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.html]

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