[Haskell-cafe] Type error

Alfonso Acosta alfonso.acosta at gmail.com
Fri Apr 6 12:11:13 EDT 2007

Hi all,

I have a type problem in my code which I dont know how to solve (and
I'm not really sure why is caused). I have made a simplified example,
can anyone have a look at it?

Thanks in advance,

The error is:

    Ambiguous type variable `a' in the constraint:
      `HDPrimType a'
        arising from use of `supplySig' at Example.hs:24:47-55
    Probable fix: add a type signature that fixes these type variable(s)

Here is the code

module Example where

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
-- This can seem ugly,
-- it would be easier for us having different types for plugSig and supplySig
-- but the final user would find it much more difficult to deal with
-- supplysig
connectIx six s dix d = plugSig six s $ (push2 supplySig) dix d

push2 :: (a -> b -> c -> d) -> b -> c -> a -> d
push2 f = (\b c a -> f a b c)

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