Once you remove those constructors, when you are operating on 'p' (with poke, I assume) it doesn't know whether you are operating on a ScPipeParams ImageXY or a ScPipeParams Image3d'.

The best solution is probably to make instances for Storable ImageXY, and Storable Image3d' and then Storable a => Storable (ScPipeParams a) which utilizes the the Storable instance of whichever parameter 'a' you've provided.

If that is not to your liking. I think you can use RecordWildCards extension or maybe it was RecordPuns to allow you to write it something like one of the following. I'm not quite sure whether these would work or not, but it might be worth a try.

scPipeOpen2 d p@(ScPipeParamsImage3d' {}) =
scPipeOpen2 d p@(ScPipeParamsImage3d' {...}) =

On Fri, Aug 26, 2016 at 10:31 AM, PICCA Frederic-Emmanuel <frederic-emmanuel.picca@synchrotron-soleil.fr> wrote:

Hello

I have these types

data ImageXY
data Image3d'

data ScPipeParams a where
ScPipeParamsImageXY :: ScBitSize -> Channel -> Modulo -> Sc3du -> Roi -> Accumulation -> ScPipeParams ImageXY
ScPipeParamsImage3d' :: ScBitSize -> Channel -> Modulo -> Sc3du -> Roi -> Accumulation -> ScPipeParams Image3d'

and now I write a method

scPipeOpen2' (ScDevDesc dev) p = alloca $ \params -> do
poke params p
res <- c_sc_pipe_open2 dev (scPipeType p) params
checkError res PipeId

scPipeOpen2 :: ScDevDesc -> ScPipeParams a -> IO (PipeId)
scPipeOpen2 d p@(ScPipeParamsImageXY _ _ _ _ _ _) = scPipeOpen2' d p
scPipeOpen2 d p@(ScPipeParamsImage3d' _ _ _ _ _ _) = scPipeOpen2' d p

it works fine

In order to avoid these _ _ _ _ _ _, I tryed to removed the @(Constructor ____) since I just pass the p parameter to the ' method.
But in that case , I get this error message

C:\Users\TEMPO\Downloads\tdc\tdc\srcLib.hsc:247:19:
No instance for (Storable (ScPipeParams a))
arising from a use of scPipeOpen2'
In the expression: scPipeOpen2' d p
In an equation for `scPipeOpen2':
scPipeOpen2 d p = scPipeOpen2' d p

Indeed I already defined two instance of Storable

instance Storable (ScPipeParams ImageXY) where
...

and

instance Storable (ScPipeParams Image3d') where
...

Should I use a special extension in order to be able to write only

scPipeOpen2 d p = scPipeOpen2' d p

thanks for your help

Frederic
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