Re: [Haskell-cafe] Putting constraints on "internal" type variables in GADTs

2011-11-08 14:59, Felipe Almeida Lessa skrev:

On Tue, Nov 8, 2011 at 11:49 AM, Anupam Jain wrote:

...

I can work around this by changing my data type declaration to include Show constraints but I don't want to restrict my data type to only Showable things just so I could have a "Show" instance for debugging -

Only ∷ Show o ⇒ o → T o TT ∷ (Show o1, Show o2) ⇒ T o1 → (o1 → o2) → T o2

What else can I do to declare a Show instance for my datatype?

[...]

I think you may do something more complicated with the new ConstraintKinds extesions, something like

data T c o where Only :: o -> T o TT :: c o1 => T o1 -> (o1 -> o2) -> T o2

instance Show o => Show (T Show o) where ...

This is completely untested. And even if it works, I don't know if it is useful =).

If you don't have the development version of GHC, this can be done without ConstraintKinds using the Sat class available in Syntactic (cabal install syntactic). I attach such a solution where the GADT is defined as follows: data T ctx o where Only :: Sat ctx o => o -> T ctx o TT :: Sat ctx o1 => T ctx o1 -> (o1 -> o2) -> T ctx o2 Whether this solution is too complicated is up to you to decide :) / Emil