functional dependencies

[Ashley Yakeley]

At 2002-07-23 09:06, Simon Peyton-Jones wrote:

>Dead right!  Imagine there was a method in class D:
>
>	class C a b => D a where
>	   op :: a -> b
>
>The type of 'op' is
>
>	op :: D a => a -> b
>
>You can't really expect that the 'b' here is determined by 'a'!

Agreed. If you were going to allow "D a", they would have to be 
considered different "b"s.

>Still, I suppose that if the methods in class D mention 
>only 'a', then it is strange to require D to be parameterised over
>'b'.  Thus, this seems more reasonable:
>
>	class C a b => D a where
>	  op :: a -> a
>
>Even this seems odd, because we would get the deduction
>
>	D a |- C a b
>
>(i.e. from a (D a) dictionary you can get a (C a b) dictionary)
>and that's only true for a particular 'b'. 

Well that's OK. From a (D a) dictionary, you get a (C a b) dictionary, 
where you're given "a", and you can find "b" from the fundep.

-- 
Ashley Yakeley, Seattle WA