Perl::Critic::Policy::ControlStructures::ProhibitPostfixControls3pm

Affiliation

This Policy is part of the core Perl::Critic distribution.

Overview

Conway discourages using postfix control structures ("if", "for", "unless", "until", "when", "while") because they hide control flow. The "unless" and "until" controls are particularly evil because they lead to double-negatives that are hard to comprehend. The only tolerable usage of a postfix "if"/"when" is when it follows a loop break such as "last", "next", "redo", or "continue".

    do_something() if $condition;           # not ok
    if ($condition) { do_something() }      # ok
    do_something() while $condition;        # not ok
    while ($condition) { do_something() }   # ok
    do_something() unless $condition;       # not ok
    do_something() unless ! $condition;     # really bad
    if (! $condition) { do_something() }    # ok
    do_something() until $condition;        # not ok
    do_something() until ! $condition;      # really bad
    while (! $condition) { do_something() } # ok
    do_something($_) for @list;             # not ok
    LOOP:
    for my $n (0..100) {
        next if $condition;                 # ok
        last LOOP if $other_condition;      # also ok
        next when m< 0 \z >xms;             # fine too
    }

Configuration

A set of constructs to be ignored by this policy can specified by giving a value for 'allow' of a string of space-delimited keywords: "if", "for", "unless", "until", "when", and/or "while". An example of specifying allowed flow-control structures in a .perlcriticrc file:

    [ControlStructures::ProhibitPostfixControls]
    allow = for if until

By default, all postfix control keywords are prohibited.

The set of flow-control functions that are exempt from the restriction can also be configured with the 'flowcontrol' directive in your .perlcriticrc file:

    [ControlStructures::ProhibitPostfixControls]
    flowcontrol = warn die carp croak cluck confess goto exit

This is useful if you're using additional modules that add things like "assert" or "throw".

Notes

The "die", "croak", and "confess" functions are frequently used as flow-controls just like "next" or "last". So this Policy does permit you to use a postfix "if" when the statement begins with one of those functions. It is also pretty common to use "warn", "carp", and "cluck" with a postfix "if", so those are allowed too.

The "when" keyword was added to the language after Perl Best Practices was written. This policy treats "when" the same way it does "if", i.e. it's allowed after flow-control constructs. Thanks to brian d foy for the inspiration <http://www.effectiveperlprogramming.com/blog/543>.