Here's my guess. I can't be sure without class instances and some function types that are not in code. sh' <- lift $ runMaybeT $ shapeND d :: Pipe a (XrdMeshFrame sh) IO (Maybe sh?) ... f <- lift $ runMaybeT $ rowND d (fromIndex sh' i') -- <- sh' is (Maybe sh), guessing that fromIndex does not take a Maybe. On Mon, Jan 23, 2017 at 3:48 AM, PICCA Frederic-Emmanuel wrote:

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 _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners