Can you have Typeable as an extra constraint? If so:
{-# LANGUAGE ExistentialQuantification #-}
import Data.Typeable
data Baz = forall a. (Eq a, Typeable a) => Baz a
instance Eq Baz where Baz x == Baz y = case cast y of Just y' -> x == y' Nothing -> False
ghci> Baz 4 == Baz 4
True
ghci> Baz 4 == Baz 5
False
ghci> Baz 4 == Baz 'a'
False
On 25 March 2010 15:07, Ozgur Akgun
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
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