up vote 4 down vote favorite

It sounds trivial but I can't find out what is supposed to do.

following is my type definition:

data CDeq q a = Shallow (q a)
              | Deep{ hq   ⦂ q a
                    , susp ⦂ CDeq q (q a)
                    , tq   ⦂ q a }

and I want it to have an instance of Show.

since GHC doesn't allow me to deriving here, I just try to write myself one:

instance (Show a, Show ????) => Show (CDeq q a) where
    ....

but I got stuck.

I don't know how to represent that for all type v, (q v) can be shown in haskell.

I can't just do following simply:

instance (Show a, Show (q a)) => Show (CDeq q a) where
    ....

since to show CDeq q (q a), Show (q (q a)) is required, then Show (q (q (q a))) is required, then on and on.

So I am wondering is there a syntax such that I can express the meaning I stated up there?

I once thought forall could be the solution to this, but it doesn't work:

instance (Show a, forall v. Show (q v)) => Show (CDeq q a)
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up vote 8 down vote accepted

There's a class Show1 in Prelude.Extras to represent "for all type v, (q v) can be shown".

class Show1 f where
  showsPrec1 :: Show a => Int -> f a -> ShowS
  default showsPrec1 :: (Show (f a), Show a) => Int -> f a -> ShowS
  showsPrec1 = showsPrec
  ...

You can use this to write a show instance for CDeq q a.

instance (Show a, Show1 q) => Show (CDeq q a) where
    ....

Where you would use show or showsPrec on a q x you'll instead use show1 or showsPrec1.

If you use these, you should also provide instances for CDeq q.

instance (Show1 q) => Show1 (CDeq q) where
    showsPrec1 = showsPrec
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1  
does that imply no syntax help or proper solution for that? what should i do if i define instance for another class without such variation sibling? – HuStmpHrrr Aug 7 '15 at 22:13
2  
There's nothing I know of. There's a Data.Constraint.Forall module in the constraints package. There isn't much documentation to go with it; the only person I'm sure would be able to answer whether it can replace classes like Show1 is Edward Kmett. He left a tantalizing hint that it might be possible in this answer to a related question. – Cirdec Aug 7 '15 at 23:04

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