Re: [Haskell-beginners] How to link two Types

22 Feb 2017


      Maybe TypeFamilies would work for you?  I can only give you a
barebones outline of what it might look like.

{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, TypeFamilies #-}
import Control.Monad.Trans.Maybe

data DataFrameH5 a = DataFrameH5
data DataFrameH5Path = DataFrameH5Path

class Frame t where
  type Key t
  len :: t -> IO (Maybe Int)
  row :: t -> Int -> MaybeT IO (Key t)

instance Frame (DataFrameH5 a) where
  type Key (DataFrameH5 a) = a
  len DataFrameH5 = return . Just $ undefined
  row DataFrameH5 idx = MaybeT $ do
    return undefined

On Wed, Feb 22, 2017 at 10:27 AM, PICCA Frederic-Emmanuel
<frederic-emmanuel.picca@synchrotron-soleil.fr> wrote:
...
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
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Re: [Haskell-beginners] How to link two Types

David McBride