[Haskell-cafe] Multiparameter class error

[Alfonso Acosta]

Thanks, the functional dependency solved the problem

On 2/21/07, Yitzchak Gale <gale at sefer.org> wrote:
> Hi Alfonso,
>
> You wrote:
> >     Could not deduce (Synchronous s f11)
> >       from the context (Synchronous s f1)
> > \begin{code}
> > class Synchronous s f1  where
> >  mapSY          :: f1 a b   -> s a -> s b
> >  delaySY         :: a        -> s a -> s a
> >  sourceSY       :: f1 a a -> a    -> s a
> >  sourceSY f s0 = o
> >   where
> >      o                  = delaySY s0 s
> >      s                  = mapSY f o
> > \end{code}
> >
> > Can anyone explain a bit further than GHC what am I doing wrong?
>
> Every method of a multiparameter class must refer
> to every parameter in its signature. Otherwise, there
> is no way for the compiler to know which instance
> of the class you want when you use the method.
>
> There are two ways to get around this restriction.
> One is to use a functional dependency:
>
> class Synchronous s f1 | s -> f1 where
>
> That promises that for each type s, you will
> only define an instance Synchronous s f1
> for at most a single type f1. Now, whenever
> you mention s in a type signature, it is as if
> you also mentioned f1.
>
> If you can't keep that promise, then you will
> have to use a phantom parameter. Change
> the type of delaySY to
>
> delaySY         :: f1 -> a -> s a -> s a
>
> and ignore the f1 parameter when you implement
> delaySY:
>
> delaySY _ x y = ...
>
> Then, when you use delaySY, you specify the
> type f1 by writing:
>
> delaySY (undefined :: T) ...
>
> Regards,
> Yitz
>