Hi oleg,
On 4/9/07, oleg@pobox.com
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