Hi,
'isBoxed (Carton func)' fails because there is no way whatsoever that any
expression context will disambiguate the x y type variables of func's type.
That is, the type of `isBoxed (Box func)` does not expose x (and y -- the
latter being irrelevant because of the fundep); so there is no way that the
type system could ever figure out which x (and y) was meant; hence overloading
will never ever get resolved (check out the hugs type error, which tells you
exactly that).
One could argue in favor of incoherent choice of instances.
(I don't!)
Say, if you had:
{-# OPTIONS -fallow-undecidable-instances #-}
{-# OPTIONS -fallow-incoherent-instances #-}
instance Func x x -- or some other generic instance
It will work fine:
ghci> isBoxed (Carton func)
HFalse
However, why would you want to wrap such a function anyhow, if you are not
going to use it? :-) Perhaps, this is a case of superficial, say unintended
polymorphism.
If you *really* wanted to box (or to carton) a polymorphic type-class bounded
function, then you may need to do something like this:
data WFunc = WFunc (forall x y. Func x y => (x -> y))
hugs-or-ghci> isBoxed (Carton (WFunc func))
HFalse
This works fine ... but this is probably *not* what you wanted in the first
place. Also, this technique is extremely limiting because now you would need to
fabricate wrappers like WFunc for each new polymorphic expression (when the
constraints or the type differ).
I am keen to hear more about your problem, and to help accordingly.
(Haskell-café?)
Regards,
Ralf
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On
> Behalf Of Jared Warren
> Sent: Wednesday, October 26, 2005 7:01 PM
> To: Haskell Mailing List
> Subject: [Haskell] Instances That Ignore Type Constraints? (HList-related)
>
>
> > data Box x = Box x
> > data Carton x = Carton x
> > class Func x y | x -> y where func :: x -> y
>
> > class IsBoxed f b | f -> b where isBoxed :: f -> b
> > instance IsBoxed (Box x) HTrue where isBoxed _ = hTrue
> > instance IsBoxed (Carton x) HFalse where isBoxed _ = hFalse
>
> `isBoxed (Box func)` fails in GHC with "Ambiguous type variables
> `x', `y' in the constraint: `Func x y' arising from use of `func'...".
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