[Haskell-beginners] How to link two Types

[PICCA Frederic-Emmanuel]

Hello, I wrote this code

data DataFrameH5 a
    = DataFrameH5
      (Nxs a) -- Nexus file
      (DataSource H5) -- gamma
      (DataSource H5) -- delta
      (DataSource H5) -- wavelength
      PoniGenerator -- ponie generator

class Frame t a where
  len :: t -> IO (Maybe Int)
  row :: t -> Int -> MaybeT IO (DifTomoFrame a DIM1)

instance Frame (DataFrameH5 DataFrameH5Path) DataFrameH5Path where
  len (DataFrameH5 _ _ (DataSourceH5 _ d) _ _) = lenH5Dataspace d

  row d@(DataFrameH5 nxs' g d' w ponigen) idx = do
    n <- lift $ len d
    let eof = fromJust n - 1 == idx
    let mu = 0.0
    let komega = 0.0
    let kappa = 0.0
    let kphi = 0.0
    gamma <- g `atIndex'` (ix1 0)
    delta <- d' `atIndex'` (ix1 idx)
    wavelength <- w `atIndex'` (ix1 0)
    let source = Source (head wavelength *~ nano meter)
    let positions = concat [mu, komega, kappa, kphi, gamma, delta]
    -- print positions
    let geometry =  Geometry K6c source positions Nothing
    let detector = ZeroD
    m <- lift $ geometryDetectorRotationGet geometry detector
    poniext <- lift $ ponigen (MyMatrix HklB m) idx
    return $ DifTomoFrame { difTomoFrameNxs = nxs'
                          , difTomoFrameIdx = idx
                          , difTomoFrameEOF = eof
                          , difTomoFrameGeometry = geometry
                          , difTomoFramePoniExt = poniext
                          }

has you can see my t type contains also the a reference to the a one
So when I create the instance, I need to write two times the DataFrameH5Path

I would like to know how to write the same class with only 

class Frame t where
  len :: t -> IO (Maybe Int)
  row :: t -> Int -> MaybeT IO (DifTomoFrame <extract type a from type t> DIM1)

thanks for your help

Frederic