[Haskell-cafe] how to simplify ressources access and pipes steam

[Compl Yue]

Maybe fork another thread, keep reading data and putting into a shared `MVar` / `TMVar`, then current thread keep taking from that var?

> On 2020-05-20, at 14:23, PICCA Frederic-Emmanuel <frederic-emmanuel.picca at synchrotron-soleil.fr> wrote:
> 
> Hello, I am writing a software which  do some data treatement from hdf5 files.
> 
> In order to access the data which are stored in arrays, I need to open and closes ressources.
> This is the purpose of the withHdf5PathP function.
> 
> given a file f and a UhvPath I end up with this
> 
> withUhvPathP :: (MonadSafe m, Location l) => l -> UhvPath -> ((Int -> IO Geometry) -> m r) -> m r
> withUhvPathP f (UhvPath m o d g w) gg =
>      withHdf5PathP f m $ \m' ->
>      withHdf5PathP f o $ \o' ->
>      withHdf5PathP f d $ \d'->
>      withHdf5PathP f g $ \g' ->
>      withHdf5PathP f w $ \w' -> gg (\j -> do
>                                      mu <- get_position m' j
>                                      omega <- get_position o' j
>                                      delta <- get_position d' j
>                                      gamma' <- get_position g' j
>                                      wavelength <- getValueWithUnit w' 0 angstrom
>                                      let positions = Data.Vector.Storable.fromList [mu, omega, delta, gamma']
>                                      let source = Source wavelength
>                                      pure $ Geometry Uhv source positions Nothing)
> 
> 
> then I use this like this
> 
> forever $ do
>    (Chunk fp from to) <- await
>    withFileP (openH5 fp) $ \f ->
>      withHdf5PathP f imgs $ \dimgs ->
>      withUhvPathP f dif $ \getDiffractometer ->
>      withSamplePathP f samp $ \getSample ->
>      forM_ [from..to-1] (\j -> yield =<< liftIO
>                               (DataFrameHkl
>                                <$> pure j
>                                <*> get_image' det dimgs j
>                                <*> getDiffractometer j
>                                <*> getSample j))
> 
> so once I open the resources, I use the getDiffratometer ((Int -> IO Geometry) -> m r) function give the position on the stream and it return the value at the j position.
> the purpose of this is to open the ressources only once at the begining and then send the values in a Pipe via the yield function of pipes package.
> 
> I hope, I was clear.
> 
> 1) I find the withUhvPathP sort of ugly, and I would like to know if it could be written more elegantly, because I will need to write a lot's of these function for different types.
> 
> 2) It is the right way to design a solution to my problem (open ressources and send each values into a stream).
> 
> thanks for you attention.
> 
> Frederic
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