sort






sort − perl pragma to control sort() behaviour


    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
    }

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.

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:
















                             ‐2‐


       { 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;
  }