[Haskell-cafe] Re: Multiparamater class type-inference error

28 Jan 2007


      Forgot to mention that no error arises if I explicitly give the type
signature of fstSY

fstSY        :: Signal (a,b) -> Signal a


On 1/29/07, Alfonso Acosta  wrote:
...
Hi all,
Sorry if this is too GHC-specific. I'm getting this strange
instantiation error when compiling the following code with GHC 6.6:
No instance for (Synchronous s ((a, b) -> a) (a, b) a)
      arising from use of `mapSY' at SynchronousLib.lhs:342:8-16
    Possible fix:
      add an instance declaration for
      (Synchronous s ((a, b) -> a) (a, b) a)
    In the expression: mapSY fst
    In the definition of `fstSY': fstSY = mapSY fst
---
data Signal a = NullS
              | a :- Signal a deriving (Eq)
class Synchronous s f1 a b | f1 -> a , f1 -> b where
 mapSY          :: f1 -> s  a -> s b
 delaySY        :: a ->  s  a -> s a
instance Synchronous Signal (a->b) a b where
  mapSY _ NullS = NullS
  mapSY f (x:-xs)       = f x :- (mapSY f xs)
  delaySY e es = e:-es
fstSY = mapSY fst
{--
    No instance for (Synchronous s ((a, b) -> a) (a, b) a)
      arising from use of `mapSY' at SynchronousLib.lhs:342:8-16
    Possible fix:
      add an instance declaration for
      (Synchronous s ((a, b) -> a) (a, b) a)
    In the expression: mapSY fst
    In the definition of `fstSY': fstSY = mapSY fst
Failed, modules loaded: AbsentExt, Vector, Queue, Signal. --}
--
No error arises if fstSY is declared as
fstSY a = mapSY fst a
Is it GHC bug or am I supposed to declare it like that to assist type inference?
Thanks in advance,
Alfonso Acosta