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Revised proposal for const/var cleanup, with
Ken's suggestion for the "iota" extension to tuples. DELTA=171 (88 added, 57 deleted, 26 changed) OCL=20460 CL=20544
This commit is contained in:
Robert Griesemer
parent
a0a14b98fa
commit
7354b864b5
189
doc/go_spec.txt
189
doc/go_spec.txt
@ -4,7 +4,7 @@ The Go Programming Language Specification (DRAFT)
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Robert Griesemer, Rob Pike, Ken Thompson
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----
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(November 17, 2008)
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(December 4, 2008)
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This document is a semi-formal specification of the Go systems
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@ -85,9 +85,6 @@ Open issues:
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with nil
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[ ] consider syntactic notation for composite literals to make them parseable w/o type information
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(require ()'s in control clauses)
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[ ] global var decls: "var a, b, c int = 0, 0, 0" is ok, but "var a, b, c = 0, 0, 0" is not
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(seems inconsistent with "var a = 0", and ":=" notation)
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[ ] const decls: "const a, b = 1, 2" is not allowed - why not? Should be symmetric to vars.
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Decisions in need of integration into the doc:
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@ -98,6 +95,9 @@ Decisions in need of integration into the doc:
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Closed:
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[x] global var decls: "var a, b, c int = 0, 0, 0" is ok, but "var a, b, c = 0, 0, 0" is not
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(seems inconsistent with "var a = 0", and ":=" notation)
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[x] const decls: "const a, b = 1, 2" is not allowed - why not? Should be symmetric to vars.
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[x] new(arraytype, n1, n2): spec only talks about length, not capacity
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(should only use new(arraytype, n) - this will allow later
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extension to multi-dim arrays w/o breaking the language) - documented
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@ -146,6 +146,7 @@ Contents
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Predeclared identifiers
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Exported declarations
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Const declarations
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Iota
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Type declarations
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Variable declarations
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Export declarations
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@ -168,7 +169,6 @@ Contents
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Operands
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Constants
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Qualified identifiers
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Iota
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Composite Literals
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Function Literals
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@ -275,7 +275,6 @@ but is usually defined close to its first use. Productions and code
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examples are indented.
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Source code representation
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----
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@ -661,22 +660,40 @@ A constant declaration binds an identifier to the value of a constant
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expression (§Constant expressions).
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ConstDecl = "const" Decl<ConstSpec> .
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ConstSpec = identifier [ CompleteType ] [ "=" Expression ] .
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ConstSpec = IdentifierList [ CompleteType ] [ "=" ExpressionList ] .
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const pi float = 3.14159265
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IdentifierList = identifier { "," identifier } .
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ExpressionList = Expression { "," Expression } .
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A constant declaration binds a list of identifiers (the names of the constants)
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to the values of a list of constant expressions. The number of identifiers must
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be equal to the number of expressions, with the i'th identifier on the left
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corresponding to the i'th expression on the right. If CompleteType is omitted,
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the types of the constants are the types of the corresponding expressions;
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different expressions may have different types. If CompleteType is present,
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the type of all constants is the type specified, and the types of all
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expressions in ExpressionList must be assignment-compatible with the
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constant type.
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const pi float64 = 3.14159265358979323846
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const e = 2.718281828
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const (
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one int = 1;
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two = 3
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size int64 = 1024;
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eof = -1;
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)
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const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo"
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const u, v float = 0, 3 // u = 0.0, v = 3.0
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The constant expression may be omitted, in which case the expression is
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the last expression used after the reserved word "const". If no such expression
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exists, the constant expression cannot be omitted.
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As a special case, within a parenthesized "const" declaration list the
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ExpressionList may be omitted from any but the first declaration. Such an empty
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ExpressionList is equivalent to the textual substitution of the first preceding
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non-empty ExpressionList in the same "const" declaration list.
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That is, omitting the list of expressions is equivalent to repeating the
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previous list. The number of identifiers must be equal to the number of
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expressions in the previous list.
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Together with the "iota" constant generator (§Iota),
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implicit repetition permits light-weight declaration of enumerated
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values:
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Together with the "iota" constant generator implicit repetition of
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ExpressionLists permit light-weight declaration of enumerated values (§Iota):
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const (
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Sunday = iota;
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@ -688,12 +705,6 @@ values:
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Partyday;
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)
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The initializing expression of a constant may contain only other
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constants. This is illegal:
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var i int = 10;
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const c = i; // error
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The initializing expression for a numeric constant is evaluated
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using the principles described in the section on numeric literals:
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constants are mathematical values given a size only upon assignment
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@ -702,7 +713,7 @@ may require precision significantly larger than any concrete type
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in the language. Thus the following is legal:
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const Huge = 1 << 100;
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var Four int8 = Huge >> 98;
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const Four int8 = Huge >> 98;
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A given numeric constant expression is, however, defined to be
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either an integer or a floating point value, depending on the syntax
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@ -717,18 +728,82 @@ yields a floating point constant of value 2.5 (1.5 + 1); its
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constituent expressions are evaluated using different rules for
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division.
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If the type is specified, the resulting constant has the named type.
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If the type is missing from the constant declaration, the constant
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If the type is missing from a numeric constant declaration, the constant
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represents a value of abitrary precision, either integer or floating
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point, determined by the type of the initializing expression. Such
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a constant may be assigned to any variable that can represent its
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value accurately, regardless of type. For instance, 3 can be
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assigned to any int variable but also to any floating point variable,
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while 1e12 can be assigned to a float32, float64, or even int64.
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It is erroneous to assign a value with a non-zero fractional
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part to an integer, or if the assignment would overflow or
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underflow.
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assigned to any integer variable but also to any floating point variable,
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while 1e12 can be assigned to a "float32", "float64", or even "int64".
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It is erroneous to assign a value with a non-zero fractional part
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to an integer, or if the assignment would overflow or underflow.
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Iota
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----
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Within a constant declaration, the predeclared operand "iota" represents
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successive elements of an integer sequence. It is reset to 0 whenever the
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reserved word "const" appears in the source and increments with each
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semicolon. For instance, "iota" can be used to construct a set of related
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constants:
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const ( // iota is set to 0
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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; // a == 1 (iota has been reset)
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b = 1 << iota; // b == 2
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c = 1 << iota; // c == 4
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)
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const (
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u = iota * 42; // u == 0 (ideal integer)
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v float = iota * 42; // v == 42.0 (float)
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w = iota * 42; // w == 84 (ideal integer)
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)
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const x = iota; // x == 0 (iota has been reset)
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const y = iota; // y == 0 (iota has been reset)
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Within an ExpressionList, the value of all "iota"'s is the same because "iota"
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is only incremented at each semicolon:
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const (
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base0, mask0 int64 = 1 << iota, i << iota - 1; // base0 == 1, mask0 = 0
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base1, mask1 int64 = 1 << iota, i << iota - 1; // base1 == 2, mask1 = 1
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base2, mask2 int64 = 1 << iota, i << iota - 1; // base2 == 4, mask2 = 3
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)
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Since the ExpressionList in constant declarations repeats implicitly
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if omitted, some of the examples above can be abbreviated:
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const (
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enum0 = iota;
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enum1;
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enum2
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)
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const (
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a = 1 << iota;
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b;
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c;
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)
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const (
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u = iota * 42;
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v float;
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w;
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)
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const (
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base0, mask0 int64 = 1 << iota, i << iota - 1;
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base1, mask1 int64;
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base2, mask2 int64;
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)
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Type declarations
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@ -774,16 +849,13 @@ of the variable.
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VarDecl = "var" Decl<VarSpec> .
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VarSpec = IdentifierList ( CompleteType [ "=" ExpressionList ] | "=" ExpressionList ) .
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IdentifierList = identifier { "," identifier } .
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ExpressionList = Expression { "," Expression } .
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var i int
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var u, v, w float
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var k = 0
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var x, y float = -1.0, -2.0
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var (
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i int;
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u, v = 2.0, 3.0
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u, v, s = 2.0, 3.0, "bar"
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)
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If the expression list is present, it must have the same number of elements
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@ -803,13 +875,13 @@ of the variable is "int" or "float" respectively:
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The syntax
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SimpleVarDecl = identifier ":=" Expression .
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SimpleVarDecl = IdentifierList ":=" ExpressionList .
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is shorthand for
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var identifier = Expression.
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"var" ExpressionList = ExpressionList .
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i := 0
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i, j := 0, 10;
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f := func() int { return 7; }
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ch := new(chan int);
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@ -1557,47 +1629,6 @@ TODO(gri) expand this section.
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PackageName = identifier .
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Iota
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----
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Within a declaration, the predeclared operand "iota"
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represents successive elements of an integer sequence.
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It is reset to zero whenever the reserved word "const"
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introduces a new declaration and increments as each identifier
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is declared. 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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Since the expression in constant declarations repeats implicitly
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if omitted, the first two examples above can be abbreviated:
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const (
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enum0 = iota; // sets enum0 to 0, etc.
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enum1;
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enum2
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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; // sets b to 2
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c; // sets c to 4
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)
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Composite Literals
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----
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