Hello In fact I realize that my real problem is during the 'values' generation of my example. I have a class like this class Frame t where len :: t -> IO (Maybe Int) row :: t -> Int -> IO (Maybe (DifTomoFrame DIM1)) And I create an instance for my dataframe comming from an hdf5 file. some time there is Nan values returned by the get_position method. I decided to return a Maybe Double and Nan -> Nothing instance Frame DataFrameH5 where len d = lenH5Dataspace (h5delta d) row d idx = do Just 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 Just gamma <- get_position' (h5gamma d) 0 Just delta <- get_position' (h5delta d) idx Just 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 <- geometryDetectorRotationGet geometry detector poniext <- ponigen d (MyMatrix HklB m) idx return $ Just DifTomoFrame { difTomoFrameNxs = nxs' , difTomoFrameIdx = idx , difTomoFrameEOF = eof , difTomoFrameGeometry = geometry , difTomoFramePoniExt = poniext } where get_position' a b = do v <- get_position a b return $ if any isNaN v then Nothing else Just v I iterate for each idx of my dataframe So I would like row to return Nothing as soon as the get_position' return Nothing but when I use this code, I get the error and it stop my program instead of skipping the point. ________________________________________ De : Beginners [beginners-bounces@haskell.org] de la part de Francesco Ariis [fa-ml@ariis.it] Envoyé : mardi 17 janvier 2017 15:19 À : The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell Objet : Re: [Haskell-beginners] how to skip pattern match error when applying a mapM_ On Tue, Jan 17, 2017 at 02:03:02PM +0000, PICCA Frederic-Emmanuel wrote:

Hello,

Here a reduction of my problem

values :: IO [IO (Maybe Int)] values = do let v = [Just 1, Just 2, Just 3, Nothing, Just 5, Nothing, Just 7] :: [Maybe Int] return $ map return v

main :: IO () main = do vs <- values nvs <- mapM_ go vs print nvs where go :: IO (Maybe Int) -> IO Int go v' = do Just v <- v' return v

Hello Frédéric, `Just v <- v'` doesn't silently skip Nothing values, but it's a full fledged pattern match (and one reason why I dislike `do notation` as a syntactic sugar). A way to solve the problem is to take advantage of `sequence` and `catMaybes` (from `Data.Maybe`). λ> :t sequence sequence :: (Monad m) => [m a] -> m [a] -- I cheated a bit on the signature, but the gist -- of it is: from a list of monadic actions, to -- one monadic action returning a list of results. λ> :t catMaybes catMaybes :: [Maybe a] -> [a] With that your main gets simpler: main :: IO () main = do vs <- values -- vs :: [IO (Maybe Int)] sv <- sequence vs -- sequence vs :: IO [Maybe Int] -- sv :: [Maybe Int] print (M.catMaybes sv) Does this help? _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

Just gamma <- get_position' (h5gamma d) 0 Just delta <- get_position' (h5delta d) idx Just wavelength <- get_position' (h5wavelength d) 0

is asking for a trouble down the road. Use a case to pattern match on nothing, (or `maybe`, or LambdaCase if you are into extensions).

I tought that was the purpose of the Monad to avoid writting these boillerplate ? What I am missing ? Cheers Frederic

If e.g. 'gamma' is 'Nothing', then the following expressions aren't evaluated and the whole "do-block" returns 'Nothing'.

I just saw that 'row' isn't operating in the 'Maybe' monad but in the 'IO' monad, so this wont work. I don't know if 'row' contains any side effects that should also be executed if the returned 'Maybe' is 'Nothing', if this isn't the case, then you might be able to switch the return type of 'row' from 'IO (Maybe ...)' to 'Maybe (IO ...)' and then you could get the described behaviour for 'Maybe'. Greetings, Daniel

now how can I know the behaviour in between the line of a Monad. I aimagine that this is the purpose of the bind method (>>=).

Yes, that's the case.

Where is this defined for Maybe and IO ?

You just look at the Monad instance for the type. IO is a bit special, but here is the one for Maybe[1]. Greetings, Daniel [1] http://hackage.haskell.org/package/base-4.9.1.0/docs/src/GHC.Base.html#line-...

thanks and what is the purpose of

fail _ = nothing

’fail’ is called for pattern match errors, pretty much the error you’ve got from ’Just x <- ...’. The IO Monad instance raises the exception you've seen. For the Maybe Monad instance just 'Nothing' is returned. Greetings, Daniel

Hello, thanks for the informations 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 len :: IO (Maybe Int) get_position' :: a -> b -> IO (Maybe Double) instance Frame DataFrameH5 where len d = lenH5Dataspace (h5delta d) row d idx = do n' <- len d case n' of (Just n) -> do 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 case gamma' of (Just gamma) -> do delta' <- get_position' (h5delta d) idx case delta' of (Just delta) -> do wavelength' <- get_position' (h5wavelength d) 0 case wavelength' of (Just wavelength) -> do 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 <- geometryDetectorRotationGet geometry detector poniext <- ponigen d (MyMatrix HklB m) idx return $ Just DifTomoFrame { difTomoFrameNxs = nxs' , difTomoFrameIdx = idx , difTomoFrameEOF = eof , difTomoFrameGeometry = geometry , difTomoFramePoniExt = poniext } Nothing -> return Nothing Nothing -> return Nothing Nothing -> return Nothing Nothing -> return Nothing where get_position' a b = do v <- get_position a b return $ if any isNaN v then Nothing else Just v Thanks for your help Frederic

fantastic I like this a lot :)) Thanks Frederic There is plenty of nice Monad transformer like that :)) I just need to learn when to use them. Cheers Frederic