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

SYNOPSIS
    You once wrote:

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

    Now you can write:

     use PerlX::Maybe;
 
     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:

     use PerlX::Maybe;

     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.

    `provided_deref $condition, $r, @rest`
        Like `provided` but dereferences the second argument into list
        context:

         my $bob = Person->new(
                                     name        => $name,
                                     address     => $addr,
           provided length($tel),    phone       => $tel,
           provided $email =~ /\@/,  email       => $email,
           provided_deref $employee, sub {
                                     employee_id => $employee->employee_id,
                               maybe department  => $employee->department,
                                   },
                                     unique_id   => $id,
         );

        The second argument may be a HASH or ARRAY reference. It may also be a
        CODE reference, which will be called in list context. If it is a
        blessed object, it will be treated as if it were a HASH reference
        (internally it could be another type of reference with overloading). A
        code reference can be used if evaluation of the second argument should
        only occur if the condition is met (e.g. to prevent method calls on an
        uninitialised value).

        This function is not exported by default.

    `provided_deref_with_maybe $condition, $r, @rest`
        Like `provide_deref` but will perform `maybe` on each key-value pair
        in the dereferenced values.

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

        Also, if the second argument is a blessed object, it will also skip
        any 'private' attributes (keys starting with an underscore).

        It not only "just works", it "DWIM"s!

        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.

    Currently there are no XS implementations of the `provided_deref` and
    `provided_deref_with_maybe` functions. Contributions welcome.

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

  Exporting
    Only `maybe` is exported by default. You can request other functions by
    name:

      use PerlX::Maybe "maybe", "provided";

    Or to export everything:

      use PerlX::Maybe ":all";

    If Exporter::Tiny is installed, you can rename imports:

      use PerlX::Maybe "maybe" => { -as => "perhaps" };

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>.

    `provided_deref` and `provided_deref_with_maybe` by Theo van Hoesel.

COPYRIGHT AND LICENCE
    This software is copyright (c) 2012-2013, 2018 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.