Is it time to start deprecating FunDeps?

oleg at oleg at
Thu May 2 09:14:25 CEST 2013

> In your class Sum example,
>         class Sum x y z | x y -> z, x z -> y
> your own solution has a bunch of helper classes

First of all, on the top of other issues, I haven't actually shown an
implementation in the message on Haskell'. I posed this as a general

In special cases like below

    > class Sum2 a b c | a b -> c, a c -> b
    > instance Sum2 Z b b
    > instance (Sum2 a' b c') => Sum2 (S a') b (S c')

    > -- note that in the FunDeps, var a is not a target
    > -- so the instances discriminate on var a
I didn't doubt the translation would go through because there is a
case analysis on a. But the general case can be more complex. For

class Sum2 a b c | a b -> c, a c -> b
instance Sum2 Z Z Z
instance Sum2 O Z O
instance Sum2 Z O O
instance Sum2 O O Z

> In your overlap example you introduce a definition that won't compile!
> >
> > {- -- correctly prohibited!
> > instance x ~ Bool => C1 [Char]  x where
> >     foo = const True
> > -}
> You expect too much if you think a mechanical translation will 'magic' a
> non-compiling program into something that will compile.

> I do expect equality constraints to not play well with overlaps. Combining
> FunDeps with overlaps is also hazardous. I'm only claiming that EC's will
> be at least as good.

I don't understand the remark. The code marked `correctly prohibited'
is in the comments. There are no claims were made about that code. If
you found that comment disturbing, please delete it. It won't affect the
the example: the types were improved in t11 but were not
improved in t21. Therefore, EC's are not just as good. Functional
dependencies seem to do better.

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