incompatible signatur syntax within instance definition

[Christian Maeder]

I've just noticed that I used ghc with -fglasgow-exts.
Without extensions hugs, ghc und nhc98 consistently need the constraint
in the type signature (below)
	showsl :: Show a => List a -> ShowS

Switching the extensions on, breaks this code, however (ghc only).
Christian

I wrote:
> Hi,
> 
> if I try to supply a signatur for the local function "showsl" below, 
> then ghc rejects a constraint (Show a) whereas hugs (and nhc98) needs 
> this constraint.
> 
> What should be the correct notation? (apart from omitting any signature)
> 
> Cheers Christian
> 
> (BTW, I would appreciate if the (instantiated) signature of a class 
> function could be given/repeated in an instance, but that's another 
> matter.)
> 
> 
> hugs error message for "showsl :: List a -> ShowS"
> ERROR "MyList.hs":11 - Cannot justify constraints in explicitly typed 
> binding
> *** Expression    : showsl
> *** Type          : List a -> ShowS
> *** Given context : ()
> *** Constraints   : Show a
> 
> ghc error message for "showsl :: Show a => List a -> ShowS"
> ...../MyList.hs:10:
>     All of the type variables in the constraint `Show a' are already in 
> scope
>     (at least one must be universally quantified here)
>     In the type: (Show a) => List a -> String -> String
>     While checking the type signature for `showsl'
>     In the definition of `showsPrec':
>     showsPrec _ l
>             = (showString "[") . ((showsl l) . (showString "]"))
>             where
>             showsl :: forall. (Show a) => List a -> ShowS
>             showsl (Cons x Nil) = shows x
>             showsl (Cons x xs) = (shows x) . ((showString ",") . (showsl 
> xs))
>     In the definition for method `showsPrec'
> Failed, modules loaded: none.
> 
> 
> module MyList where
> 
> data List a = Nil | Cons a (List a)
> 
> instance Show a => Show (List a) where
>     showsPrec _  Nil = showString "[]"
>     showsPrec _ l =
>     showString "[" . showsl l . showString "]"
>         where -- showsl :: List a -> ShowS            -- for ghc
>           -- showsl :: Show a => List a -> ShowS  -- for hugs
>           showsl (Cons x Nil) = shows x
>           showsl (Cons x xs) =
>               shows x . showString "," . showsl xs
>           -- undefined for Nil