Send Beginners mailing list submissions to
beginners@haskell.org
To subscribe or unsubscribe via the World Wide Web, visit
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
or, via email, send a message with subject or body 'help' to
beginners-requ...@haskell.org
You can reach the person managing the list at
beginners-ow...@haskell.org
When replying, please edit your Subject line so it is more specific
than "Re: Contents of Beginners digest..."
Today's Topics:
1. Re: how to skip pattern match error when applying a mapM_
(Manuel Vázquez Acosta)
2. Re: how to skip pattern match error when applying a mapM_
(PICCA Frederic-Emmanuel)
3. Re: how to skip pattern match error when applying a mapM_
(Daniel Trstenjak)
----------------------------------------------------------------------
Message: 1
Date: Tue, 17 Jan 2017 18:31:55 -0500
From: Manuel Vázquez Acosta <mva....@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] how to skip pattern match error when
applying a mapM_
Message-ID: <87eg016z6c....@pavla.com>
Content-Type: text/plain
>From [1]:
-- | Fail with a message. This operation is not part of the
-- mathematical definition of a monad, but is invoked on pattern-match
-- failure in a @do@ expression.
--
-- As part of the MonadFail proposal (MFP), this function is moved
-- to its own class 'MonadFail' (see "Control.Monad.Fail" for more
-- details). The definition here will be removed in a future
-- release.
fail :: String -> m a
fail s = errorWithoutStackTrace s
Best regards,
Manuel (also a newbie)
[1] http://hackage.haskell.org/package/base-4.9.1.0/docs/src/GHC.Base.html#Monad
> thanks and what is the purpose of
>
> fail _ = nothing
>
> Cheers
>
> Fred
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
------------------------------
Message: 2
Date: Wed, 18 Jan 2017 10:28:30 +0000
From: PICCA Frederic-Emmanuel
<frederic-emmanuel.pi...@synchrotron-soleil.fr>
To: "The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell" <beginners@haskell.org>
Subject: Re: [Haskell-beginners] how to skip pattern match error when
applying a mapM_
Message-ID:
<a2a20ec3b8560d408356cac2fc148e53bb329...@sun-dag3.synchrotron-soleil.fr>
Content-Type: text/plain; charset="us-ascii"
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
------------------------------
Message: 3
Date: Wed, 18 Jan 2017 12:53:26 +0100
From: Daniel Trstenjak <daniel.trsten...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] how to skip pattern match error when
applying a mapM_
Message-ID: <20170118115326.GA7930@octa>
Content-Type: text/plain; charset=us-ascii
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
------------------------------
Subject: Digest Footer
_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
------------------------------
End of Beginners Digest, Vol 103, Issue 16
******************************************
