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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[Felipe Almeida Lessa]

On Tue, Nov 8, 2011 at 11:49 AM, Anupam Jain <[hidden email]> wrote:
> 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.

That means 'o1' is an existencial variable.

> 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?

That's the only way of showing o1.  Note that you don't need 'Show o2'
constraint on 'TT', so this would suffice:

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

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

Without the inner Show constraint on TT you can't do any showing with
o1.  Since it is an existencial, it could be anything, even things
that don't have Show instances.



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 =).

HTH,

--
Felipe.

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[Tillmann Rendel-5]

Hi,

Anupam Jain wrote:
> -- 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) ⊕ ")"

As you noticed, the last line doesn't work because t1 is of some unknown
type o1, and we know nothing about o1. In particular, we don't know how
to show values of type t1, neither at compile time nor at runtime. I
can't see a way around that.

However, for debugging, you could do:

   show (TT t1 f) = "TT (" ++ show (f t1) ++ ")"

This is not perfect, but at least it should work.

   Tillmann

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[Emil Axelsson-2]

2011-11-08 14:59, Felipe Almeida Lessa skrev:
> On Tue, Nov 8, 2011 at 11:49 AM, Anupam Jain<[hidden email]>  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


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