1sort(3pm)              Perl Programmers Reference Guide              sort(3pm)
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NAME

6       sort - perl pragma to control sort() behaviour
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SYNOPSIS

9           use sort 'stable';          # guarantee stability
10           use sort 'defaults';        # revert to default behavior
11           no  sort 'stable';          # stability not important
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13           my $current;
14           BEGIN {
15               $current = sort::current();     # identify prevailing pragmata
16           }
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DESCRIPTION

19       With the "sort" pragma you can control the behaviour of the builtin
20       "sort()" function.
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22       A stable sort means that for records that compare equal, the original
23       input ordering is preserved.  Stability will matter only if elements
24       that compare equal can be distinguished in some other way.  That means
25       that simple numerical and lexical sorts do not profit from stability,
26       since equal elements are indistinguishable.  However, with a comparison
27       such as
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29          { substr($a, 0, 3) cmp substr($b, 0, 3) }
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31       stability might matter because elements that compare equal on the first
32       3 characters may be distinguished based on subsequent characters.
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34       Whether sorting is stable by default is an accident of implementation
35       that can change (and has changed) between Perl versions.  If stability
36       is important, be sure to say so with a
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38         use sort 'stable';
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40       The "no sort" pragma doesn't forbid what follows, it just leaves the
41       choice open.  Thus, after
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43         no sort 'stable';
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45       sorting may happen to be stable anyway.
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CAVEATS

48       As of Perl 5.10, this pragma is lexically scoped and takes effect at
49       compile time. In earlier versions its effect was global and took effect
50       at run-time; the documentation suggested using "eval()" to change the
51       behaviour:
52
53         { eval 'no sort "stable"';      # stability not wanted
54           print sort::current . "\n";
55           @a = sort @b;
56           eval 'use sort "defaults"';   # clean up, for others
57         }
58         { eval 'use sort qw(defaults stable)';     # force stability
59           print sort::current . "\n";
60           @c = sort @d;
61           eval 'use sort "defaults"';   # clean up, for others
62         }
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64       Such code no longer has the desired effect, for two reasons.  Firstly,
65       the use of "eval()" means that the sorting algorithm is not changed
66       until runtime, by which time it's too late to have any effect.
67       Secondly, "sort::current" is also called at run-time, when in fact the
68       compile-time value of "sort::current" is the one that matters.
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70       So now this code would be written:
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72         { no sort "stable";      # stability not wanted
73           my $current;
74           BEGIN { $current = sort::current; }
75           print "$current\n";
76           @a = sort @b;
77           # Pragmas go out of scope at the end of the block
78         }
79         { use sort qw(defaults stable);     # force stability
80           my $current;
81           BEGIN { $current = sort::current; }
82           print "$current\n";
83           @c = sort @d;
84         }
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88perl v5.30.1                      2019-11-29                         sort(3pm)
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