[Haskell-beginners] What is wrong with my types signatures.

[PICCA Frederic-Emmanuel]

Hello here a snipset code

class Shape sh => FrameND t sh where
  shapeND :: t -> MaybeT IO sh
  rowND :: t -> sh -> MaybeT IO (XrdMeshFrame sh)

I have an instance of FrameND

then I created a function which use this class

framesND :: (Shape sh, FrameND a sh) => Pipe a (XrdMeshFrame sh) IO ()
framesND = do
  d <- await
  sh' <- lift $ runMaybeT $ shapeND d
  let n = size sh'
  forM_ [0..n-1] (\i' -> do
                     f <- lift $ runMaybeT $ rowND d (fromIndex sh' i')
                     when (isJust f) (yield (fromJust f)))

But when I compile it; I get this error message


src/Hkl/XRD.hs:613:55:
    Could not deduce (sh ~ Maybe a0)
    from the context (Shape sh, FrameND a sh)
      bound by the type signature for
                 framesND :: (Shape sh, FrameND a sh) =>
                             Pipe a (XrdMeshFrame sh) IO ()
      at src/Hkl/XRD.hs:606:13-70
      `sh' is a rigid type variable bound by
           the type signature for
             framesND :: (Shape sh, FrameND a sh) =>
                         Pipe a (XrdMeshFrame sh) IO ()
           at src/Hkl/XRD.hs:606:13
    Expected type: Maybe (XrdMeshFrame sh)
      Actual type: Maybe (XrdMeshFrame (Maybe a0))
    In the first argument of `fromJust', namely `f'
    In the first argument of `yield', namely `(fromJust f)'
    In the second argument of `when', namely `(yield (fromJust f))'


So I do not undestand why the typse system guess Maybe a0 instead of sh in f

If I read this

f <- lift $ runMaybeT $ rowND d (fromIndex sh' i')

runMaybeT return a (Maybe (XrdMeshFrame sh)) and then I lift it into the Pipe.

So

What is wrong ?

thanks


Frederic