[Haskell-beginners] How to link two Types

[David McBride]

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 at 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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