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
}
Perl Programmers Reference Guide sort(3)
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;
}
ATTRIBUTES
See attributes(7) for descriptions of the following attributes:
+---------------+-----------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+-----------------------+
|Availability | runtime/perl-532 |
+---------------+-----------------------+
|Stability | Pass-through volatile |
+---------------+-----------------------+
NOTES
Source code for open source software components in Oracle Solaris can
be found at https://www.oracle.com/downloads/opensource/solaris-source-
code-downloads.html.
This software was built from source available at
https://github.com/oracle/solaris-userland. The original community
source was downloaded from
http://www.cpan.org/src/5.0/perl-5.32.0.tar.gz.
Further information about this software can be found on the open source
community website at https://www.perl.org/.
perl v5.32.0 2020-06-14 sort(3)