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Man Pages


Manual Reference Pages  -  PERLX::MAYBE (3)

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NAME

PerlX::Maybe - return a pair only if they are both defined

CONTENTS

SYNOPSIS

You once wrote:



 my $bob = Person->new(
    defined $name ? (name => $name) : (),
    defined $age ? (age => $age) : (),
 );



Now you can write:



 my $bob = Person->new(
    maybe name => $name,
    maybe age  => $age,
 );



DESCRIPTION

Moose classes (and some other classes) distinguish between an attribute being unset and the attribute being set to undef. Supplying a constructor arguments like this:



 my $bob = Person->new(
    name => $name,
    age => $age,
 );



Will result in the name and age attributes possibly being set to undef (if the corresponding $name and $age variables are not defined), which may violate the Person class’ type constraints.

(Note: if you are the author of the class in question, you can solve this using MooseX::UndefTolerant. However, some of us are stuck using non-UndefTolerant classes written by third parties.)

To ensure that the Person constructor does not try to set a name or age at all when they are undefined, ugly looking code like this is often used:



 my $bob = Person->new(
    defined $name ? (name => $name) : (),
    defined $age ? (age => $age) : (),
 );



or:



 my $bob = Person->new(
    (name => $name) x!!(defined $name),
    (age  => $age)  x!!(defined $age),
 );



A slightly more elegant solution is the maybe function.

    Functions

maybe $x => $y, @rest This function checks that $x and $y are both defined. If they are, it returns them both as a list; otherwise it returns the empty list.

If @rest is provided, it is unconditionally appended to the end of whatever list is returned.

The combination of these behaviours allows the following very sugary syntax to just work.



 my $bob = Person->new(
         name      => $name,
         address   => $addr,
   maybe phone     => $tel,
   maybe email     => $email,
         unique_id => $id,
 );



This function is exported by default.

provided $condition, $x => $y, @rest Like maybe but allows you to use a custom condition expression:



 my $bob = Person->new(
                             name      => $name,
                             address   => $addr,
   provided length($tel),    phone     => $tel,
   provided $email =~ /\@/,  email     => $email,
                             unique_id => $id,
 );



This function is not exported by default.

PerlX::Maybe::IMPLEMENTATION Indicates whether the XS backend PerlX::Maybe::XS was loaded.

    XS Backend

If you install PerlX::Maybe::XS, a faster XS-based implementation will be used instead of the pure Perl functions. My basic benchmarking experiments seem to show this to be around 30% faster.

    Environment

The environment variable PERLX_MAYBE_IMPLEMENTATION may be set to "PP" to prevent the XS backend from loading.

BUGS

Please report any bugs to <http://rt.cpan.org/Dist/Display.html?Queue=PerlX-Maybe>.

SEE ALSO

Syntax::Feature::Maybe, PerlX::Maybe::XS.

MooseX::UndefTolerant, PerlX::Perform, Exporter.

AUTHOR

Toby Inkster <tobyink@cpan.org>.

COPYRIGHT AND LICENCE

This software is copyright (c) 2012-2013 by Toby Inkster.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

DISCLAIMER OF WARRANTIES

THIS PACKAGE IS PROVIDED AS IS AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.
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perl v5.20.3 PERLX::MAYBE (3) 2014-09-23

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