[Haskell-cafe] How to write a Monad instance for this type

[Tobias Brandt]

Hello Frederic,

you might want to take a look at /free monads/ 
<http://www.haskellforall.com/2012/06/you-could-have-invented-free-monads.html> 
and the free library <http://hackage.haskell.org/package/free-5.1.1>.


Cheers,

Tobias

On 9/2/19 9:20 PM, PICCA Frederic-Emmanuel wrote:
> Hello, I try to write a monad instance for this type which  represent the content of an hdf5 file.
> This file format is similar to a filesystem, where directories are Group and files Dataset.
>
> I end up with this type.
>
> data Hdf5M a
>    = H5Root (Hdf5M a)
>    | H5Group ByteString [Hdf5M a]  -- A group can contain other groups and/or datasets
>    | forall sh b. (NativeType b, Shape sh) => H5Dataset ByteString (Array F sh b)
>
> type Hdf5 = Hdf5M ()
>
> hdf5 :: Hdf5 -> Hdf5
> hdf5 = H5Root
>
> group :: ByteString -> [Hdf5] -> Hdf5
> group g = H5Group g
>
> dataset :: (NativeType b, Shape sh) => ByteString -> (Array F sh b) -> Hdf5
> dataset = H5Dataset
>
> I tryed to inspire myself from the blaze-html Markup type.
>
> And I would like to be able to describe a contain like this
>
> hdf5 $ do
>        group "name" $ do
>              dataset "name1" array1
>              dataset "name2" array2
>              group "other-name" $ do
>                       etc...
>
> instead of
>
> H5Root ( H5Group "name" [ dataset ]...)
>
> the final idea, is to be able to serialize object via a
>
> class ToHdf5 where
>      toHdf5 :: a -> Hdf5
>
> So my question is someone can help me design this ?
>
> thanks for your help
>
> Frederic
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