[Haskell-cafe] Possible bug?

[Peter Verswyvelen]

Thanks Ryan.
I'm always struggling with functional dependencies since to be honest - I
don't really understand how the type inferer figures out all the types and I
didn't take the time to study it yet. Your email will help me a bit further
with this.

My functional dependency was c -> m v. It can't be m v -> c since for the
same model and view type , you can have many controllers types.

On Sat, Feb 14, 2009 at 11:38 PM, Ryan Ingram <ryani.spam at gmail.com> wrote:

> 2009/2/13 Peter Verswyvelen <bugfact at gmail.com>:
> > No the error I got was
> > Could not deduce (Controller m v c)
> > from the context (Controller m v c2)
> >       arising from a use of `MVC' at NM8\GUI\PanZoom.hs:126:32-65
> >     Possible fix:
> >       add (Controller m v c) to the context of the constructor `MVC'
> >     In the expression: MVC m v (PZC s z (unsafeCoerce c))
> >     In the definition of `panZoomedMVC'':
> >         panZoomedMVC' s z (MVC m v c) = MVC m v (PZC s z (unsafeCoerce
> c))
> > I got this after adding the type signature of
> > panZoomedMVC' :: (Controller m v c, PanZoomable z) =>
> >                  State -> z -> MVC m v -> MVC m v
>
> No function with the type signature of panZoomedMVC' can be called
> (unless there is a functional dependency that uniquely determines c
> from m and v).  It's ambiguous; there's no way to know which instance
> to call.
>
> GHC allows such a function to get an inferred type, but then when it
> comes time to call it (and provide the Controller instance) or type
> check it against a provided signature, it cannot resolve the ambiguity
> and you get that error.
>
> What is happening in this case is something along these lines:
>
> 1) Infer a type and constraints for panZoomedMVC':
>
> Constraints:
>   Controller t1 t2 t3
>   PanZoomable t4
>
> Type:
>   State -> t4 -> MVC t1 t2 -> MVC t1 t2
>
> 2) Unify the inferred type signature with your provided signature
>
> Constraints:
>   Controller m v t3
>   PanZoomable z
>
> Type:
>    State -> z -> MVC m v -> MVC m v
>
> 3) Verify that constraints are sufficient.  This fails, because the
> use of Controller in the function (Controller m v t3) doesn't match
> the use provided by your constraint (Controller m v c).
>
> However, leaving out the type signature doesn't help you; it just
> delays your problem.  Because of the ambiguity, panZoomedMVC' cannot
> be called; you'll get the error at the callsite instead.
>
> To solve this problem, either add a dummy argument that fixes "c", or
> add a functional dependency or associated type to Controller that
> fixes c based on m and v.  For example:
>
> > data Proxy a = Proxy
> > panZoomedMVC' :: (Controller m v c, PanZoomable z) =>
> >                  Proxy c -> State -> z -> MVC m v -> MVC m v
> > panZoomedMVC' _ s z mvc = ...
>
> Then you can pass the proper "Proxy" when calling the function to make
> the typechecker happy.
>
> or
>
> > class Controller m v c | m v -> c where ...
>
> or
>
> > class Controller m v where
> >    type Control m v
> >    ...
>
>  -- ryan
>
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