NAME
perlsnippets - A collection of Perl idioms or short pieces of Perl code
VERSION
This document describes version 0.004 of perlsnippets (from Perl distribution perlsnippets), released on 2019-06-06.
DESCRIPTION
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.
Keywords: idiom, snippet, pattern
ARY (Arrays)
ARY/PICK (Arrays / Picking Array Elements)
ARY/PICK/1 (Check whether a value exists in an array, without any module)
my $foo_exists = grep { $_ eq 'foo' } @ary;Another alternative is using smartmatch:
my $foo_exists = 'foo' ~~ @ary;but smartmatch is now in experimental status and might be removed in the future. Plus the above is not exactly equivalent to the snippet and might be "too smart" for your needs (read perlop for more details).
Another alternative is using List::Util's first:
my $foo_exists = first { $_ eq 'foo' } @ary;The benefit of the above alternative is that it short-circuits (finishes after it finds the first match) while grep evaluates the whole array even though it already finds a match (although the speed difference might not matter except for big arrays). But this alternative requires a module (albeit a core one). And beware of traps like:
my $foo_exists = first { $_ == 0 } @ary; # wil return false even though array contains 0ARY/PICK/2 (Check whether a value exists in an array, without any module, with short-circuiting)
my $foo_exists; $_ eq 'foo' ? ($foo_exists++, last) : 0 for @ary;
my $foo_exists = do { my $res; $_ eq 'foo' ? ($res++, last) : 0 for @ary; $res };Since grep is not short-circuiting, to short-circuit we need to employ a loop (e.g. for + last, while). This snippet offers a few styled variations. Some require multiple statements (declare variable + set it) while other offer single statement but requires a do {} block and double declaration. All are ugly enough that you might as well use first() for clarity.
ARY/PICK/3 (Pick a random element from array)
my $elem = $ary[rand @ary];If you need to random-pick multiple elements from an array, you can use samples from List::MoreUtils. If you cannot use a module for some reason, you can copy-paste the implementation from List::MoreUtils::PP.
ARY/SORT (Arrays / Sorting Arrays)
ARY/SORT/1 (Schwartzian transform)
my @sorted = map { $_->[0] }
sort { $a->[1] <=> $b->[1] }
map { [$_, gen_key_for($_)] }
@ary;This is the infamous Schwartzian transform named after Randal Schwartz who popularized it during the early days of Perl 5 in 1994. Basically, it's a one-statement way to sort an array using a key. Examples of functions to generate a key: length($_) (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 (Slurp 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 (Slurp a file content into a string)
my $content = do { local $/; open my $fh, "<", $filename; <$fh> };IO/FILE/3 (Slurp 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 (Slurp 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 (Require 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 (Require 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: $? in perlvar.
OBJ (Objects)
REF (References)
SUB (Subroutines)
SUB/ARG (Subroutines / Subroutine Arguments)
SUB/ARG/1 (Assign hash arguments)
my %args = @_;SUB/ARG/2 (Assign hash arguments in method)
my ($self, %args) = @_;SUB/ARG/3 (Assign hash arguments in method, shift $self)
my ($self, %args) = (shift, @_);This variant shifts $self from @_ first, so you can do this later:
$self->yet_other_method(...);
$self->SUPER::method(@_);HOMEPAGE
Please visit the project's homepage at https://metacpan.org/release/perlsnippets.
SOURCE
Source repository is at https://github.com/perlancar/perl-perlsnippets.
BUGS
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.
SEE ALSO
The Idioms subchapter in the Modern Perl book. http://modernperlbooks.com/
Perl Cookbook, 2nd edition. http://shop.oreilly.com/product/9780596003135.do
AUTHOR
perlancar <perlancar@cpan.org>
COPYRIGHT AND LICENSE
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.