Re: [Haskell-cafe] existentially quantified data types - restrictions

I think type witnesses *might* be relevant to your interests. On Thu, Mar 25, 2010 at 11:13 AM, andy morris wrote:

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

...

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

On 25 March 2010 15:07, Ozgur Akgun wrote: 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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-- Alex R