anatoli wrote:

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

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data Z = Z data S a = S a

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

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

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

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

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

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data Q a b = Q a b

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

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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

This looks like a bug that I've been aware of for a while, where context reduction isn't getting iterated enough to rattle out all of the functional dependencies. I think this has an easy fix, and will take a look at it at some point. --Jeff