[Haskell-cafe] Multiparameter class error

21 Feb 2007

      Hi,

I'm a newbie to multiparameter classes and I'm getting this error from
GHC when compiling the following class definition:

    Could not deduce (Synchronous s f11)
      from the context (Synchronous s f1)
      arising from use of `delaySY'
    Possible fix:
      add (Synchronous s f11) to the class or instance method `sourceSY'
    In the expression: delaySY s0 s
    In the definition of `o': o = delaySY s0 s
    In the definition of `sourceSY':
        sourceSY f s0
                   = o
                   where
                       o = delaySY s0 s
                       s = mapSY f

\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?

Thanks,

Alfonso Acosta