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

22 Feb 2017

      ...
It is hard to tell from your code what you intend, but it works
however you want it to, so long as it type checks.
...
class Frame a where
  type Whatever a
  len :: Whatever a -> IO (Maybe Int)
  row :: Whatever a -> MaybeT IO (DifTomoFrame a DIM1)
...
instance Frame DataFrameH5Path  where
  type Whatever DataFrameH5Path = DataFrameH5
  len = undefined -- :: DataFrameH5 -> IO (Maybe Int)
  row = undefined -- :: DataFrameH5 -> Int -> MaybeT (DifTomoFrame
DataFrameH5Path DIM1)
In fact what I try realy to do is this.

data DataFrameH5Path
    = DataFrameH5Path
      (DataItem H5) -- image
      (DataItem H5) -- gamma
      (DataItem H5) -- delta
      (DataItem H5) -- wavelength
    deriving (Show)

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

withDataFrameH5 :: (Frame a, MonadSafe m) => File -> Nxs (Key a) -> PoniGenerator -> (a -> m r) -> m r
withDataFrameH5 h nxs'@(Nxs _ _ (DataFrameH5Path _ g d w)) gen = bracket (liftIO before) (liftIO . after)
  where
    -- before :: File -> DataFrameH5Path -> m DataFrameH5
    before :: IO a
    before =  DataFrameH5
              <$> return nxs'
              <*> openDataSource h g
              <*> openDataSource h d
              <*> openDataSource h w
              <*> return gen

    after :: a -> IO ()
    after (DataFrameH5 _ g' d' w' _) = do
      closeDataSource g'
      closeDataSource d'
      closeDataSource w'

I open and hdf5 file and I need to read a bunch of data from this file.
the DataFrameH5 is a sort of resource like a File handler.
I need a location in the file in order to acce the data, then I need to close the file
So I store in the H5 type these resource, that I can release at the end.

Ideally I would like to have only The H5Path type and hide the H5 one but I do not know how to do this.

I have in fact different H5Path types which necessitate each time there corresponding H5 type.

So I want a one for one relation between the H5 <-> H5Path type.

Cheers

Frederic

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

PICCA Frederic-Emmanuel