focal (3) sort.3perl.gz

NAME

       sort - perl pragma to control sort() behaviour

SYNOPSIS

           use sort 'stable';          # guarantee stability
           use sort 'defaults';        # revert to default behavior
           no  sort 'stable';          # stability not important

           my $current;
           BEGIN {
               $current = sort::current();     # identify prevailing pragmata
           }

DESCRIPTION

       With the "sort" pragma you can control the behaviour of the builtin "sort()" function.

       A stable sort means that for records that compare equal, the original input ordering is preserved.
       Stability will matter only if elements that compare equal can be distinguished in some other way.  That
       means that simple numerical and lexical sorts do not profit from stability, since equal elements are
       indistinguishable.  However, with a comparison such as

          { substr($a, 0, 3) cmp substr($b, 0, 3) }

       stability might matter because elements that compare equal on the first 3 characters may be distinguished
       based on subsequent characters.

       Whether sorting is stable by default is an accident of implementation that can change (and has changed)
       between Perl versions.  If stability is important, be sure to say so with a

         use sort 'stable';

       The "no sort" pragma doesn't forbid what follows, it just leaves the choice open.  Thus, after

         no sort 'stable';

       sorting may happen to be stable anyway.

CAVEATS

       As of Perl 5.10, this pragma is lexically scoped and takes effect at compile time. In earlier versions
       its effect was global and took effect at run-time; the documentation suggested using "eval()" to change
       the behaviour:

         { eval 'no sort "stable"';      # stability not wanted
           print sort::current . "\n";
           @a = sort @b;
           eval 'use sort "defaults"';   # clean up, for others
         }
         { eval 'use sort qw(defaults stable)';     # force stability
           print sort::current . "\n";
           @c = sort @d;
           eval 'use sort "defaults"';   # clean up, for others
         }

       Such code no longer has the desired effect, for two reasons.  Firstly, the use of "eval()" means that the
       sorting algorithm is not changed until runtime, by which time it's too late to have any effect. Secondly,
       "sort::current" is also called at run-time, when in fact the compile-time value of "sort::current" is the
       one that matters.

       So now this code would be written:

         { no sort "stable";      # stability not wanted
           my $current;
           BEGIN { $current = sort::current; }
           print "$current\n";
           @a = sort @b;
           # Pragmas go out of scope at the end of the block
         }
         { use sort qw(defaults stable);     # force stability
           my $current;
           BEGIN { $current = sort::current; }
           print "$current\n";
           @c = sort @d;
         }