On 16/09/05, Tom Hawkins
Hello,
Is it possible to overload (==) to a type other than a -> a -> Bool?
I have an abstract datatype that somewhat behaves like a C integer: a comparison returns a boolean represented as the same datatype:
(==) :: my_type -> my_type -> my_type
Thanks for any help!
-Tom
You largely wouldn't want to -- the thing that makes the usual type for (==) useful is that lots of functions make use of the Bool result with if/then/else. If you change the Eq typeclass, then a large chunk of the standard library is going to break, and it won't be all that easy to correct, because if-then-else-expressions all expect a Bool. You'd want to add a typeclass for boolean-interpretable values, and fix up ghc to use it with if-expressions. Haskell doesn't use ad-hoc polymorphism, so you're out of luck if you want to keep the usual equality test around and have it overlap with your function of another type. Further, there wouldn't be much point to that kind of polymorphism, since functions requiring (==) to give a Boolean result would simply break, and that would be nearly all of them. You can of course give the new kind of comparison another name though. (===) isn't taken, for instance. You can even develop a similar typeclass around it if you like. (Look how Eq is written in the prelude.) It may also be useful to have a function around which projects one of your values down to a Bool. If you want to do that generically, you may also want a class method which does that. Here's an example of the sort of class you might want. class Equative t where (===) :: t -> t -> t isTrue :: t -> Bool isFalse :: t -> Bool -- Minimal complete definition: (===) and one of isTrue or isFalse. isFalse = not . isTrue isTrue = not . isFalse hope this helps, - Cale