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Robert Griesemer
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doc/go_lang.txt
214
doc/go_lang.txt
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@ -1,4 +1,5 @@
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The Go Programming Language
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(March 7, 2008)
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This document is an informal specification/proposal for a new systems programming
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language.
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@ -490,21 +491,19 @@ TypeName = QualifiedIdent.
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Array types
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[TODO: this section needs work regarding the precise difference between
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regular and dynamic arrays]
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static, open and dynamic arrays]
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An array is a structured type consisting of a number of elements which
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are all of the same type, called the element type. The number of
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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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are designated by indices which are integers between 0 and the length - 1.
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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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arrays, and dynamic arrays can only be used as parameter types or in a
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pointer type (for instance, a struct may not contain a dynamic array
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field, but only a pointer to an open array).
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an optional array length. If the length is present, it is part of the type.
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Arrays without a length specification are called open arrays.
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Any array may be assigned to an open array variable with the
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same element type. Typically, open arrays are used as
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formal parameters for functions.
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ArrayType = { '[' ArrayLength ']' } ElementType.
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ArrayLength = Expression.
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@ -515,6 +514,11 @@ ElementType = Type.
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[64] struct { x, y: int32; }
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[1000][1000] float64
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The length of an array can be discovered at run time using the
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built-in special function len():
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len(a)
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Array literals
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@ -920,61 +924,38 @@ export directive.
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ExportDecl = 'export' ExportIdentifier { ',' ExportIdentifier } .
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ExportIdentifier = QualifiedIdent .
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export sin, cos
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export Math.abs
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export sin, cos
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export Math.abs
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[ TODO complete this section ]
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Expressions
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Expression syntax is based on that of C.
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Expression syntax is based on that of C but with fewer precedence levels.
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Operand = Literal | Designator | UnaryExpr | '(' Expression ')' | Call.
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UnaryExpr = unary_op Expression
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unary_op = '!' | '-' | '^' | '&' | '<' .
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Designator = QualifiedIdent { Selector }.
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Selector = '.' identifier | '[' Expression [ ':' Expression ] ']'.
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Call = Operand '(' ExpressionList ')'.
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Expression = BinaryExpr | UnaryExpr | PrimaryExpr .
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BinaryExpr = Expression binary_op Expression .
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UnaryExpr = unary_op Expression .
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2
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a[i]
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"hello"
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f("abc")
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p.q.r
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a.m(zot, bar)
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<chan_ptr
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~v
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m["key"]
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(x+y)
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For selectors and function invocations, one level of pointer dereferencing
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is provided automatically. Thus, the expressions
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PrimaryExpr =
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identifier | Literal | '(' Expression ')' | 'iota' |
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Call | Conversion |
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Expression '[' Expression [ ':' Expression ] ']' | Expression '.' identifier .
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(*a)[i]
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(*m)["key"]
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(*s).field
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(*f)()
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can be simplified to
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Call = Expression '(' [ ExpressionList ] ')' .
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Conversion = TypeName '(' [ ExpressionList ] ')' .
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a[i]
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m["key"]
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s.field
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f()
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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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SimpleExpr = Term { add_op Term }.
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Term = Operand { mul_op Operand }.
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relation = '==' | '!=' | '<' | '<=' | '>' | '>='.
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binary_op = log_op | rel_op | add_op | mul_op .
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log_op = '||' | '&&' .
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rel_op = '==' | '!=' | '<' | '<=' | '>' | '>='.
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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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unary_op = '+' | '-' | '!' | '^' | '<' | '>' | '*' | '&' .
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Field selection ('.') binds tightest, followed by indexing ('[]') and then calls and conversions.
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The remaining precedence levels are as follows (in increasing precedence order):
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Precedence Operator
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1 ||
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@ -982,13 +963,8 @@ Precedence Operator
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3 == != < <= > >=
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4 + - | ^
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5 * / % << >> &
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23 + 3*x[i]
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x <= f()
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a >> ~b
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f() || g()
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x == y || <chan_ptr > 0
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6 + - ! ^ < > * & (unary)
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For integer values, / and % satisfy the following relationship:
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(a / b) * b + a % b == a
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@ -997,15 +973,67 @@ 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. 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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cannot be mixed in an expression without explicit conversion.
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Unary '^' corresponds to C '~' (bitwise complement).
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The shift operators implement arithmetic shifts for signed integers,
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and logical shifts for unsigned integers. The property of negative
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shift counts are undefined. Unary '^' corresponds to C '~' (bitwise
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complement).
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There is no '->' operator. Given a pointer p to a struct, one writes
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p.f to access field f of the struct. Similarly. given an array or map pointer, one
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writes p[i], given a function pointer, one writes p() to call the function.
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Other operators behave as in C.
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The 'iota' keyword is discussed in the next section.
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Primary expressions
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x
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2
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(s + ".txt")
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f(3.1415, true)
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Point(1, 2)
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m["foo"]
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s[i : j + 1]
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obj.color
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Math.sin
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f.p[i].x()
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General expressions
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+x
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23 + 3*x[i]
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x <= f()
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^a >> b
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f() || g()
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x == y + 1 && <chan_ptr > 0
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The constant generator 'iota'
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Within a declaration, each appearance of the keyword 'iota' represents a successive
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element of an integer sequence. It is reset to zero whenever the keyword 'const', 'type'
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or 'var' introduces a new declaration. For instance, 'iota' can be used to construct
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a set of related constants:
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const (
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enum0 = iota; // sets enum0 to 0, etc.
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enum1 = iota;
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enum2 = iota
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)
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const (
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a = 1 << iota; // sets a to 1 (iota has been reset)
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b = 1 << iota; // sets b to 2
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c = 1 << iota; // sets c to 4
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)
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const x = iota; // sets x to 0
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const y = iota; // sets y to 0
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Statements
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@ -1014,14 +1042,16 @@ Statements control execution.
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Statement =
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Declaration |
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ExpressionStat | IncDecStat | CompoundStat |
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Assignment |
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SimpleStat | CompoundStat |
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GoStat |
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ReturnStat |
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IfStat | SwitchStat |
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ForStat | RangeStat |
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BreakStat | ContinueStat | GotoStat | LabelStat .
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SimpleStat =
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ExpressionStat | IncDecStat | Assignment | SimpleVarDecl .
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Expression statements
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@ -1055,17 +1085,22 @@ from the declaration to the end of the compound statement.
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Assignments
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Assignment = SingleAssignment | TupleAssignment | Send .
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SimpleAssignment = Designator '=' Expression .
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TupleAssignment = DesignatorList '=' ExpressionList .
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SimpleAssignment = Designator assign_op Expression .
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TupleAssignment = DesignatorList assign_op ExpressionList .
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Send = '>' Expression = Expression .
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assign_op = [ add_op | mul_op ] '=' .
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The designator must be an l-value such as a variable, pointer indirection,
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or an array indexing.
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x = 1
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*p = f()
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a[i] = 23
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As in C, arithmetic binary operators can be combined with assignments:
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j <<= 2
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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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@ -1243,7 +1278,7 @@ InitStat = SimpleStat .
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Condition = Expression .
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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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A SimpleStat is a simple statement such as an assignment, 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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@ -1350,14 +1385,45 @@ PackageClause = 'package' PackageName .
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Import declarations
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A program can access exported items from another package using
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an import declaration:
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A program can gain access to exported items from another package
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through an import declaration:
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ImportDecl = 'import' [ PackageName ] PackageFileName .
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ImportDecl = 'import' [ '.' | PackageName ] PackageFileName .
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PackageFileName = string_lit .
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An import statement makes the exported contents of the named
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package file accessible in this package.
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[ TODO complete this section ]
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In the following discussion, assume we have a package in the
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file "/lib/math", called package Math, which exports functions sin
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and cos.
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In the general form, with an explicit package name, the import
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statement declares that package name as an identifier whose
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contents are the exported elements of the imported package.
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For instance, after
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import M "/lib/math"
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the contents of the package /lib/math can be accessed by
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M.cos, M.sin, etc.
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In its simplest form, with no package name, the import statement
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implicitly uses the imported package name itself as the local
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package name. After
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import "/lib/math"
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the contents are accessible by Math.sin, Math.cos.
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Finally, if instead of a package name the import statement uses
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an explicit period, the contents of the imported package are added
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to the current package. After
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import . "/lib/math"
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the contents are accessible by sin and cos. In this instance, it is
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an error if the import introduces name conflicts.
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Program
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@ -1372,5 +1438,3 @@ Program = PackageClause { ImportDecl } { Declaration } .
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TODO: type switch?
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TODO: select
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TODO: words about slices
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TODO: words about channel ops, tuple returns
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TODO: words about map ops, tuple returns
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