2011-11-08 14:59, Felipe Almeida Lessa skrev:
On Tue, Nov 8, 2011 at 11:49 AM, Anupam Jain<ajn...@gmail.com> 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
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
import Language.Syntactic
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
-- | Representation of a 'Show' constraint
data ShowCtx
instance Show a => Sat ShowCtx a
where
data Witness ShowCtx a = Show a => ShowWit
witness = ShowWit
show' :: forall a . Sat ShowCtx a => a -> String
show' a = case witness :: Witness ShowCtx a of
ShowWit -> show a
instance Show (T ShowCtx o) where
show (Only o) = "Only " ++ (show' o)
show (TT t1 f) = "TT (" ++ (show' t1) ++ ")"
t :: Sat ctx Int => T ctx Bool
t = TT (Only (3 :: Int)) even
test = show (t :: T ShowCtx Bool)
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