incompatible signatur syntax within instance definition

[Christian Maeder]

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
-------------- next part --------------
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, nhc98  
		  showsl (Cons x Nil) = shows x
		  showsl (Cons x xs) = 
		      shows x . showString "," . showsl xs
		  -- undefined for Nil