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go spec: clarify rules for shifts

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Note: This is not a spec change.

The spec was not clear on the result type of
constant shift expressions. Made it more
explicit and added additional examples.

Also: Remove paragraph on send expressions (they
are statements, now).

Fixes #1708.

R=rsc, r, iant, r
CC=golang-dev
https://golang.org/cl/4517074
This commit is contained in:
Robert Griesemer 2011-05-23 14:12:42 -07:00
parent 09dd5bf13b
commit 32d127823f
1 changed files with 36 additions and 31 deletions
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67
doc/go_spec.html
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@ -1,5 +1,5 @@
<!-- title The Go Programming Language Specification -->
<!-- subtitle Version of May 15, 2011 -->
<!-- subtitle Version of May 23, 2011 -->
<!--
TODO
@ -10,7 +10,6 @@ TODO
[ ] clarify what a field name is in struct declarations
(struct{T} vs struct {T T} vs struct {t T})
[ ] need explicit language about the result type of operations
[ ] may want to have some examples for the types of shift operations
[ ] should string(1<<s) and float32(1<<s) be valid?
[ ] should probably write something about evaluation order of statements even
though obvious
@ -2702,42 +2701,34 @@ unary_op = "+" | "-" | "!" | "^" | "*" | "&amp;" | "&lt;-" .
<p>
Comparisons are discussed <a href="#Comparison_operators">elsewhere</a>.
For other binary operators, the operand types must be <a href="#Type_identity">identical</a>
unless the operation involves channels, shifts, or untyped <a href="#Constants">constants</a>.
unless the operation involves shifts or untyped <a href="#Constants">constants</a>.
For operations involving constants only, see the section on
<a href="#Constant_expressions">constant expressions</a>.
</p>
<p>
In a channel send, the first operand is always a channel and the second
must be a value <a href="#Assignability">assignable</a>
to the channel's element type.
</p>
<p>
Except for shift operations,
if one operand is an untyped <a href="#Constants">constant</a>
Except for shift operations, if one operand is an untyped <a href="#Constants">constant</a>
and the other operand is not, the constant is <a href="#Conversions">converted</a>
to the type of the other operand.
</p>
<p>
The right operand in a shift operation must have unsigned integer type
The right operand in a shift expression must have unsigned integer type
or be an untyped constant that can be converted to unsigned integer type.
</p>
<p>
If the left operand of a non-constant shift operation is an untyped constant,
the type of constant is what it would be if the shift operation were replaced by
the left operand alone.
If the left operand of a non-constant shift expression is an untyped constant,
the type of the constant is what it would be if the shift expression were
replaced by its left operand alone.
</p>
<pre>
var s uint = 33
var i = 1&lt;&lt;s // 1 has type int
var j = int32(1&lt;&lt;s) // 1 has type int32; j == 0
var u = uint64(1&lt;&lt;s) // 1 has type uint64; u == 1&lt;&lt;33
var f = float32(1&lt;&lt;s) // illegal: 1 has type float32, cannot shift
var g = float32(1&lt;&lt;33) // legal; 1&lt;&lt;33 is a constant shift operation; g == 1&lt;&lt;33
var i = 1&lt;&lt;s // 1 has type int
var j int32 = 1&lt;&lt;s // 1 has type int32; j == 0
var k = uint64(1&lt;&lt;s) // 1 has type uint64; k == 1&lt;&lt;33
var m int = 1.0&lt;&lt;s // legal: 1.0 has type int
var u = 1.0&lt;&lt;s // illegal: 1.0 has type float64, cannot shift
var v float32 = 1&lt;&lt;s // illegal: 1 has type float32, cannot shift
var w int64 = 1.0&lt;&lt;33 // legal: 1.0&lt;&lt;33 is a constant shift expression
</pre>
<h3 id="Operator_precedence">Operator precedence</h3>
@ -2869,8 +2860,8 @@ be replaced by a bitwise "and" operation:
<p>
The shift operators shift the left operand by the shift count specified by the
right operand. They implement arithmetic shifts if the left operand is a signed
integer and logical shifts if it is an unsigned integer. The shift count must
be an unsigned integer. There is no upper limit on the shift count. Shifts behave
integer and logical shifts if it is an unsigned integer.
There is no upper limit on the shift count. Shifts behave
as if the left operand is shifted <code>n</code> times by 1 for a shift
count of <code>n</code>.
As a result, <code>x &lt;&lt; 1</code> is the same as <code>x*2</code>
@ -3382,21 +3373,35 @@ respectively. Except for shift operations, if the operands of a binary operation
are an untyped integer constant and an untyped floating-point constant,
the integer constant is converted to an untyped floating-point constant
(relevant for <code>/</code> and <code>%</code>).
Similarly,
untyped integer or floating-point constants may be used as operands
Similarly, untyped integer or floating-point constants may be used as operands
wherever it is legal to use an operand of complex type;
the integer or floating point constant is converted to a
complex constant with a zero imaginary part.
</p>
<p>
Applying an operator to untyped constants results in an untyped
A constant <a href="#Comparison_operators">comparison</a> always yields
a constant of type <code>bool</code>. If the left operand of a constant
<a href="#Operators">shift expression</a> is an untyped constant, the
result is an integer constant; otherwise it is a constant of the same
type as the left operand, which must be of integer type
(§<a href="#Arithmetic_operators">Arithmetic operators</a>).
Applying all other operators to untyped constants results in an untyped
constant of the same kind (that is, a boolean, integer, floating-point,
complex, or string constant), except for
<a href="#Comparison_operators">comparison operators</a>, which result in
a constant of type <code>bool</code>.
complex, or string constant).
</p>
<pre>
const a = 2 + 3.0 // a == 5.0 (floating-point constant)
const b = 15 / 4 // b == 3 (integer constant)
const c = 15 / 4.0 // c == 3.75 (floating-point constant)
const d = 1 &lt;&lt; 3.0 // d == 8 (integer constant)
const e = 1.0 &lt;&lt; 3 // e == 8 (integer constant)
const f = int32(1) &lt;&lt; 33 // f == 0 (type int32)
const g = float64(2) &gt;&gt; 1 // illegal (float64(2) is a typed floating-point constant)
const h = "foo" &gt; "bar" // h == true (type bool)
</pre>
<p>
Imaginary literals are untyped complex constants (with zero real part)
and may be combined in binary