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Go spec: map indexing never raises a runtime exception.

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Also: Actual map key must be assignment-compatible with
formal map key type.

Fixes #357.

R=r, iant, rsc, ken2
CC=golang-dev
https://golang.org/cl/673042
This commit is contained in:
Robert Griesemer 2010-03-23 14:01:51 -07:00
parent a65a56ec1f
commit 29f1ca528b
1 changed files with 28 additions and 25 deletions
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53
doc/go_spec.html
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@ -2394,9 +2394,10 @@ where <code>A</code> is an <a href="#Array_types">array type</a>,
or for <code>a</code> of type <code>S</code> where <code>S</code> is a <a href="#Slice_types">slice type</a>:
</p>
<ul>
<li><code>x</code> must be an integer value and <code>0 &lt;= x &lt; len(a)</code>
<li><code>x</code> must be an integer value and <code>0 &lt;= x &lt; len(a)</code></li>
<li><code>a[x]</code> is the array element at index <code>x</code> and the type of
<code>a[x]</code> is the element type of <code>A</code>
<code>a[x]</code> is the element type of <code>A</code></li>
<li>if the index <code>x</code> is out of range, a run-time exception occurs</li>
</ul>
<p>
@ -2404,10 +2405,11 @@ For <code>a</code> of type <code>T</code>
where <code>T</code> is a <a href="#String_types">string type</a>:
</p>
<ul>
<li><code>x</code> must be an integer value and <code>0 &lt;= x &lt; len(a)</code>
<li><code>x</code> must be an integer value and <code>0 &lt;= x &lt; len(a)</code></li>
<li><code>a[x]</code> is the byte at index <code>x</code> and the type of
<code>a[x]</code> is <code>byte</code>
<code>a[x]</code> is <code>byte</code></li>
<li><code>a[x]</code> may not be assigned to
<li>if the index <code>x</code> is out of range, a run-time exception occurs</li>
</ul>
<p>
@ -2415,38 +2417,38 @@ For <code>a</code> of type <code>M</code>
where <code>M</code> is a <a href="#Map_types">map type</a>:
</p>
<ul>
<li><code>x</code>'s type must be compatible with the key type of <code>M</code>
and the map must contain an entry with key <code>x</code> (but see special forms below)
<li><code>a[x]</code> is the map value with key <code>x</code>
and the type of <code>a[x]</code> is the value type of <code>M</code>
<li><code>x</code>'s type must be
<a href="#Assignment_compatibility">assignment compatible</a>
with the key type of <code>M</code></li>
<li>if the map contains an entry with key <code>x</code>,
<code>a[x]</code> is the map value with key <code>x</code>
and the type of <code>a[x]</code> is the value type of <code>M</code></li>
<li>if the map does not contain such an entry,
<code>a[x]</code> is the <a href="#The_zero_value">zero value</a>
for the value type of <code>M</code></li>
</ul>
<p>
Otherwise <code>a[x]</code> is illegal. If the index or key is out of range evaluating
an otherwise legal index expression, a run-time exception occurs.
Otherwise <code>a[x]</code> is illegal.
</p>
<p>
However, if an index expression on a map <code>a</code> of type <code>map[K] V</code>
is used in an assignment or initialization of the form
An index expression on a map <code>a</code> of type <code>map[K]V</code>
may be used in an assignment or initialization of the special form
</p>
<pre>
r, ok = a[x]
r, ok := a[x]
var r, ok = a[x]
v, ok = a[x]
v, ok := a[x]
var v, ok = a[x]
</pre>
<p>
the result of the index expression is a pair of values with types
<code>(V, bool)</code>.
If the key is present in the map,
the expression returns the pair <code>(a[x], true)</code>;
otherwise it returns <code>(Z, false)</code> where <code>Z</code> is
the <a href="#The_zero_value">zero value</a> for <code>V</code>.
No run-time exception occurs in this case.
The index expression in this construct thus acts like a function call
returning a value and a boolean indicating success. (§<a href="#Assignments">Assignments</a>)
where the result of the index expression is a pair of values with types
<code>(V, bool)</code>. In this form, the value of <code>ok</code> is
<code>true</code> if the key <code>x</code> is present in the map, and
<code>false</code> otherwise. The value of <code>v</code> is the value
<code>a[x]</code> as in the single-result form.
</p>
<p>
@ -2454,7 +2456,7 @@ Similarly, if an assignment to a map has the special form
</p>
<pre>
a[x] = r, ok
a[x] = v, ok
</pre>
<p>
@ -2464,6 +2466,7 @@ the entry for key <code>x</code> is deleted from the map; if
a regular assignment to an element of the map.
</p>
<h3 id="Slices">Slices</h3>
<p>