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clarify conversions vs type guards.

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allow conversions between equal types.

R=r
DELTA=15  (4 added, 4 deleted, 7 changed)
OCL=25618
CL=25630
This commit is contained in:
Russ Cox 2009-03-03 08:10:25 -08:00
parent d169268a1a
commit e8b43190bb
1 changed files with 11 additions and 11 deletions
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22
doc/go_spec.html
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@ -3609,32 +3609,36 @@ T(value)
</pre>
<p>
where <code>T</code> is the type name of an arithmetic type or string (§Basic types),
and <code>value</code> is the value of an expression that can be converted to a value
where <code>T</code> is a type
and <code>value</code> is an expression
that can be converted to a value
of result type <code>T</code>.
<p>
The following conversion rules apply:
</p>
<ul>
<li>
1) Between integer types. If the value is a signed quantity, it is
1) Between equal types. The conversion always succeeds.
</li>
<li>
2) Between integer types. If the value is a signed quantity, it is
sign extended to implicit infinite precision; otherwise it is zero
extended. It is then truncated to fit in the result type size.
For example, <code>uint32(int8(0xFF))</code> is <code>0xFFFFFFFF</code>.
The conversion always yields a valid value; there is no signal for overflow.
</li>
<li>
2) Between integer and floating point types, or between floating point
3) Between integer and floating point types, or between floating point
types. To avoid overdefining the properties of the conversion, for
now it is defined as a ``best effort'' conversion. The conversion
always succeeds but the value may be a NaN or other problematic
result. <font color=red>TODO: clarify?</font>
</li>
<li>
3) Strings permit two special conversions.
4) Strings permit two special conversions.
</li>
<li>
3a) Converting an integer value yields a string containing the UTF-8
4a) Converting an integer value yields a string containing the UTF-8
representation of the integer.
(TODO: this one could be done just as well by a library.)
@ -3644,7 +3648,7 @@ string(0x65e5) // "\u65e5"
</li>
<li>
3b) Converting an array or slice of bytes yields a string whose successive
4b) Converting an array or slice of bytes yields a string whose successive
bytes are those of the array/slice.
<pre>
@ -3658,10 +3662,6 @@ There is no linguistic mechanism to convert between pointers and integers.
The <code>unsafe</code> package
implements this functionality under
restricted circumstances (§Package <code>unsafe</code>).
<font color=red>
TODO: Do we allow interface/ptr conversions in this form or do they
have to be written as type guards? (§Type guards)
</font>
</p>