[Haskell-beginners] how to skip pattern match error when applying a mapM_

[Daniel Trstenjak]

Hi Frederic,

> after investigating, I could not switch  the IO and the Maybe
> I need to process the IO in order to know if I have a Just or a Nothing
> So this is a IO (Maybe ...)
> 
> I just would like to know if there is a better way to write this
> knowing

There's the MaybeT[1] monad transformer for this use case.

Instead of having:
   row :: t -> Int -> IO (Maybe (DifTomoFrame DIM1))

you would have:
   row :: t -> Int -> MaybeT IO (DifTomoFrame DIM1)


So 'row' might look like:

   row d idx = do
     n <- len d
     let eof = n - 1 == idx
     let nxs' = h5nxs d
     let mu = 0.0
     let komega = 0.0
     let kappa = 0.0
     let kphi = 0.0
     gamma <- get_position' (h5gamma d) 0
     delta <- get_position' (h5delta d) idx
     wavelength <- get_position' (h5wavelength d) 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 d (MyMatrix HklB m) idx
     return $ DifTomoFrame { difTomoFrameNxs = nxs'
                           , difTomoFrameIdx = idx
                           , difTomoFrameEOF = eof
                           , difTomoFrameGeometry = geometry
                           , difTomoFramePoniExt = poniext
                           }


This assumes that the functions 'len', 'get_position' also return a 'MaybeT IO (...)'.
Functions in the IO monad - like 'geometryDetectorRotationGet' and 'ponigen' -
have to be "lifted" into the IO monad by 'lift'.

To get at the 'Maybe' result of 'row' you're using the 'runMaybeT' function in the IO monad:

   main :: IO ()
   main = do
      ...
      result <- runMaybeT (row t int)
      ...


Greetings,
Daniel


[1] https://hackage.haskell.org/package/transformers-0.5.2.0/docs/Control-Monad-Trans-Maybe.html