Dear Cafe, I need to use a language feature which is explicitly documented to be a restriction, and -even worse- I think I reasonably need to use it. f2 (Baz1 a b) (Baz1 p q) = a==q
It's ok to say a==b or p==q, but a==q is wrong because it equates the two distinct types arising from the two Baz1 constructors. [from *7.4.4.4. Restrictions* at http://www.haskell.org/ghc/docs/latest/html/users_guide/data-type-extensions... ]
To simplify, let's say Baz is the only constructor of a data type, data Baz = forall a. Eq a => Baz a -- | this cannot be done: instance Eq (Baz a) where (Baz x) == (Baz y) = x == y I am quite tempted to use show functions for this equality comparison, but after trying to have a nicely type framework I really don't want to do that. What I simply want is, haskell to be able to compare them if they belong to the same type, and return False otherwise. (not that haskelly way of doing things, I know.) Any suggestions better than the following are very welcome: (==) = (==) `on` show Regards, -- Ozgur Akgun