Problem with functional dependencies
Daniel Gorín
dgorin at dc.uba.ar
Fri Nov 16 19:26:17 EST 2007
Hi, Chris
Thanks for your answer. I guess that my intuitions of what functional
dependencies and context meant were not very accurate (see below)
>>> class C m f n | m -> n, f -> n where
>>> c :: m -> f -> Bool
>
> The "m->n" functional dependency means that I tell you
> "C x _ z" is an instance then you whenever you match "x" that you
> must have the corresponding "z".
That's what I thought..
>>>
>>> instance C (M n) (F n) n where
>>> c _ _ = True
>
> This promises that "C x _ z" with x=="M n" has z==n
I agree....
>>> instance C m (F N) N => C m F' N where
>>> c m (F' f) = c m f
>
> By the "m->n" functional dependency, the above implies that _any_
> "m" must map
> to the type M2.N: "m -> M2.N"
>
> This kills you in M3...
Here I was expecting the context "C m (F N) N" to work as a logical
guard, something like:
'for all m such that "C m (F N) N" holds, "C m F' N" must hold too'
and since '"C m (F N) N" holds' would already imply 'm -> N', then "C
m F' N" would not produce any contradiction.
I guess this view doesn't hold when FlexibleInstances is on....
Anyway, it makes (kind of) sense now...
> By the way, if you make the class C fundep declaration into:
>
>> class C m f n | m f -> n where
>
> then it compiles. This means ((M n) and (F n) imply N) and ("any
> m" and F'
> imply N') which no longer conflict.
Thanks again for the tip, I will try it out!
Daniel
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