The problem is that an implementation of (===) must return a result of type b for *any* type b which is an instance of Boolean. For example, if I made data X = A | B instance Boolean X where ... then I would be able to have: eqX :: MyEq a => a -> a -> X eqX = (===) That's all fine and probably the reason why you are going through these hoops. However, look at your definition for MyEq Bool: a === b = a == b Now, "a == b" has type Bool. What would happen if I wanted an X instead? You would need something like class Boolean a where (/\) :: a -> a -> a fromBool :: Bool -> a Your instance would then become instance MyEq Bool where a === b = fromBool (a == b) You may want to take a look at the Awesome Prelude [1], which implement this and much more. Cheers! =) [1] https://github.com/tomlokhorst/AwesomePrelude -- Felipe.