NAME
Data::Predicate
SYNOPSIS
my $predicate = get_a_predicate(); #Say this returns true if an object is defined & a number
$predicate->apply(1); #returns true
$predicate->apply('a'); #returns false
$predicate->apply(undef); #returns false
my @data = (1, 'a', undef, 2, 3);
$predicate->filter(\@data); #returns [1,2,3]
$predicate->filter_transform(\@data, sub { $_*2 }); #returns [2,4,6]
$predicate->all_true([1,2,3]); #Returns true
$predicate->all_false([qw(a b c)]); #Returns true DESCRIPTION
This idea of predicates is taken from "/commons.apache.org/collections/'" in <a href='http:Apache Commons collections</a>> and "/code.google.com/p/google-collections/'" in <a href='http:google-collections</a>>.
Predicates are a way of composing logic so it eventually reports a true/false for a given value. The criteria for coming up with this reponse could be quite complex but always the answer has to be boolean. The predicates can also do filtering of an array reference of items and do transformations on those items when a predicate returns true.
Predicates also allow for more complex and/or statements to be built. For example:
use Data::Predicate::Predicates qw(:all);
my $or_predicate = p_or(map { p_string_equals($_) } qw(a b c d e f) );
my $and_predicate = p_and( p_defined(), $or_predicate );
my $truth = $and_predicate->apply('a');
$truth = $and_predicate->apply('d');
$truth = $and_predicate->apply('f');
This is functionally equivalent to:
my ($v) = @_;
if(defined $v && ( $v eq 'a' || $v eq 'b' || $v eq 'c' || $v eq 'd' || $v eq 'e' || $v eq 'f')) {
}This or code will also begin to shortcut operations in the same way the above if statement would have done.
However one word of warning is that you should not use predicates for matters where simple logic will suffice. Do not use it when doing simple if tests or simple list grepping. Predicates are not a Perlism and will make your code harder to read to the un-initiated.
WHAT DO YOU WANT TO USE
9 times out of 10 you want Data::Predicate::Predicates which provides a set of useful pre-built predicates. If this does not have a predicate to suit your mood then apply this role to a class of your own and start writing.
MOUSE?
Mouse is my post-modern meta-object protocol aware object builder of choice. Moose is better but I program in Mouse. If you want it in Moose then fork & produce a Moose version; it should be functionally equivalent.
METHODS
apply()
The core method; give it a reference and the apply method will return true or false depending on the predicate contents. This is just a Mouse::Role and a required method for anything which uses this Role.
filter()
p_defined()->filter([undef,1,2]); #Will return [1,2]More complex version of the apply which runs over a given array reference of items. The synopsis has a very good example of what it can do.
filter_transform()
p_defined()->filter_transform([undef,1,2], sub { $_*2 }); #Will return [2,4]An evolution of the filter code which when the predicate evaluates to true this code will then trigger a transformaiton process of the value and return a new list filled with these values.
all_true()
Returns true if all values in the given array reference were true. A quick way of deciding to accept a set of values without writing a loop.
all_false()
Returns false if all values in the given array reference were false. A quick way of deciding to reject a set of values without writing a loop.
AUTHOR
Andrew Yates
VERSION
1.0
LICENCE
Copyright (C) 2010 "EBI"
This program was developed as part of work carried out by EMBL.
This program is free software; you can redistribute it and/or modify it under the terms of the Artistic License.
See http://dev.perl.org/licenses/ for more information.