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perlsnippets - A collection of Perl idioms or short pieces of Perl code


This document describes version 0.003 of perlsnippets (from Perl distribution perlsnippets), released on 2019-06-04.


This distribution catalogs (in its POD pages) various idioms or short pieces of code that a Perl programmer usually uses. You can also copy-paste them to your code.

The pieces of code are often too small to refactor into modules, or sometimes they have been refactored as modules but you still want to "inline" them to avoid adding a dependency. The pieces of code are often Perl idioms (patterns of code that are specific to Perl programming), but not always.

ARY (Arrays)

ARY/SORT (Arrays / Sorting Arrays)

ARY/SORT/1 (Schwartzian transform)

 my @sorted = map  { $_->[0] }
              sort { $a->[1] <=> $b->[1] }
              map  { [$_, gen_key_for($_)] }

This is the infamous Schwartzian transform named after Randal Schwartz who popularized it during the early days of Perl 5 in 1994. Basically, it's a one-statement way to sort an array using a key. Examples of functions to generate a key: length($_) (to sort by string length), lc($_) (to do case-insensitive sorting).

Related documentation: https://en.wikipedia.org/wiki/Schwartzian_transform

HASH (Hashes)

IO (I/O)

IO/FILE (I/O / File I/O)

IO/FILE/1 (Slurping a file content into a string)

 my $content = do { local $/; <$fh> };

The above snippet slurps the whole file (which has been opened with filehandle $fh) into memory (scalar variable). The do {} block localizes the effect of $/. If you start from a filename and want to open it first:

IO/FILE/2 (Slurping a file content into a string)

 my $content = do { local $/; open my $fh, "<", $filename; <$fh> };

IO/FILE/3 (Slurping a file content into array of strings)

 my @lines = do { local $/; open my $fh, "<", $filename; <$fh> };

Like the previous snippet but you get the content as an array of lines. Each line still has their terminating newlines.

IO/FILE/4 (Slurping a file content into array of strings)

 chomp(my @lines = do { local $/; open my $fh, "<", $filename; <$fh> });

Like the previous snippet, but the lines no longer contain newlines because they are chomp()-ed.

Related modules: File::Slurper.

Related documentation: $/ in perlvar.

MOD (Modules)

MOD/LOAD/1 (Requiring a module by name from variable)

 { (my $mod_pm = "$mod.pm") =~ s!::!/!g; require $mod_pm }

You have a module name in $mod (e.g. "Foo::Bar") and want to load/require it. You cannot just use require $mod because require expects its non-bareware argument to be in the form of "Foo/Bar.pm". So the above snippet converts $mod to that form.

This is safer than eval "use $mod" or eval "require $mod" which work but necessitates you to check that $mod does not contain arbitrary and dangerous code.

Related modules: Module::Load

Related documentation: require in perlfunc.

MOD/LOAD/2 (Requiring a module and importing from it)

 require Foo::Bar; Foo::Bar->import("baz", "qux");

The above snippet loads Foo::Bar module and imports things from the module. It is the run-time equivalent of use Foo::Bar "baz", "qux";. require Foo::Bar; itself is the run-time equivalent of use Foo::Bar ();, i.e. loading a module without importing anything from it.

PROC (Process Management)

PROC/CHLD (Process / Child Process)

Some bit-fiddling and logic is needed to extract exit code from $? ($CHILD_ERROR). Process::Status makes things easier by presenting you with an object that you can query, but if you just want an exit code:

PROC/CHLD/1 (Extract information from $?)

 my ($exit_code, $signal, $core_dump) = ($? < 0 ? $? : $? >> 8, $? & 127, $? & 128);

This snippet extracts all the information contained in $?: exit code (which can be -1 to mean there is no child process being created due to an execution error, e.g. system "non-existent-command"), what signal the child process dies from, and whether the child process dumps core.

PROC/CHLD/2 (Extract exit code from $?)

 my $exit_code = $? < 0 ? $? : $? >> 8.

This snippets just extracts the exit code of child process (which can be -1 to mean that there is no child process being created due to an execution error, e.g. system "non-existent-command").

Related modules: Process::Status, Proc::ChildError.

Related documentation: $? in perlvar.

OBJ (Objects)

REF (References)

SUB (Subroutines)

SUB/ARG (Subroutines / Subroutine Arguments)


Please visit the project's homepage at https://metacpan.org/release/perlsnippets.


Source repository is at https://github.com/perlancar/perl-perlsnippets.


Please report any bugs or feature requests on the bugtracker website https://rt.cpan.org/Public/Dist/Display.html?Name=perlsnippets

When submitting a bug or request, please include a test-file or a patch to an existing test-file that illustrates the bug or desired feature.


Common Perl idioms (2004)

The Idioms subchapter in the Modern Perl book. http://modernperlbooks.com/



perlancar <perlancar@cpan.org>


This software is copyright (c) 2019 by perlancar@cpan.org.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.