[Haskell-cafe] Re: Type error

[Alfonso Acosta]

Hi oleg,

On 4/9/07, oleg at pobox.com <oleg at pobox.com> wrote:
>
> Alfonso Acosta wrote:
>
> I have a type problem in my code which I dont know how to solve
>
> > data HDSignal a = HDSignal
> > class HDPrimType a where
> > class PortIndex a where
> >
> > class SourcePort s where
> >  -- Plug an external signal to the port
> >  plugSig  :: (HDPrimType a, PortIndex ix) =>ix -> s  ->(HDSignal a -> b) -> b
> >
> > class DestPort d where
> >  -- Supply a signal to the port
> >  supplySig  :: (PortIndex ix, HDPrimType a) => HDSignal a -> ix -> d -> d
> >
> >
> > -- Connect providing indexes
> > connectIx :: (SourcePort s, PortIndex six, DestPort d, PortIndex dix) =>
> >              six -> s -> dix -> d -> d
> > connectIx six s dix d = plugSig six s $ (push2 supplySig) dix d
>
> I'm afraid the example may be a bit too simplified. The first
> question: what is the role of the |HDPrimType a| constraint in plugSig
> and supplySig? Do the implementations of those functions invoke some
> methods of the class HDPrimType?

Yes, I added the |HDPrimType a| constraint beacuse I realized I'm
forced to call a function from that class withoin supplySig . Actually
all the code was working smothly before adding the constraint which
has been te cause of my problem.

> Or the |HDPrimType a| constraint is
> just to declare that the parameter |a| of HDSignal is a member of
> HDPrimType?

Unfortunately not


> If plugSig and supplySig do use the methods of HDPrimType, one could
> use existentials:

I tried the existential approach when it was previously suggested by
Chris, but the problem is that, for some Source instances calling
methods from HDPrimType within supplySig is not enough. Thus, it
doesn't work with existentials due to their limitations.

I'm definitively stuck wit this problem :S

Cheers,

Fons