mirror of
https://github.com/golang/go
synced 2024-11-25 01:57:56 -07:00
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)
|
||||
Robert Griesemer, Rob Pike, Ken Thompson
|
||||
|
||||
----
|
||||
(November 17, 2008)
|
||||
(December 4, 2008)
|
||||
|
||||
|
||||
This document is a semi-formal specification of the Go systems
|
||||
@ -85,9 +85,6 @@ Open issues:
|
||||
with nil
|
||||
[ ] consider syntactic notation for composite literals to make them parseable w/o type information
|
||||
(require ()'s in control clauses)
|
||||
[ ] global var decls: "var a, b, c int = 0, 0, 0" is ok, but "var a, b, c = 0, 0, 0" is not
|
||||
(seems inconsistent with "var a = 0", and ":=" notation)
|
||||
[ ] const decls: "const a, b = 1, 2" is not allowed - why not? Should be symmetric to vars.
|
||||
|
||||
|
||||
Decisions in need of integration into the doc:
|
||||
@ -98,6 +95,9 @@ Decisions in need of integration into the doc:
|
||||
|
||||
|
||||
Closed:
|
||||
[x] global var decls: "var a, b, c int = 0, 0, 0" is ok, but "var a, b, c = 0, 0, 0" is not
|
||||
(seems inconsistent with "var a = 0", and ":=" notation)
|
||||
[x] const decls: "const a, b = 1, 2" is not allowed - why not? Should be symmetric to vars.
|
||||
[x] new(arraytype, n1, n2): spec only talks about length, not capacity
|
||||
(should only use new(arraytype, n) - this will allow later
|
||||
extension to multi-dim arrays w/o breaking the language) - documented
|
||||
@ -146,6 +146,7 @@ Contents
|
||||
Predeclared identifiers
|
||||
Exported declarations
|
||||
Const declarations
|
||||
Iota
|
||||
Type declarations
|
||||
Variable declarations
|
||||
Export declarations
|
||||
@ -168,7 +169,6 @@ Contents
|
||||
Operands
|
||||
Constants
|
||||
Qualified identifiers
|
||||
Iota
|
||||
Composite Literals
|
||||
Function Literals
|
||||
|
||||
@ -275,7 +275,6 @@ but is usually defined close to its first use. Productions and code
|
||||
examples are indented.
|
||||
|
||||
|
||||
|
||||
Source code representation
|
||||
----
|
||||
|
||||
@ -661,22 +660,40 @@ A constant declaration binds an identifier to the value of a constant
|
||||
expression (§Constant expressions).
|
||||
|
||||
ConstDecl = "const" Decl<ConstSpec> .
|
||||
ConstSpec = identifier [ CompleteType ] [ "=" Expression ] .
|
||||
ConstSpec = IdentifierList [ CompleteType ] [ "=" ExpressionList ] .
|
||||
|
||||
const pi float = 3.14159265
|
||||
IdentifierList = identifier { "," identifier } .
|
||||
ExpressionList = Expression { "," Expression } .
|
||||
|
||||
A constant declaration binds a list of identifiers (the names of the constants)
|
||||
to the values of a list of constant expressions. The number of identifiers must
|
||||
be equal to the number of expressions, with the i'th identifier on the left
|
||||
corresponding to the i'th expression on the right. If CompleteType is omitted,
|
||||
the types of the constants are the types of the corresponding expressions;
|
||||
different expressions may have different types. If CompleteType is present,
|
||||
the type of all constants is the type specified, and the types of all
|
||||
expressions in ExpressionList must be assignment-compatible with the
|
||||
constant type.
|
||||
|
||||
const pi float64 = 3.14159265358979323846
|
||||
const e = 2.718281828
|
||||
const (
|
||||
one int = 1;
|
||||
two = 3
|
||||
size int64 = 1024;
|
||||
eof = -1;
|
||||
)
|
||||
const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo"
|
||||
const u, v float = 0, 3 // u = 0.0, v = 3.0
|
||||
|
||||
The constant expression may be omitted, in which case the expression is
|
||||
the last expression used after the reserved word "const". If no such expression
|
||||
exists, the constant expression cannot be omitted.
|
||||
As a special case, within a parenthesized "const" declaration list the
|
||||
ExpressionList may be omitted from any but the first declaration. Such an empty
|
||||
ExpressionList is equivalent to the textual substitution of the first preceding
|
||||
non-empty ExpressionList in the same "const" declaration list.
|
||||
That is, omitting the list of expressions is equivalent to repeating the
|
||||
previous list. The number of identifiers must be equal to the number of
|
||||
expressions in the previous list.
|
||||
|
||||
Together with the "iota" constant generator (§Iota),
|
||||
implicit repetition permits light-weight declaration of enumerated
|
||||
values:
|
||||
Together with the "iota" constant generator implicit repetition of
|
||||
ExpressionLists permit light-weight declaration of enumerated values (§Iota):
|
||||
|
||||
const (
|
||||
Sunday = iota;
|
||||
@ -688,12 +705,6 @@ values:
|
||||
Partyday;
|
||||
)
|
||||
|
||||
The initializing expression of a constant may contain only other
|
||||
constants. This is illegal:
|
||||
|
||||
var i int = 10;
|
||||
const c = i; // error
|
||||
|
||||
The initializing expression for a numeric constant is evaluated
|
||||
using the principles described in the section on numeric literals:
|
||||
constants are mathematical values given a size only upon assignment
|
||||
@ -702,7 +713,7 @@ may require precision significantly larger than any concrete type
|
||||
in the language. Thus the following is legal:
|
||||
|
||||
const Huge = 1 << 100;
|
||||
var Four int8 = Huge >> 98;
|
||||
const Four int8 = Huge >> 98;
|
||||
|
||||
A given numeric constant expression is, however, defined to be
|
||||
either an integer or a floating point value, depending on the syntax
|
||||
@ -717,18 +728,82 @@ yields a floating point constant of value 2.5 (1.5 + 1); its
|
||||
constituent expressions are evaluated using different rules for
|
||||
division.
|
||||
|
||||
If the type is specified, the resulting constant has the named type.
|
||||
|
||||
If the type is missing from the constant declaration, the constant
|
||||
If the type is missing from a numeric constant declaration, the constant
|
||||
represents a value of abitrary precision, either integer or floating
|
||||
point, determined by the type of the initializing expression. Such
|
||||
a constant may be assigned to any variable that can represent its
|
||||
value accurately, regardless of type. For instance, 3 can be
|
||||
assigned to any int variable but also to any floating point variable,
|
||||
while 1e12 can be assigned to a float32, float64, or even int64.
|
||||
It is erroneous to assign a value with a non-zero fractional
|
||||
part to an integer, or if the assignment would overflow or
|
||||
underflow.
|
||||
assigned to any integer variable but also to any floating point variable,
|
||||
while 1e12 can be assigned to a "float32", "float64", or even "int64".
|
||||
It is erroneous to assign a value with a non-zero fractional part
|
||||
to an integer, or if the assignment would overflow or underflow.
|
||||
|
||||
|
||||
Iota
|
||||
----
|
||||
|
||||
Within a constant declaration, the predeclared operand "iota" represents
|
||||
successive elements of an integer sequence. It is reset to 0 whenever the
|
||||
reserved word "const" appears in the source and increments with each
|
||||
semicolon. For instance, "iota" can be used to construct a set of related
|
||||
constants:
|
||||
|
||||
const ( // iota is set to 0
|
||||
enum0 = iota; // sets enum0 to 0, etc.
|
||||
enum1 = iota;
|
||||
enum2 = iota
|
||||
)
|
||||
|
||||
const (
|
||||
a = 1 << iota; // a == 1 (iota has been reset)
|
||||
b = 1 << iota; // b == 2
|
||||
c = 1 << iota; // c == 4
|
||||
)
|
||||
|
||||
const (
|
||||
u = iota * 42; // u == 0 (ideal integer)
|
||||
v float = iota * 42; // v == 42.0 (float)
|
||||
w = iota * 42; // w == 84 (ideal integer)
|
||||
)
|
||||
|
||||
const x = iota; // x == 0 (iota has been reset)
|
||||
const y = iota; // y == 0 (iota has been reset)
|
||||
|
||||
Within an ExpressionList, the value of all "iota"'s is the same because "iota"
|
||||
is only incremented at each semicolon:
|
||||
|
||||
const (
|
||||
base0, mask0 int64 = 1 << iota, i << iota - 1; // base0 == 1, mask0 = 0
|
||||
base1, mask1 int64 = 1 << iota, i << iota - 1; // base1 == 2, mask1 = 1
|
||||
base2, mask2 int64 = 1 << iota, i << iota - 1; // base2 == 4, mask2 = 3
|
||||
)
|
||||
|
||||
Since the ExpressionList in constant declarations repeats implicitly
|
||||
if omitted, some of the examples above can be abbreviated:
|
||||
|
||||
const (
|
||||
enum0 = iota;
|
||||
enum1;
|
||||
enum2
|
||||
)
|
||||
|
||||
const (
|
||||
a = 1 << iota;
|
||||
b;
|
||||
c;
|
||||
)
|
||||
|
||||
const (
|
||||
u = iota * 42;
|
||||
v float;
|
||||
w;
|
||||
)
|
||||
|
||||
const (
|
||||
base0, mask0 int64 = 1 << iota, i << iota - 1;
|
||||
base1, mask1 int64;
|
||||
base2, mask2 int64;
|
||||
)
|
||||
|
||||
|
||||
Type declarations
|
||||
@ -774,16 +849,13 @@ of the variable.
|
||||
VarDecl = "var" Decl<VarSpec> .
|
||||
VarSpec = IdentifierList ( CompleteType [ "=" ExpressionList ] | "=" ExpressionList ) .
|
||||
|
||||
IdentifierList = identifier { "," identifier } .
|
||||
ExpressionList = Expression { "," 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
|
||||
u, v, s = 2.0, 3.0, "bar"
|
||||
)
|
||||
|
||||
If the expression list is present, it must have the same number of elements
|
||||
@ -803,13 +875,13 @@ of the variable is "int" or "float" respectively:
|
||||
|
||||
The syntax
|
||||
|
||||
SimpleVarDecl = identifier ":=" Expression .
|
||||
SimpleVarDecl = IdentifierList ":=" ExpressionList .
|
||||
|
||||
is shorthand for
|
||||
|
||||
var identifier = Expression.
|
||||
"var" ExpressionList = ExpressionList .
|
||||
|
||||
i := 0
|
||||
i, j := 0, 10;
|
||||
f := func() int { return 7; }
|
||||
ch := new(chan int);
|
||||
|
||||
@ -1557,47 +1629,6 @@ TODO(gri) expand this section.
|
||||
PackageName = identifier .
|
||||
|
||||
|
||||
Iota
|
||||
----
|
||||
|
||||
Within a declaration, the predeclared operand "iota"
|
||||
represents successive elements of an integer sequence.
|
||||
It is reset to zero whenever the reserved word "const"
|
||||
introduces a new declaration and increments as each identifier
|
||||
is declared. For instance, "iota" can be used to construct
|
||||
a set of related constants:
|
||||
|
||||
const (
|
||||
enum0 = iota; // sets enum0 to 0, etc.
|
||||
enum1 = iota;
|
||||
enum2 = iota
|
||||
)
|
||||
|
||||
const (
|
||||
a = 1 << iota; // sets a to 1 (iota has been reset)
|
||||
b = 1 << iota; // sets b to 2
|
||||
c = 1 << iota; // sets c to 4
|
||||
)
|
||||
|
||||
const x = iota; // sets x to 0
|
||||
const y = iota; // sets y to 0
|
||||
|
||||
Since the expression in constant declarations repeats implicitly
|
||||
if omitted, the first two examples above can be abbreviated:
|
||||
|
||||
const (
|
||||
enum0 = iota; // sets enum0 to 0, etc.
|
||||
enum1;
|
||||
enum2
|
||||
)
|
||||
|
||||
const (
|
||||
a = 1 << iota; // sets a to 1 (iota has been reset)
|
||||
b; // sets b to 2
|
||||
c; // sets c to 4
|
||||
)
|
||||
|
||||
|
||||
Composite Literals
|
||||
----
|
||||
|
||||
|
||||
Loading…
Reference in New Issue
Block a user