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Today's Topics:
1. (no subject) (PICCA Frederic-Emmanuel)
2. 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_
(Francesco Ariis)
4. Re: how to skip pattern match error when applying a mapM_
(PICCA Frederic-Emmanuel)
5. Re: how to skip pattern match error when applying a mapM_
(Francesco Ariis)
6. Re: how to skip pattern match error when applying a mapM_
(PICCA Frederic-Emmanuel)
----------------------------------------------------------------------
Message: 1
Date: Tue, 17 Jan 2017 10:15:24 +0000
From: PICCA Frederic-Emmanuel
<[email protected]>
To: "The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell" <[email protected]>
Subject: [Haskell-beginners] (no subject)
Message-ID:
<a2a20ec3b8560d408356cac2fc148e53bb329...@sun-dag3.synchrotron-soleil.fr>
Content-Type: text/plain; charset="iso-8859-1"
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
when I run this script, I get a runtime error
picca@diffabs6:~/tmp$ runhaskell test.hs
test.hs: user error (Pattern match failure in do expression at test.hs:13:10-15)
What I want is a go method which skip silently the (IO Nothing) values.
so when used in the mapM_ it return only the values which are returned by the
IO (Maybe Int) (stored in the values)
Thanks for your help
Frédéric
Indeed
------------------------------
Message: 2
Date: Tue, 17 Jan 2017 14:03:02 +0000
From: PICCA Frederic-Emmanuel
<[email protected]>
To: "The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell" <[email protected]>
Subject: [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="iso-8859-1"
Sorry I forgot to put a subject
________________________________________
De : Beginners [[email protected]] de la part de PICCA
Frederic-Emmanuel
Envoyé : mardi 17 janvier 2017 11:15
À : The Haskell-Beginners Mailing List - Discussion of primarily beginner-level
topics related to Haskell
Objet : [Haskell-beginners] (no subject)
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
when I run this script, I get a runtime error
picca@diffabs6:~/tmp$ runhaskell test.hs
test.hs: user error (Pattern match failure in do expression at test.hs:13:10-15)
What I want is a go method which skip silently the (IO Nothing) values.
so when used in the mapM_ it return only the values which are returned by the
IO (Maybe Int) (stored in the values)
Thanks for your help
Frédéric
Indeed
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------------------------------
Message: 3
Date: Tue, 17 Jan 2017 15:19:46 +0100
From: Francesco Ariis <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] how to skip pattern match error when
applying a mapM_
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8
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?
------------------------------
Message: 4
Date: Tue, 17 Jan 2017 14:49:11 +0000
From: PICCA Frederic-Emmanuel
<[email protected]>
To: "The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell" <[email protected]>
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="utf-8"
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 [[email protected]] de la part de Francesco Ariis
[[email protected]]
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?
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------------------------------
Message: 5
Date: Tue, 17 Jan 2017 16:26:03 +0100
From: Francesco Ariis <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] how to skip pattern match error when
applying a mapM_
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Tue, Jan 17, 2017 at 02:49:11PM +0000, PICCA Frederic-Emmanuel wrote:
> Hello
>
> In fact I realize that my real problem is during the 'values' generation of
> my example.
>
> I have a class like this
>
> [...]
A repository would help! In any case writing:
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).
------------------------------
Message: 6
Date: Tue, 17 Jan 2017 17:34:05 +0000
From: PICCA Frederic-Emmanuel
<[email protected]>
To: "The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell" <[email protected]>
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"
> 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
------------------------------
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