Re: GADT + Newtype deriving = Erroneous errors

Tomasz Zielonka wrote:

On Mon, Mar 26, 2007 at 09:18:34PM -0700, Stefan O'Rear wrote:

...

stefan@stefans:/tmp$ cat A.lhs

...

{-# OPTIONS_GHC -fglasgow-exts #-}

data IsIntT x where IsIntT :: IsIntT Int

class IsIntC a where isInt :: IsIntT a instance IsIntC Int where isInt = IsIntT

newtype Foo = Foo Int deriving(IsIntC)

I think newtype deriving should be rejected in this case. Maybe this is the real problem here?

Best regards Tomek

On reflection, I agree. The derived instance would have to be

instance IsIntC Foo where isInt :: IsIntT Foo isInt = isIntT

which cannot type check. Only

instance IsIntC Foo where isInt :: IsIntT Foo isInt = undefined

is possible.