how to make a function that is based on it's return type

gistfile1.hs

-- | how to make a function that is based on its return type?

class (Storable a) => P a where
  data Ps a 
  pStorageType :: [a] -> StorageType
  wP :: [a] -> (Ptr a -> IO b) -> IO b
  wP xs f = V.unsafeWith (V.fromList xs) f

export:: (MonadIO m, Pixel a) => PMagickWand -> Int -> m [a]
exportImagePixels len =  exportArray len ffiFunction undefined
  where
    exportArray :: (Pixel a) => Int -> (StorageType -> Ptr () -> IO b) -> [a] -> IO [a]
    exportArray s f hack = allocaArray s (\q -> f storage (castPtr q) >> peekArray s q)
      where storage = pStorageType hack