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spec: clarify evaluation order of "i, x[i] = range ..."

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Part of fix for issue 3464.

R=golang-dev, rsc, mirtchovski, iant, r
CC=golang-dev
https://golang.org/cl/6246045
This commit is contained in:
Robert Griesemer 2012-05-24 10:59:48 -07:00
parent 016d0d0900
commit 2dde4f5d29
1 changed files with 14 additions and 3 deletions
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17
doc/go_spec.html
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@ -1,6 +1,6 @@
<!--{ <!--{
"Title": "The Go Programming Language Specification", "Title": "The Go Programming Language Specification",
"Subtitle": "Version of March 17, 2012", "Subtitle": "Version of May 24, 2012",
"Path": "/ref/spec" "Path": "/ref/spec"
}--> }-->
@ -3866,7 +3866,11 @@ x, _ = f() // ignore second value returned by f()
In the second form, the number of operands on the left must equal the number In the second form, the number of operands on the left must equal the number
of expressions on the right, each of which must be single-valued, and the of expressions on the right, each of which must be single-valued, and the
<i>n</i>th expression on the right is assigned to the <i>n</i>th <i>n</i>th expression on the right is assigned to the <i>n</i>th
operand on the left. The assignment proceeds in two phases. operand on the left.
</p>
<p>
The assignment proceeds in two phases.
First, the operands of <a href="#Indexes">index expressions</a> First, the operands of <a href="#Indexes">index expressions</a>
and <a href="#Address_operators">pointer indirections</a> and <a href="#Address_operators">pointer indirections</a>
(including implicit pointer indirections in <a href="#Selectors">selectors</a>) (including implicit pointer indirections in <a href="#Selectors">selectors</a>)
@ -3885,13 +3889,20 @@ i, x[i] = 1, 2 // set i = 1, x[0] = 2
i = 0 i = 0
x[i], i = 2, 1 // set x[0] = 2, i = 1 x[i], i = 2, 1 // set x[0] = 2, i = 1
x[0], x[0] = 1, 2 // set x[0] = 1, then x[0] = 2 (so x[0] = 2 at end) x[0], x[0] = 1, 2 // set x[0] = 1, then x[0] = 2 (so x[0] == 2 at end)
x[1], x[3] = 4, 5 // set x[1] = 4, then panic setting x[3] = 5. x[1], x[3] = 4, 5 // set x[1] = 4, then panic setting x[3] = 5.
type Point struct { x, y int } type Point struct { x, y int }
var p *Point var p *Point
x[2], p.x = 6, 7 // set x[2] = 6, then panic setting p.x = 7 x[2], p.x = 6, 7 // set x[2] = 6, then panic setting p.x = 7
i = 2
x = []int{3, 5, 7}
for i, x[i] = range x { // set i, x[2] = 0, x[0]
break
}
// after this loop, i == 0 and x == []int{3, 5, 3}
</pre> </pre>
<p> <p>