[Haskell-beginners] howto Pipe

[PICCA Frederic-Emmanuel]

Hello

I try to mix my C library and the Pipe module

here the code I am using

solveTraj :: Factory -> Geometry -> Detector -> Sample -> Pipe Engine Geometry IO ()
solveTraj f g d s = do
  e <- await
  let name = engineName e
  withSample s $ \sample ->
      withDetector d $ \detector ->
          withGeometry f g $ \geometry ->
              withEngineList f $ \engines ->
                withCString name $ \cname -> do
                  c_hkl_engine_list_init engines geometry detector sample
                  engine <- c_hkl_engine_list_engine_get_by_name engines cname nullPtr
                  n <- c_hkl_engine_pseudo_axis_names_get engine >>= darrayStringLen
                  yield $ solve' engine n e >>= getSolution0

where 

 getSolution0 :: ForeignPtr HklGeometryList -> IO Geometry

And I am using this like this

runEffect $ for (each engines) >-> solveTraj factory geometry detector sample >-> P.print

where

[Engine]
engines


But When I compile the code I get this error.

src/Hkl/C.hsc:83:3:
    Couldn't match type `IO' with `Proxy () Engine () Geometry IO'
    Expected type: Proxy () Engine () Geometry IO ()
      Actual type: IO ()
    In a stmt of a 'do' block:
      withSample s
      $ \ sample
          -> withDetector d
             $ \ detector -> withGeometry f g $ \ geometry -> ...
    In the expression:
      do { e <- await;
           let name = engineName e;
           withSample s $ \ sample -> withDetector d $ \ detector -> ... }
    In an equation for `solveTraj':
        solveTraj f g d s
          = do { e <- await;
                 let name = ...;
                 withSample s $ \ sample -> withDetector d $ ... }

src/Hkl/C.hsc:91:19:
    Couldn't match type `Proxy x'0 x0 () (IO Geometry) m0' with `IO'
    Expected type: IO ()
      Actual type: Proxy x'0 x0 () (IO Geometry) m0 ()
    In a stmt of a 'do' block:
      yield $ solve' engine n e >>= getSolution0
    In the expression:
      do { c_hkl_engine_list_init engines geometry detector sample;
           engine <- c_hkl_engine_list_engine_get_by_name
                       engines cname nullPtr;
           n <- c_hkl_engine_pseudo_axis_names_get engine >>= darrayStringLen;
           yield $ solve' engine n e >>= getSolution0 }
    In the second argument of `($)', namely
      `\ cname
         -> do { c_hkl_engine_list_init engines geometry detector sample;
                 engine <- c_hkl_engine_list_engine_get_by_name
                             engines cname nullPtr;
                 .... }'


I do not understand why the yield does not produce the right type as output.

I think I missed something big :), but...

Thanks for your help

Fred