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Improve operator precedence docs regarding evaluation order

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Associativity has nothing to do with evaluation order and we do not give any guarantees regarding that.

git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@331484 c90b9560-bf6c-de11-be94-00142212c4b1
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Nikita Popov 2013-09-25 11:59:47 +00:00
parent 3ce14fde5c
commit e28fdbe16c
1 changed files with 40 additions and 15 deletions
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55
language/operators.xml
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@ -43,15 +43,29 @@
<literal>18</literal>.
</para>
<para>
When operators have equal precedence, their associativity decides
whether they are evaluated starting from the right, or starting from
the left - see the examples below.
When operators have equal precedence their associativity decides
how the operators are grouped. For example "-" is left-associative, so
<literal>1 - 2 - 3</literal> is grouped as <literal>(1 - 2) - 3</literal>
and evaluates to <literal>-4</literal>. "=" on the other hand is
right-associative, so <literal>$a = $b = $c</literal> is grouped as
<literal>$a = ($b = $c)</literal>.
</para>
<para>
Operators of equal precedence that are non-associative cannot be used
next to each other, for example <literal>1 &lt; 2 &gt; 1</literal> is
illegal in PHP. The expression <literal>1 &lt;= 1 == 1</literal> on the
other hand is legal, because the <literal>==</literal> operator has lesser
precedence than the <literal>&lt;=</literal> operator.
</para>
<para>
Use of parentheses, even when not strictly necessary, can often increase
readability of the code by making grouping explicit rather than relying
on the implicit operator precedence and associativity.
</para>
<para>
The following table lists the operators in order of precedence, with
the highest-precedence ones at the top. Operators on the same line
have equal precedence, in which case associativity decides the order
of evaluation.
have equal precedence, in which case associativity decides grouping.
<table>
<title>Operator Precedence</title>
<tgroup cols="2">
@ -211,14 +225,6 @@
</table>
</para>
<para>
For operators of equal precedence, left associativity means that
evaluation proceeds from left to right, and right associativity means
the opposite. For operators of equal precedence that are non-associative
those operators may not associate with themselves. So for example, the
statement <literal>1 &lt; 2 &gt; 1</literal>, is illegal in PHP. Whereas,
the statement <literal>1 &lt;= 1 == 1</literal> is not, because the
<constant>T_IS_EQUAL</constant> operator has lesser precedence than the
<constant>T_IS_SMALLER_OR_EQUAL</constant> operator.
<example>
<title>Associativity</title>
<programlisting role="php">
@ -239,8 +245,27 @@ echo ++$a + $a++; // may print 4 or 5
]]>
</programlisting>
</example>
Use of parentheses, even when not strictly necessary, can often increase
readability of the code.
</para>
<para>
Operator precedence and associativity only determine how expressions
are grouped, they do not specify an order of evaluation. PHP does not
(in the general case) specify in which order an expression is evaluated
and code that assumes a specific order of evaluation should be avoided,
because it can change between versions of PHP without further notice.
<example>
<title>Undefined order of evaluation</title>
<programlisting role="php">
<![CDATA[
<?php
$a = 1;
echo $a + $a++; // may print either 2 or 3
$i = 1;
$array[$i] = $i++; // may set either index 1 or 2
?>
]]>
</programlisting>
</example>
</para>
<note>
<para>