[Haskell-cafe] help designing a function

[PICCA Frederic-Emmanuel]

Hello,

I try to write a software, which takes a stack of images and produce a data cube.
Depending on the user input obtain via an .ini file, these cube are computed from different kind of projection.

For now I have only two projection QxQyQzProjection and HklProjection.
I prepare the InputXXX for the processXXX functions from the BinocularConfiguration and a function (InputType -> a)

mkInputQxQyQz :: FramesQxQyQzP a => BinocularsConfig -> (InputType -> a) -> IO (InputQxQyQz a)
processQxQyQz :: FramesQxQyQzP a => InputQxQyQz a -> IO ()

mkInputHkl :: FramesHklP a => BinocularsConfig -> (InputType -> a) -> IO (InputHkl a b)
processHkl :: FramesHklP a => InputHkl a b -> IO ()

now I want to write this kind of function, which use the right Projection depending on the type find in the BinocularsConfig

processProjection :: Show a => BinocularsConfig -> (InputType -> a) -> IO ()
processProjection c' f = do
  print c'
  case  (ptype . bProjection $ c') of
    QxQyQzProjection -> do
      i <- mkInputQxQyQz c' f
      print i
      processQxQyQz i
    HklProjection -> do
      i <- mkInputHkl c' f
      print i
      processHkl i
  return ()

Indeed when I compile this code, I have an Error explaining that a FramesHklP a  constrain is missing from the processProjection function.
and idem for the FramesQxQyQzP. so I can add both constrains, but in that case I need to implement the two interfaces for each type a.

Indeed this is wrong the `a` is for a specific projection thus I can not create an instance for each kind of Projection.

I do not know If I am clear, but I do not know how to organise my code in order to be able to write this processProjection method.

any help would be appreciate.

thanks 

Frederic