A fundep bug (?)

[anatoli]

This slightly bizarre (meta)code has to do with
the recent Dimension thingy, discussed on
haskell mailing list.

> data Z = Z
> data S a = S a

> z = Z
> sz = S Z
> ssz = S (S Z)

> class Add a b c | a b -> c where add :: a -> b -> c

> instance Add Z a a
> instance Add a b c => Add (S a) b (S c)

> class Mul a b c | a b -> c where mul :: a -> b -> c

> instance Mul Z a Z
> instance (Mul a b c, Add b c d) => Mul (S a) b d

> data Q a b = Q a b

Problem here.  This is the addition of rational
numbers: (a/b) + (c/d) = (ad+bc)/bd

> instance (Mul a d ad,
>           Mul b c bc,
>           Mul b d bd,
>           Add ad bc ad_bc) => Add (Q a b) (Q c d) (Q ad_bc bd)

The problem is, Hugs does not infer a predicate-free type for
say "add (Q sz sz) (Q sz sz)". I think it's a Hugs bug.
I tried both CVS and last stable versions.

When I turn "Explain instance resolution" on, the last thing I
get is:

 entail: () ||- Add (S Z) (S Z) a
  No instance found for Add (S Z) (S Z) a

which is strange because the last parameter of Add 
functionally depends on the rest.
-- 
anatoli (at, not speaking for) ptc (dot) com

__________________________________________________
Do You Yahoo!?
Get email at your own domain with Yahoo! Mail. 
http://personal.mail.yahoo.com/