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

[Anupam Jain]

Hi all,

I was trying to do something very simple with GADTs when I ran into this
problem -

-- My datatype
data T o where
  Only ∷ o → T o
  TT ∷ T o1 → (o1 → o2) → T o2

-- Show instance for debugging
instance Show o ⇒ Show (T o) where
  show (Only o) = "Only " ⊕ (show o)
  show (TT t1 f) = "TT (" ⊕ (show t1) ⊕ ")"


When I try to compile this I get the following -

Could not deduce (Show o1) arising from a use of `show'
    from the context (Show o)



While I understand why I get this error, I have no idea how to fix it! I
cannot put a Show constraint on o1 because that variable is not exposed in
the type of the expression.

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?

Thanks,
Anupam Jain
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