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- updated docs
SVN=111539
This commit is contained in:
Robert Griesemer
parent
719a06fd97
commit
28590a0abb
235
doc/go_lang.txt
235
doc/go_lang.txt
@ -17,8 +17,8 @@ The design is motivated by the following guidelines:
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- strongly typed
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- concise syntax avoiding repetition
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- few, orthogonal, and general concepts
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- excellent support for threading and interprocess communication
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- efficient garbage collection
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- support for threading and interprocess communication
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- garbage collection
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- container library written in Go
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- reasonably efficient (C ballpark)
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@ -34,12 +34,11 @@ individual identifiers visible to other files by marking them as
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exported; there is no "header file".
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A package collects types, constants, functions, and so on into a named
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entity that may be imported to enable its constituents be used in
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entity that may be exported to enable its constituents be used in
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another compilation unit.
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Because there are no header files, all identifiers in a package are either
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declared explicitly within the package or, in certain cases, arise from an
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import statement.
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declared explicitly within the package or arise from an import statement.
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Scoping is essentially the same as in C.
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@ -64,20 +63,9 @@ still under development.
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Typing, polymorphism, and object-orientation
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Go programs are strongly typed: each program entity has a static
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type known at compile time. Variables also have a dynamic type, which
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is the type of the value they hold at run-time. Usually, the
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dynamic and the static type of a variable are identical, except for
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variables of interface type. In that case the dynamic type of the
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variable is a pointer to a structure that implements the variable's
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(static) interface type. There may be many different structures
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implementing an interface and thus the dynamic type of such variables
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is generally not known at compile time. Such variables are called
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polymorphic.
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Also, certain expressions, in particular map and channel accesses,
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can also be polymorphic. The language provides mechanisms to
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make use of such polymorphic values type-safe.
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Go programs are strongly typed. Certain expressions, in particular map
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and channel accesses, can also be polymorphic. The language provides
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mechanisms to make use of such polymorphic values type-safe.
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Interface types are the mechanism to support an object-oriented
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programming style. Different interface types are independent of each
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@ -130,7 +118,8 @@ language support.
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Values and references
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Unless accessing expliciting through a pointer, all objects are values.
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All objects have value semantics, but its contents may be accessed
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through different pointers referring to the same object.
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For example, when calling a function with an array, the array is
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passed by value, possibly by making a copy. To pass a reference,
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one must explicitly pass a pointer to the array. For arrays in
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@ -151,9 +140,9 @@ Here is a complete example Go program that implements a concurrent prime sieve:
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============================
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package Main
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// Send the sequence 2, 3, 4, ... to channel 'c'.
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func Generate(ch *chan< int) {
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for i := 2; true; i++ {
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// Send the sequence 2, 3, 4, ... to channel 'ch'.
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func Generate(ch *chan> int) {
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for i := 2; ; i++ {
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>ch = i; // Send 'i' to channel 'ch'.
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}
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}
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@ -161,7 +150,7 @@ func Generate(ch *chan< int) {
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// Copy the values from channel 'in' to channel 'out',
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// removing those divisible by 'prime'.
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func Filter(in *chan< int, out *chan> int, prime int) {
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while true {
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for ; ; {
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i := <in; // Receive value of new variable 'i' from 'in'.
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if i % prime != 0 {
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>out = i; // Send 'i' to channel 'out'.
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@ -173,7 +162,7 @@ func Filter(in *chan< int, out *chan> int, prime int) {
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func Sieve() {
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ch := new(chan int); // Create a new channel.
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go Generate(ch); // Start Generate() as a subprocess.
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while true {
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for ; ; {
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prime := <ch;
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printf("%d\n", prime);
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ch1 := new(chan int);
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@ -248,14 +237,14 @@ hex_digit = { '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9' | 'a' |
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'A' | 'b' | 'B' | 'c' | 'C' | 'd' | 'D' | 'e' | 'E' | 'f' | 'F' } .
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letter = 'A' | 'a' | ... 'Z' | 'z' | '_' .
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For simplicity, letters and digits are ASCII. We may expand this to allow
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Unicode definitions of letters and digits.
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For simplicity, letters and digits are ASCII. We may in time allow
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Unicode identifiers.
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Identifiers
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An identifier is a name for a program entity such as a variable, a
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type, a function, etc.
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type, a function, etc. An identifier must not be a reserved word.
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identifier = letter { letter | decimal_digit } .
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@ -304,7 +293,7 @@ architecture, or int64 on a 64-bit architecture. These types are by
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definition platform-specific and should be used with the appropriate
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caution.
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Two predeclared identifiers, 'true' and 'false', represent the
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Two reserved words, 'true' and 'false', represent the
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corresponding boolean constant values.
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@ -509,7 +498,7 @@ elements of an array is called its length. The elements of an array
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are designated by indices which are integers between 0 and the length
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- 1.
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An array type specifies a set of arrays with a given element type and
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An array type specifies arrays with a given element type and
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an optional array length. The array length must be a (compile-time)
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constant expression, if present. Arrays without length specification
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are called dynamic arrays. A dynamic array must not contain other dynamic
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@ -522,6 +511,7 @@ ArrayLength = Expression.
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ElementType = Type.
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[] uint8
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[2*n] int
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[64] struct { x, y: int32; }
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[1000][1000] float64
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@ -544,13 +534,15 @@ called (key, value) pairs. For a given map,
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the keys and values must each be of a specific type.
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Upon creation, a map is empty and values may be added and removed
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during execution. The number of entries in a map is called its length.
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A map whose value type is 'any' can store values of all types.
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MapType = 'map' '[' KeyType ']' ValueType .
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KeyType = Type .
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ValueType = Type .
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ValueType = Type | 'any' .
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map [string] int
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map [struct { pid int; name string }] *chan Buffer
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map [string] any
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Map Literals
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@ -595,11 +587,14 @@ Struct literals represent struct constants. They comprise a list of
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expressions that represent the individual fields of a struct. The
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individual expressions must match those of the specified struct type.
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StructLit = StructType '{' [ ExpressionList ] '}' .
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StructLit = StructType '(' [ ExpressionList ] ')' .
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StructType = TypeName .
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The type name must be that of a defined struct type.
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Point(2, 3)
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ColoredPoint(4, 4, "green")
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Pointer types
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@ -626,12 +621,12 @@ Upon creation, a channel can be used both to send and to receive; it
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may be restricted only to send or to receive; such a restricted channel
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is called a 'send channel' or a 'receive channel'.
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ChannelType = 'chan' [ '<' | '>' ] [ Type ] .
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ChannelType = 'chan' [ '<' | '>' ] ValueType .
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chan // a generic channel
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chan any // a generic channel
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chan int // a channel that can exchange only ints
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chan> float // a channel that can only be used to send floats
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chan< // a channel that can receive (only) values of any type
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chan< any // a channel that can receive (only) values of any type
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Channel variables always have type pointer to channel.
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It is an error to attempt to dereference a channel pointer.
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@ -682,6 +677,8 @@ Block = '{' [ StatementList ] '}' .
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A function literal can be invoked
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or assigned to a variable of the corresponding function pointer type.
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For now, a function literal can reference only its parameters, global
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variables, and variables declared within the function literal.
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// Function literal
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func (a, b int, z float) bool { return a*b < int(z); }
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@ -700,10 +697,13 @@ a method indicates the type of the struct by declaring a receiver of type
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the declaration
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func (p *Point) distance(float scale) float { return scale * (p.x*p.x + p.y*p.y) }
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func (p *Point) distance(float scale) float {
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return scale * (p.x*p.x + p.y*p.y);
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}
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creates a method of type Point. Note that methods are not declared
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within their struct type declaration. They may appear anywhere.
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within their struct type declaration. They may appear anywhere and
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may be forward-declared for commentary.
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When invoked, a method behaves like a function whose first argument
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is the receiver, but at the call site the receiver is bound to the method
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@ -736,9 +736,9 @@ MethodDecl = identifier Parameters [ Result ] ';' .
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Close();
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}
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Any struct that has, as a subset, the methods of that interface is
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said to implement the interface. For instance, if two struct types
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S1 and S2 have the methods
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Any struct whose interface has, possibly as a subset, the complete
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set of methods of an interface I is said to implement interface I.
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For instance, if two struct types S1 and S2 have the methods
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func (p *T) Read(b Buffer) bool { return ... }
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func (p *T) Write(b Buffer) bool { return ... }
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@ -860,7 +860,8 @@ Function and method declarations
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Functions and methods have a special declaration syntax, slightly
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different from the type syntax because an identifier must be present
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in the signature.
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in the signature. For now, functions and methods can only be declared
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at the global level.
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FunctionDecl = 'func' NamedSignature ( ';' | Block ) .
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NamedSignature = [ Receiver ] identifier Parameters [ Result ] .
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@ -900,7 +901,7 @@ Functions and methods can be forward declared by omitting the body:
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Export declarations
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Globally declared identifiers may be exported, thus making the
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Global identifiers may be exported, thus making the
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exported identifer visible outside the package. Another package may
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then import the identifier to use it.
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@ -966,10 +967,11 @@ can be simplified to
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Expression = Conjunction { '||' Conjunction }.
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Conjunction = Comparison { '&&' Comparison }.
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Comparison = SimpleExpr [ relation SimpleExpr ].
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relation = '==' | '!=' | '<' | '<=' | '>' | '>='.
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SimpleExpr = Term { add_op Term }.
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add_op = '+' | '-' | '|' | '^'.
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Term = Operand { mul_op Operand }.
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relation = '==' | '!=' | '<' | '<=' | '>' | '>='.
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add_op = '+' | '-' | '|' | '^'.
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mul_op = '*' | '/' | '%' | '<<' | '>>' | '&'.
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The corresponding precedence hierarchy is as follows:
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@ -996,13 +998,14 @@ and
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(a / b) is "truncated towards zero".
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The shift operators implement arithmetic shifts for signed integers,
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and logical shifts for unsigned integers.
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and logical shifts for unsigned integers. The property of negative
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shift counts are undefined.
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There are no implicit type conversions except for
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constants and literals. In particular, unsigned and signed integers
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cannot be mixed in an expression w/o explicit casting.
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Unary '^' corresponds to C '~' (bitwise negate).
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Unary '^' corresponds to C '~' (bitwise complement).
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Statements
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@ -1016,7 +1019,7 @@ Statement =
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GoStat |
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ReturnStat |
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IfStat | SwitchStat |
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WhileStat | ForStat | RangeStat |
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ForStat | RangeStat |
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BreakStat | ContinueStat | GotoStat | LabelStat .
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@ -1066,22 +1069,39 @@ or an array indexing.
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A tuple assignment assigns the individual elements of a multi-valued operation,
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such function evaluation or some channel and map operations, into individual
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variables. Tuple assignment is simultaneous.
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For example,
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variables. For instance, a tuple assignment such as
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v1, v2, v3 = e1, e2, e3
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assigns the expressions e1, e2, e3 to temporaries and then assigns the temporaries
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to the variables v1, v2, v3. Thus
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a, b = b, a
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exchanges the values of a and b.
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exchanges the values of a and b. The tuple assignment
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x, y = f()
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calls the function f, which must return 2 values and assigns them to x and y.
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As a special case, retrieving a value from a map, when written as a two-element
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tuple assignment, assign a value and a boolean. If the value is present in the map,
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the value is assigned and the second, boolean variable is set to true. Otherwise,
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the variable is unchanged, and the boolean value is set to false.
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value, present = map_var[key]
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Analogously, receiving a value from a channel can be written as a tuple assignment.
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value, success = <chan_var
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If the receive operation would block, the boolean is set to false. This provides to avoid
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blocking on a receive operation.
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Sending on a channel is a form of assignment. The left hand side expression
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must denote a channel pointer value.
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>chan_ptr = value
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In assignments, the type of the expression must match the type of the designator.
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@ -1136,13 +1156,11 @@ first form of return statement is used:
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If statements
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[ NOTE We propose a simplified control syntax ]
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If statements have the traditional form except that the
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condition need not be parenthesized and the statements
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condition need not be parenthesized and the "then" statement
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must be in brace brackets.
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IfStat = 'if' [ SimpleVarDecl ';' ] Expression Block [ 'else' ( Block | IfStat ) ] .
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IfStat = 'if' [ SimpleVarDecl ';' ] Expression Block [ 'else' Statement ] .
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if x > 0 {
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return true;
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@ -1165,13 +1183,20 @@ Switch statements
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Switches provide multi-way execution.
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SwitchStat = 'switch' [ SimpleVarDecl ';' ] [ Expression ] '{' CaseList '}' .
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CaseList = ( 'case' ExpressionList | 'default' ) ':' { Statement | 'fallthrough' ';' } .
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SwitchStat = 'switch' [ [ SimpleVarDecl ';' ] [ Expression ] ] '{' { CaseClause } '}' .
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CaseClause = CaseList { Statement } [ 'fallthrough' ] .
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CaseList = Case { Case } .
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Case = ( 'case' ExpressionList | 'default' ) ':' .
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Note that the expressions do not need to be constants. They will
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be evaluated top to bottom until the first successful non-defauit case.
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If none matches and there is a default case, the default case is
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executed.
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There can be at most one default case in a switch statement.
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The 'fallthrough' keyword indicates that the control should flow from
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the end of this case clause to the first statement of the next clause.
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The expressions do not need to be constants. They will
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be evaluated top to bottom until the first successful non-default case is reached.
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If none matches and there is a default case, the statements of the default
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case are executed.
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switch tag {
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default: s3()
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@ -1187,13 +1212,13 @@ the variable is initialized once before the switch is entered.
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case x < 0: return -x
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default: return x
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}
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Cases do not fall through unless explicitly marked with a 'fallthrough' statement.
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switch a {
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case 1:
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b();
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fallthrough;
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fallthrough
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case 2:
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c();
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}
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@ -1207,43 +1232,36 @@ If the expression is omitted, it is equivalent to 'true'.
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}
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While statements
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A while statement is the usual loop construct.
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WhileStat = 'while' [ SimpleVarDecl ';' ] Expression Block .
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while a < b {
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a++
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}
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A while statement may include the declaration of a single temporary variable.
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The scope of the declared variable extends to the end of the while statement, and
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the variable is initialized once before the loop is entered.
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while x := <ch_ptr; y < x {
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y++
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}
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For statements
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For statements are as in C except the first clause can be a simplified variable
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declaration.
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For statements are a combination of the 'for' and 'while' loops of C.
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ForStat = 'for' [ InitStatement ] ';' [ Condition ] ';' [ Continuation ] Block .
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InitStatement = SimpleVarDecl | Expression .
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ForStat = 'for' [ Condition | ForClause ] Block .
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ForClause = [ InitStat ] ';' [ Condition ] ';' [ PostStat ] .
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InitStat = SimpleStat .
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Condition = Expression .
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Continuation = Expression | IncDecStatement .
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PostStat = SimpleStat .
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A SimpleStat is a simple statement such as an assignemnt, a SimpleVarDecl,
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or an increment or decrement statement. Therefore one may declare a loop
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variable in the init statement.
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for i := 0; i < 10; i++ {
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printf("%d\n", i);
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printf("%d\n", i)
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}
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A 'for' statement with just a condition executes until the condition becomes
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false. Thus it is the same as C 'while' statement.
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for a < b {
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a *= 2
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}
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If the condition is absent, it is equivalent to 'true'.
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for ;; {
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f();
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for {
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f()
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}
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@ -1261,7 +1279,7 @@ to range over the keys of the map; two identifiers range over the keys and corre
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values. For arrays and strings, the behavior is analogous for integer indices (the keys) and
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array elements (the values).
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a := [ 1, 2, 3];
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a := [ 1, 2, 3 ];
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m := [ "fo" : 2, "foo" : 3, "fooo" : 4 ]
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range i := a {
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@ -1275,19 +1293,29 @@ array elements (the values).
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Break statements
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Within a for or while loop a break statement terminates execution of the loop.
|
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[ TODO Do they work in switches? If not - we avoid an ambiguity ]
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Within a 'for' or 'switch' statement, a 'break' statement terminates execution of
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the innermost 'for' or 'switch' statement.
|
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|
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BreakStat = 'break' .
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BreakStat = 'break' [ identifier ].
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If there is an identifier, it must be the label name of an enclosing 'for' or' 'switch'
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statement, and that is the one whose execution terminates.
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|
||||
L: for i < n {
|
||||
switch i {
|
||||
case 5: break L
|
||||
}
|
||||
}
|
||||
|
||||
|
||||
Continue statements
|
||||
|
||||
Within a for or while loop a continue statement begins the next iteration of the
|
||||
loop. Within a while loop, the continue jumps to the condition; within a for loop
|
||||
it jumps to the continuation statement.
|
||||
Within a 'for' loop a continue statement begins the next iteration of the
|
||||
loop at the post statement.
|
||||
|
||||
ContinueStat = 'continue' .
|
||||
ContinueStat = 'continue' [ identifier ].
|
||||
|
||||
The optional identifier is analogous to that of a 'break' statement.
|
||||
|
||||
|
||||
Goto statements
|
||||
@ -1301,12 +1329,13 @@ GotoStat = 'goto' identifier .
|
||||
|
||||
Label statement
|
||||
|
||||
A label statement serves as the target of a goto statement.
|
||||
[ TODO This invention is likely to resolve grammatical problems ]
|
||||
A label statement serves as the target of a 'goto', 'break' or 'continue' statement.
|
||||
|
||||
LabelStat = 'label' identifier ':' .
|
||||
LabelStat = identifier ':' .
|
||||
|
||||
label Error:
|
||||
Error:
|
||||
|
||||
There are various restrictions [TBD] as to where a label statement can be used.
|
||||
|
||||
|
||||
Packages
|
||||
@ -1325,7 +1354,7 @@ A program can access exported items from another package using
|
||||
an import declaration:
|
||||
|
||||
ImportDecl = 'import' [ PackageName ] PackageFileName .
|
||||
PackageFileName = '"' { utf8_char } '"' .
|
||||
PackageFileName = string_lit .
|
||||
|
||||
|
||||
[ TODO complete this section ]
|
||||
|
||||
Loading…
Reference in New Issue
Block a user