[Haskell-cafe] Multi-parameter type class woes

[Alexander Dunlap]

On Sun, Dec 14, 2008 at 8:10 PM, Mario Blažević <mblazevic at stilo.com> wrote:
>> I'll take a swing at this one:
>>
>> instance Container (Maybe x) [x] where
>> wrapper = isNothing
>> . . .
>>
>> That isn't a sensible definition of 'wrapper', but I believe without
>> trying to compile it is completely legal.  Which wrapper do you use?
>>
>> You /don't/ have a different matching Container instance, but without the
>> functional dependency you /might/, and ghc barfs.
>
>
>    But liftWrap doesn't require any particular instance, it's a
> generic function accepting any pair of types for which there is
> an instance of Container. Instance selection (as I understand it)
> shouldn't come into play until one applies liftWrap to a
> particular type, and indeed it does cause problems there: note
> the type annotations on the last line. That part I understand
> and accept, or at least have learned to live with.

The problem is that y is not mentioned in the signature of wrapper.
When you call wrapper x, there could be many different instances of
Container x y with the same x, so GHC doesn't know which version to
call. You can fix this problem either by adding a functional
dependency or by splitting wrapper out into its own class (Wrapper x,
e.g.) so all of the type variables in the class head are mentioned in
its type and the instance can be determined by the call.

Thanks for asking this question, by the way. I had known about this
issue but had never really realized why it happened. Now that I have
thought about it, I understand it too. :)

Hope that helps,
Alex