same type, or if they satisfy the interface type (much more precision needed here!)
Compound statements
CompoundStat = '{' { Statement } '}' .
Expression statements
ExpressionStat = Expression .
IncDec statements
IncDecStat = Expression ( ' ' | '--' ) .
If statements
IfStat = 'if' ( [ Expression ] '{' { IfCaseList } '}' ) | ( Expression '{' { Statement } '}' [ 'else' { Statement } ] ). IfCaseList = ( 'case' ExpressionList | 'default' ) ':' { Statement } .
if x < y { return x; } else { return y; }
if tag { case 0, 1: s1(); case 2: s2(); default: ; }
if { case x < y: f1(); case x < z: f2(); }
While statements
WhileStat = 'while' ( [ Expression ] '{' { WhileCaseList } '}' ) | ( Expression '{' { Statement } '}' ). WhileCaseList = 'case' ExpressionList ':' { Statement } .
while { case i < n: f1(); case i < m: f2(); }
For statements
NEEDS TO BE COMPLETED
ForStat = 'for' ...
Range statements
Range statements denote iteration 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 := [ 1, 2, 3]; m := [ "fo" : 2, "foo" : 3, "fooo" : 4 ]
range i := a { f(a[i]); }
range k, v := m { assert(len(k) == v); }
Return statements
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; }
func complex_f1() (re float, im float) { return -7.0, -4.0; }
The second is to provide names for the return values and assign them explicitly in the function; the return statement will then provide no values:
func complex_f2() (re float, im float) { re = 7.0; im = 4.0; return; }
It is legal to name the return values in the declaration even if the first form of return statement is used:
func complex_f2() (re float, im float) { return 7.0, 4.0; }
Expressions
Expression = Conjunction { '||' Conjunction }. Conjunction = Comparison { '&&' Comparison }. Comparison = SimpleExpr [ relation SimpleExpr ]. relation = '==' | '!=' | '<' | '<=' | '>' | '>='. SimpleExpr = Term { add_op Term }. add_op = ' ' | '-' | '|' | '^'. Term = Factor { mul_op Factor }. mul_op = '*' | '/' | '%' | '<<' | '>>' | '&'.
The corresponding precedence hierarchy is as follows: (5 levels of precedence is about the maximum people can keep comfortably in their heads. The experience with C and C shows that more then that usually requires explicit manual consultation...). [gri: I still think we should consider 0 levels of binary precedence: All operators are on the same level, but parentheses are required when different operators are mixed. That would make it really easy, and really clear. It would also open the door for straight-forward introduction of user-defined operators, which would be rather useful.]
Precedence Operator 1 || 2 && 3 == != < <= > >= 4 - | ^ 5 * / % << >> &
For integer values, / and % satisfy the following relationship:
(a / b) * b a % b == a
and
(a / b) is "truncated towards zero".
The shift operators implement arithmetic shifts for signed integers, and logical shifts for unsigned integers. TBD: is there any range checking on s in x >> s, or x << s ?
[gri: We decided on a couple of issues here that we need to write down more nicely]
- There are no implicit type conversions except for constants/literals. In particular, unsigned and signed integers cannot be mixed in an expression w/o explicit casting.
- Unary '^' corresponds to C '~' (bitwise negate).
- Arrays can be subscripted (a[i]) or sliced (a[i : j]). A slice a[i : j] is a new array of length (j - i), and consisting of the elements a[i], a[i 1], ... a[j - 1]. [gri/r: Is the slice array bounds check hard (leading to an error), or soft (truncating) ?]. Furthermore: Array slicing is very tricky! Do we get a copy (a new array) or a new array descriptor? This is open at this point. There is a simple way out of the mess: Structured types are always passed by reference, and there is no value assignment for structured types. It gets very complicated very quickly.
[gri: Syntax below is incomplete - what about method invocation?]
Factor = Literal | Designator | '!' Expression | '-' Expression | '^' Expression | '&' Expression | '(' Expression ')' | Call. Designator = QualifiedIdent { Selector }. Selector = '.' identifier | '[' Expression [ ':' Expression ] ']'. Call = Factor '(' ExpressionList ')'.
[gri: We need a precise definition of a constant expression]
Compilation units
The unit of compilation is a single file. A compilation unit consists of a package specifier followed by a list of import declarations followed by a list of global declarations.
CompilationUnit = { ImportDecl } { GlobalDeclaration }. GlobalDeclaration = Declaration.
Exports
Globally declared identifiers may be exported, thus making the exported identifer visible outside the package. Another package may then import the identifier to use it.
Export directives must only appear at the global level of a compilation unit (at least for now). That is, one can export compilation-unit global identifiers but not, for example, 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.
ExportDirective = 'export' ExportIdentifier { ',' ExportIdentifier } . ExportIdentifier = identifier .
export sin, cos;
One may export variables and types, but (at least for now), not aliases. [r: what is needed to make aliases exportable? issue is transitivity.]
Exporting a variable does not automatically export the type of the variable. For illustration, consider the program fragment:
package P; export v1, v2, p; struct S { a int; b int; } var v1 S; var v2 S; var p *S;
Notice that S is not exported. Another source file may contain:
import P; alias v1 P.v1; alias v2 P.v2; alias p P.p;
This program can use v and p but not access the fields (a and b) of structure type S explicitly. For instance, it could legally contain
if p == nil { } if v1 == v2 { }
but not
if v.a == 0 { }
