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 -> ShowSSwitching 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
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