[Haskell-cafe] existentially quantified data types - restrictions

[andy morris]

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 <ozgurakgun at gmail.com> wrote:
> 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
>
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