Safe::Isa(3pm)

NAME
Safe::Isa - Call isa, can, does and DOES safely on things that may not be objects

SYNOPSIS
  use strict;
  use warnings;

  { package Foo; sub new { bless({}, $_[0]) } }
  { package Bar; our @ISA = qw(Foo); sub bar { 1 } }

  my $foo = Foo->new;
  my $bar = Bar->new;
  my $blam = [ 42 ];

  # basic isa usage -

  $foo->isa('Foo');  # true
  $bar->isa('Foo');  # true
  $blam->isa('Foo'); # BOOM

  $foo->can('bar');  # false
  $bar->can('bar');  # true
  $blam->can('bar'); # BOOM

  # Safe::Isa usage -

  use Safe::Isa;

  $foo->$_isa('Foo');  # true
  $bar->$_isa('Foo');  # true
  $blam->$_isa('Foo'); # false, no boom today

  $foo->$_can('bar');  # false
  $bar->$_can('bar');  # true
  $blam->$_can('bar'); # false, no boom today

Similarly:

  $maybe_an_object->$_does('RoleName'); # true or false, no boom today
  $maybe_an_object->$_DOES('RoleName'); # true or false, no boom today

And just in case we missed a method or two:

  $maybe_an_object->$_call_if_object(name => @args);
  $maybe_an_object->$_call_if_can(name => @args);

Or to re-use a previous example for purposes of explication:

  $foo->$_call_if_object(isa => 'Foo');  # true
  $bar->$_call_if_object(isa => 'Foo');  # true
  $blam->$_call_if_object(isa => 'Foo'); # false, no boom today

DESCRIPTION
How many times have you found yourself writing:

  if ($obj->isa('Something')) {

and then shortly afterwards cursing and changing it to:

  if (Scalar::Util::blessed($obj) and $obj->isa('Something')) {

Right. That's why this module exists.

Since perl allows us to provide a subroutine reference or a method name to the -> operator when used as a method call, and a subroutine doesn't require the invocant to actually be an object, we can create safe versions of isa, can and friends by using a subroutine reference that only tries to call the method if it's used on an object. So:

  my $isa_Foo = $maybe_an_object->$_call_if_object(isa => 'Foo');

is equivalent to

  my $isa_Foo = do {
    if (Scalar::Util::blessed($maybe_an_object)) {
      $maybe_an_object->isa('Foo');
    } else {
      undef;
    }
  };

Note that we don't handle trying class names, because many things are valid class names that you might not want to treat as one (like say "Matt") - the "is_module_name" function from Module::Runtime is a good way to check for something you might be able to call methods on if you want to do that.

We are careful to make sure that scalar/list context is preserved for the method that is eventually called.

EXPORTS

  $_isa
  $maybe_an_object->$_isa('Foo');

If called on an object, calls "isa" on it and returns the result, otherwise returns nothing.

  $_can
  $maybe_an_object->$_can('Foo');

If called on an object, calls "can" on it and returns the result, otherwise returns nothing.

  $_does
  $maybe_an_object->$_does('Foo');

If called on an object, calls "does" on it and returns the result, otherwise returns nothing.

  $_DOES
  $maybe_an_object->$_DOES('Foo');

If called on an object, calls "DOES" on it and returns the result, otherwise returns nothing.

  $_call_if_object
  $maybe_an_object->$_call_if_object(method_name => @args);

If called on an object, calls "method_name" on it and returns the result, otherwise returns nothing.

  $_call_if_can
  $maybe_an_object->$_call_if_can(name => @args);

If called on an object, calls "can" on it; if that returns true, then calls "method_name" on it and returns the result; if any condition is false returns nothing.

SEE ALSO
I gave a lightning talk on this module (and curry and Import::Into) at YAPC::NA 2013 <https://www.youtube.com/watch?v=wFXWV2yY7gE&t=46m05s>.

AUTHOR
mst - Matt S. Trout (cpan:MSTROUT) <mst@shadowcat.co.uk>

CONTRIBUTORS
None yet. Well volunteered? :)

COPYRIGHT
Copyright (c) 2012 the Safe::Isa "AUTHOR" and "CONTRIBUTORS" as listed above.

LICENSE
This library is free software and may be distributed under the same terms as perl itself.