/ 
Perl-Critic-Policy-BuiltinFunctions-ProhibitChainingSameListSub-0.001
/ Perl::Critic::Policy::BuiltinFunctions::ProhibitChainingSameListSub

NAME

Perl::Critic::Policy::BuiltinFunctions::ProhibitChainingSameListSub - Walk a list once: do not feed map into map, or grep into grep.

VERSION

version 0.001

Perl::Critic::Policy::BuiltinFunctions::ProhibitChainingSameListSub

A list function whose list is the result of another call to the same function walks the list twice and builds a list in between that nobody keeps:

my @names = map { lc } map { $_->name } @users;          # reported
my @names = map { lc $_->name } @users;                   # one pass

my @live  = grep { !$_->deleted } grep { $_->active } @rows;   # reported
my @live  = grep { $_->active && !$_->deleted } @rows;          # one pass

The two blocks go into one, joined as the function calls for: one expression after the other for map, && for grep.

PROHIBITED

map { f($_) } map { g($_) } @x;
map( { f($_) } map { g($_) } @x );
map f($_), map g($_), @x;
grep { a($_) } grep { b($_) } @x;
sort { $a <=> $b } sort @x;
any { a($_) } any { b($_) } @x;             # List::Util, and its kin
List::Util::first { a($_) } first { b($_) } @x;

ALLOWED

map { f($_) } grep { g($_) } @x;            # two different functions
map { f($_) } @x, map { g($_) } @y;         # the second is one list of several
map { [ map { f($_) } @$_ ] } @rows;        # nested, not chained
$obj->map( sub { ... } )->map( sub { ... } );   # methods, not the builtin

CONFIGURATION

functions

Space separated list of function names to check, which adds to the built-in list rather than replacing it. A name with a package matches only a call that names that package; a name without one matches a call with or without a package.

[BuiltinFunctions::ProhibitChainingSameListSub]
functions = pairmap My::Util::each_item

The built-in list is:

map grep sort first any all none notall

CAVEATS

A map whose inner block returns more than one element per item flattens as it goes, and combining the two blocks means writing that flattening into one. It is still one pass, but when the result reads worse than the chain, say ## no critic (ProhibitChainingSameListSub) and why.

sort chained with sort is reported because the first sort's order is thrown away by the second, whatever the blocks say.

METHODS

supported_parameters

initialize_if_enabled

default_severity

default_themes

applies_to

violates

BUGS

Please report any bugs or feature requests on the bugtracker website https://github.com/teodesian/perl-critic-policy-prohibitchainingsamelistsub/issues

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.

AUTHORS

Current Maintainers:

  • George S. Baugh <george@troglodyne.net>

COPYRIGHT AND LICENSE

Copyright (c) 2026 Troglodyne LLC

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.