[Haskell-cafe] Type families versus functional dependencies question

Alexey Rodriguez mrchebas at gmail.com
Thu Jul 3 14:56:16 EDT 2008

On Thu, Jul 3, 2008 at 7:31 PM, Claus Reinke <claus.reinke at talk21.com>

> GHC gives the error:
>>   Couldn't match expected type `T f1 f1 a'
>>          against inferred type `T f f a'
>>   In the expression: blah x
>>   In the definition of `wrapper': wrapper x = blah x
> actually, GHC gives me "could not deduce Blah f a from Blah f1 a"
> first. It seems that desugaring type function notation into an additional
> constraint helps, so there's something odd going on:

Silly me, I didn't paste the whole type error. Yes, GHC gives both. I should
add that I tested this under GHC 6.8.2. But this is known not to work with a
(one/two months old) GHC head.



>   class Blah f a where blah :: a -> T f f a
>   class A f where type T f :: (* -> *) -> * -> *
>   wrapper :: forall a f tf . (Blah f a,T f~tf) => a -> tf f a
>   wrapper x = blah x
> You're relying on that second f to determine the first, which
> then allows T f to determine tf f a. Looks a bit like cyclic
> programming at the type level?-) Whereas the desugared
> view is that we may not know the type constructor tf yet,
> but whatever it is, its first parameter fixes f.
> Yet another take on it: tf, the result of T f f a, needs to be
> determined by the context, rather than the type function,
> and type functions are traditionally bad at reasoning backwards. The extra
> indirection separates determining
> f from applying T f.
> I think I'd prefer if that naive desugaring of type function
> always worked, without such differences.
> Worth a ticket?
> Claus
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