NAME

FP::Abstract::Equal - equality protocol

SYNOPSIS

 package FPEqualExample::Foo {
     sub new { my $class= shift; bless [@_], $class }
     sub FP_Equal_equal {
         my ($a, $b)=@_;
         # If you know you've got numbers in here only:
         $$a[0] == $$b[0]
         # For generic values, you would instead:
         #use FP::Equal;
         #equal($$a[0], $$b[0])
     }
 }

 use FP::Equal qw(equal); use FP::List;

 ok equal( list(10,20,30)->map
               (sub{ equal(FPEqualExample::Foo->new(20),
                           FPEqualExample::Foo->new($_[0])) }),
           list('', 1, ''));

DESCRIPTION

Objects implementing this protocol can be compared using the functions from `FP::Equal`, primarily `equal`.

`equal` forces promises before doing further comparisons or passing them to `FP_Equal_equal` (only the immediate layer, not deeply). `FP_Equal_equal` is only ever called with the two arguments (self and one method argument) being references of, currently, the same type (`equal` handles the other cases internally) (TODO: how to handle subtypes?). In better(?) words, `FP_Equal_equal` implementations can rely on the second argument supporting the same operations that the first one does (TODO: even into the future once accepting subtyping? This is *alpha*.) Likewise, `FP_Equal_equal` is not called if the arguments are both the same reference (in this case `equal` simply returns true).

TODO

Handle circular data structures.

SEE ALSO

FP::Equal