DOES() allows you to test for allomorphism, where objects perform similar duties despite different lineages. isa() allows you to test for inheritance, where objects perform similar duties due to a common lineage.

What's allomorphism? You've probably encountered it already in Perl 5. Consider, for example, a normal hash reference, a blessed hash, a tied hash reference, and a blessed scalar with hash dereferencing overloading. You have a Perl function that receives as an argument an entity which is one of those four items. You want to access a key and value within that entity as if it were a hash reference--but you don't know precisely what it is.

To see if you can dereference the reference safely, you have a couple of options. You could use ref to see if the entity is just a hash reference, but that only works for the first one. You could use UNIVERSAL::isa() to check if the entity seems to be a built-in hash (but that definitely fails for the last item and it's really bad style). You could use reftype() from Scalar::Util, but that definitely won't catch the final item.

Besides that, checking the type is the wrong question--you just want to know if you can dereference the entity as a hash. The real question is "does this entity behave as I expect a hash to behave", not "how does this entity achieve its behavior"?

In a similar way when dealing with objects (and, in Perl 6, built-in data types which act as objects), DOES() asks "Does this object or class perform this role? I don't care how it does it. I just want to know if it does."

See Roles: Composable Units of Object Behavior for more information.

Update: Made the example clearer as an example.

In reply to Re: I don't understand UNIVERSAL::DOES() by chromatic
in thread I don't understand UNIVERSAL::DOES() by rlb3

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