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.