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.IX Title "Perl::Critic::Policy::Variables::ProhibitReusedNames 3pm"
.TH Perl::Critic::Policy::Variables::ProhibitReusedNames 3pm "2018-07-27" "perl v5.26.2" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.SH "NAME"
Perl::Critic::Policy::Variables::ProhibitReusedNames \- Do not reuse a variable name in a lexical scope
.SH "AFFILIATION"
.IX Header "AFFILIATION"
This Policy is part of the core Perl::Critic
distribution.
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
It's really hard on future maintenance programmers if you reuse a
variable name in a lexical scope. The programmer is at risk of
confusing which variable is which. And, worse, the programmer could
accidentally remove the inner declaration, thus silently changing the
meaning of the inner code to use the outer variable.
.PP
.Vb 4
\&    my $x = 1;
\&    for my $i (0 .. 10) {
\&        my $x = $i+1;  # not OK, "$x" reused
\&    }
.Ve
.PP
With \f(CW\*(C`use warnings\*(C'\fR in effect, Perl will warn you if you reuse a
variable name at the same scope level but not within nested scopes.  Like so:
.PP
.Vb 2
\&    % perl \-we \*(Aqmy $x; my $x\*(Aq
\&    "my" variable $x masks earlier declaration in same scope at \-e line 1.
.Ve
.PP
This policy takes that warning to a stricter level.
.SH "CAVEATS"
.IX Header "CAVEATS"
.SS "Crossing subroutines"
.IX Subsection "Crossing subroutines"
This policy looks across subroutine boundaries.  So, the following may
be a false positive for you:
.PP
.Vb 7
\&    sub make_accessor {
\&        my ($self, $fieldname) = @_;
\&        return sub {
\&            my ($self) = @_;  # false positive, $self declared as reused
\&            return $self\->{$fieldname};
\&        }
\&    }
.Ve
.PP
This is intentional, though, because it catches bugs like this:
.PP
.Vb 4
\&    my $debug_mode = 0;
\&    sub set_debug {
\&        my $debug_mode = 1;  # accidental redeclaration
\&    }
.Ve
.PP
I've done this myself several times \*(-- it's a strong habit to put that
\&\*(L"my\*(R" in front of variables at the start of subroutines.
.SS "Performance"
.IX Subsection "Performance"
The current implementation walks the tree over and over.  For a big
file, this can be a huge time sink.  I'm considering rewriting to
search the document just once for variable declarations and cache the
tree walking on that single analysis.
.SH "CONFIGURATION"
.IX Header "CONFIGURATION"
This policy has a single option, \f(CW\*(C`allow\*(C'\fR, which is a list of names to
never count as duplicates.  It defaults to containing \f(CW$self\fR and
\&\f(CW$class\fR.  You add to this by adding something like this to your
\&\fI.perlcriticrc\fR:
.PP
.Vb 2
\&    [Variables::ProhibitReusedNames]
\&    allow = $self $class @blah
.Ve
.SH "AUTHOR"
.IX Header "AUTHOR"
Chris Dolan <cdolan@cpan.org>
.PP
This policy is inspired by
<http://use.perl.org/~jdavidb/journal/37548>.  Java does not allow
you to reuse variable names declared in outer scopes, which I think is
a nice feature.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright (c) 2008\-2013 Chris Dolan
.PP
This program is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.  The full text of this license
can be found in the \s-1LICENSE\s0 file included with this module.