/ 
Perl-Critic-Policy-Subroutines-ProhibitTailRecursion-0.001
/ Perl::Critic::Policy::Subroutines::ProhibitTailRecursion

NAME

Perl::Critic::Policy::Subroutines::ProhibitTailRecursion - Do not call a sub from itself: Perl gives every call a stack frame.

VERSION

version 0.001

Perl::Critic::Policy::Subroutines::ProhibitTailRecursion

Perl does not optimize a tail call. Every call makes a new stack frame, the last call of a sub too, so a sub that calls itself uses one frame for each level of its input, and a deep enough input fills the stack:

sub walk {
    my ( $node, @seen ) = @_;
    walk( $_, @seen, $node ) for children($node);    # violates
    return;
}

my $walk = sub {
    __SUB__->($_) for children( $_[0] );             # violates
};

Walk the input with a loop instead. Push what is still to be visited onto an array, and visit it in a c-style for loop, which sees what the loop pushes while it runs. A foreach over the array does not:

my @todo = ($root);
for ( my $i = 0; $i < scalar(@todo); $i++ ) {
    push( @todo, children( $todo[$i] ) );
}

What it reports

Inside a named sub, a call to that sub by its name: walk(...), walk @args, &walk(...), &walk, and Pkg::walk(...) in its own package. It also reports a call of the same name as a method on the invocant of the sub, $self->walk, $class->walk, __PACKAGE__->walk or $self->Pkg::walk, which is how a method recurses. A call anywhere in the body counts, in a loop, in the block of a map or a sort, or in an anonymous sub, because each one adds a frame when it runs.

Anywhere, a call through __SUB__: __SUB__->(...) and &{ __SUB__ }(...).

What it leaves alone

  • goto &walk and goto __SUB__. goto replaces the frame of the sub rather than adding one, so it is the one tail call that Perl makes cheap.

  • \&walk, a reference and not a call.

  • A call to the same name on any other object, and $self->SUPER::walk, which can reach a different sub.

  • A named sub declared inside another. It is a sub of its own, and it is checked on its own.

  • Calls from one sub to another that calls the first. This policy looks at one sub at a time.

CONFIGURATION

There is nothing to configure.

METHODS

What Perl::Critic::Policy asks of a policy, answered here rather than called from anywhere.

supported_parameters

None.

default_severity

Medium: the code works until an input is deep enough, and then it fails.

default_themes

performance.

applies_to

A named sub, for a call to it by name, and a word, for __SUB__.

violates

One violation for each call of the sub to itself.

BUGS

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