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go spec: specify constant conversions

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This is not a language change.

Added paragraphs specifying which conversions
yield results that are constants.

R=r, rsc, iant, ken
CC=golang-dev
https://golang.org/cl/4515176
This commit is contained in:
Robert Griesemer 2011-06-13 16:47:33 -07:00
parent eee70b07c1
commit 2769356dda
1 changed files with 55 additions and 15 deletions
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70
doc/go_spec.html
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@ -528,7 +528,8 @@ A constant value is represented by an
<a href="#Character_literals">character</a>, or
<a href="#String_literals">string</a> literal,
an identifier denoting a constant,
a <a href="#Constant_expressions">constant expression</a>, or
a <a href="#Constant_expressions">constant expression</a>,
a <a href="#Conversions">conversion</a> with a result that is a constant, or
the result value of some built-in functions such as
<code>unsafe.Sizeof</code> applied to any value,
<code>cap</code> or <code>len</code> applied to
@ -3227,8 +3228,42 @@ If the type starts with an operator it must be parenthesized:
</pre>
<p>
A value <code>x</code> can be converted to type <code>T</code> in any
of these cases:
A <a href="#Constants">constant</a> value <code>x</code> can be converted to
type <code>T</code> in any of these cases:
</p>
<ul>
<li>
<code>x</code> is representable by a value of type <code>T</code>.
</li>
<li>
<code>x</code> is an integer constant and <code>T</code> is a
<a href="#String_types">string type</a>.
The same rule as for non-constant <code>x</code> applies in this case
(§<a href="#Conversions_to_and_from_a_string_type">Conversions to and from a string type</a>).
</li>
</ul>
<p>
Converting a constant yields a typed constant as result.
</p>
<pre>
uint(iota) // iota value of type uint
float32(2.718281828) // 2.718281828 of type float32
complex128(1) // 1.0 + 0.0i of type complex128
string('x') // "x" of type string
string(0x266c) // "♬" of type string
MyString("foo" + "bar") // "foobar" of type MyString
string([]byte{'a'}) // not a constant: []byte{'a'} is not a constant
(*int)(nil) // not a constant: nil is not a constant, *int is not a boolean, numeric, or string type
int(1.2) // illegal: 1.2 cannot be represented as an int
string(65.0) // illegal: 65.0 is not an integer constant
</pre>
<p>
A non-constant value <code>x</code> can be converted to type <code>T</code>
in any of these cases:
</p>
<ul>
@ -3262,15 +3297,27 @@ of these cases:
</ul>
<p>
Specific rules apply to conversions between numeric types or to and from
a string type.
Specific rules apply to (non-constant) conversions between numeric types or
to and from a string type.
These conversions may change the representation of <code>x</code>
and incur a run-time cost.
All other conversions only change the type but not the representation
of <code>x</code>.
</p>
<p>
There is no linguistic mechanism to convert between pointers and integers.
The package <a href="#Package_unsafe"><code>unsafe</code></a>
implements this functionality under
restricted circumstances.
</p>
<h4>Conversions between numeric types</h4>
<p>
For the conversion of non-constant numeric values, the following rules apply:
</p>
<ol>
<li>
When converting between integer types, if the value is a signed integer, it is
@ -3296,13 +3343,12 @@ of precision, but <code>float32(x + 0.1)</code> does not.
</ol>
<p>
In all conversions involving floating-point or complex values,
In all non-constant conversions involving floating-point or complex values,
if the result type cannot represent the value the conversion
succeeds but the result value is
implementation-dependent.
succeeds but the result value is implementation-dependent.
</p>
<h4>Conversions to and from a string type</h4>
<h4 id="Conversions_to_and_from_a_string_type">Conversions to and from a string type</h4>
<ol>
<li>
@ -3360,12 +3406,6 @@ If the string is empty, the result is <code>[]int(nil)</code>.
</li>
</ol>
<p>
There is no linguistic mechanism to convert between pointers and integers.
The package <a href="#Package_unsafe"><code>unsafe</code></a>
implements this functionality under
restricted circumstances.
</p>
<h3 id="Constant_expressions">Constant expressions</h3>