languages/go
languages/go
1
0
Fork 0
mirror of https://github.com/golang/go synced 2024-09-05 16:04:50 +00:00
Code Issues Packages Projects Releases Wiki Activity

spec: rules for index expressions, len, cap, with type parameter types

Browse source 
We want to support some special cases for index expressions, len, and
cap on operands of type parameters (such as indexing a value constrained
by byte slices and strings), hence the extra rules.

Change-Id: I4a07dc7e64bb47361b021d606c52eae1784d5430
Reviewed-on: https://go-review.googlesource.com/c/go/+/366814
Trust: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Dan Scales <danscales@google.com>
This commit is contained in:
Robert Griesemer 2021-11-23 20:24:40 -08:00
parent 37a5d720d4
commit 68da368a4e
1 changed files with 32 additions and 1 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

33
doc/go_spec.html
View file

@ -2269,7 +2269,6 @@ An identifier is exported if both:
All other identifiers are not exported.
</p>
<h3 id="Uniqueness_of_identifiers">Uniqueness of identifiers</h3>
<p>
@ -3715,6 +3714,26 @@ For <code>a</code> of <a href="#Map_types">map type</a> <code>M</code>:
for the element type of <code>M</code></li>
</ul>
<p>
For <code>a</code> of <a href="#Type_parameters">type parameter type</a> <code>P</code>:
</p>
<ul>
<li><code>P</code> must have <a href="#Structure_of_interfaces">specific types</a>.</li>
<li>The index expression <code>a[x]</code> must be valid for values
of all specific types of <code>P</code>.</li>
<li>The element types of all specific types of <code>P</code> must be identical.
In this context, the element type of a string type is <code>byte</code>.</li>
<li>If there is a map type among the specific types of <code>P</code>,
all specific types must be map types, and the respective key types
must be all identical.</li>
<li><code>a[x]</code> is the array, slice, or string element at index <code>x</code>,
or the map element with key <code>x</code> of the type argument
that <code>P</code> is instantiated with, and the type of <code>a[x]</code> is
the type of the (identical) element types.</li>
<li><code>a[x]</code> may not be assigned to if the specific types of <code>P</code>
include string types.
</ul>
<p>
Otherwise <code>a[x]</code> is illegal.
</p>
@ -6468,12 +6487,24 @@ len(s) string type string length in bytes
[]T slice length
map[K]T map length (number of defined keys)
chan T number of elements queued in channel buffer
type parameter see below
cap(s) [n]T, *[n]T array length (== n)
[]T slice capacity
chan T channel buffer capacity
type parameter see below
</pre>
<p>
If the argument type is a <a href="#Type_parameters">type parameter</a> <code>P</code>,
<code>P</code> must have <a href="#Structure of interfaces">specific types</a>, and
the call <code>len(e)</code> (or <code>cap(e)</code> respectively) must be valid for
each specific type of <code>P</code>.
The result is the length (or capacity, respectively) of the argument whose type
corresponds to the type argument with which <code>P</code> was
<a href="#Instantiations">instantiated</a>.
</p>
<p>
The capacity of a slice is the number of elements for which there is
space allocated in the underlying array.