- Beginners - Haskell.org

weird constraint
by Dimitri DeFigueiredo 20 Jan '19

20 Jan '19

Could someone help me make sense of this Num constraint given by GHCi? I was expecting to get: inc . const :: Num a => a -> b -> a rather than: inc . const :: Num (b -> a) => a -> b -> a Here's the transcript: GHCi, version 8.6.3: http://www.haskell.org/ghc/ :? for help Prelude> inc x = x + 1 Prelude> :t inc inc :: Num a => a -> a Prelude> :t const const :: a -> b -> a Prelude> :t inc . const inc . const :: Num (b -> a) => a -> b -> a Prelude> Thanks, Dimitri -- 2E45 D376 A744 C671 5100 A261 210B 8461 0FB0 CA1F

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FFI for PBC - Windows and Nixes
by Sumit Raja 17 Jan '19

17 Jan '19

Hi, I am stuck with an odd problem that I don't know how to begin to resolve. I have developed an FFI wrapper around the PBC library. I have a number of tests thatcheck simple operations and some of the more simple examples from the PBC C examples. On FreeBSD and Linux (Mint) all tests pass successfully but on Windows (mingw64) many fail with what look like data corruption errors. I suspect it is something to do with alignment or size calculations but I have no idea how to proceed. The hsc file is fairly standard: data PBCPairing = PBCPairing data PBCElement = PBCElement data PBCField = PBCField data PBCPreProcElement = PBCPreProcElement instance Storable PBCPairing where sizeOf _ = #{size struct pairing_s} alignment _ = #{alignment struct pairing_s} peek = notImplemented poke = notImplemented instance Storable PBCElement where sizeOf _ = #{size struct element_s} alignment _ = #{alignment struct element_s} peek = notImplemented poke = notImplemented instance Storable PBCPreProcElement where sizeOf _ = #{size struct element_pp_s} alignment _ = #{alignment struct element_pp_s} peek = notImplemented poke = notImplemented instance Storable PBCField where sizeOf _ = #{size struct field_s} alignment _ = #{alignment struct field_s} peek = notImplemented poke = notImplemented Generated sizeOf is the same and alignment is 8 across all OSes. I am using inline-c for the FFI. PBC was built using msys2's version of mingw64. All the C examples work correctly in the msys2 shell. Does any one have any ideas on how best to approach this issue? Thanks Sumit

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Re: [Haskell-beginners] -no-pie
by Sumit Raja 15 Jan '19

15 Jan '19

Hi Dennis, Seems like the GHC settings file is incorrect. There is an SO answer that explains how to change this on OSX: https://stackoverflow.com/questions/50386787/cabal-install-gcc-failed-in-ph… and a defect (https://github.com/haskell/haskell-platform/issues/304) This may not be your exact situation but you can adjust settings accordingly based on the path provided (/Library/Frameworks/GHC.framework/Versions/8.4.2-x86_64/usr/lib/ghc-8.4.2/settings). Regards Sumit On Mon, 14 Jan 2019 at 23:47, <beginners-request(a)haskell.org> wrote: > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Sun, 13 Jan 2019 08:39:42 -0800 > From: Dennis Raddle <dennis.raddle(a)gmail.com> > To: Haskell Beginners <beginners(a)haskell.org> > Subject: [Haskell-beginners] -no-pie > Message-ID: > <CAKxLvooVcHrQ1h_L5a0z_wLJNpM_g9R8D55oExyb%3DwAQ6J86vA(a)mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > I'm on MacOS Sierra. I'm using stack, but I also sometimes use the system > GHC for situations where stack is problematic (one of my applications > spawns GHC as part of a work-loop cycle and it's faster to run the system > GHC). > > My system GHC is 8.4.2. > > I just started getting an error on compilation with the system GC: > clang: error unknown argument: -no-pie > > Does I just need to update the system GHC or is this a symptom of something > deeper? >

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-no-pie
by Dennis Raddle 14 Jan '19

14 Jan '19

I'm on MacOS Sierra. I'm using stack, but I also sometimes use the system GHC for situations where stack is problematic (one of my applications spawns GHC as part of a work-loop cycle and it's faster to run the system GHC). My system GHC is 8.4.2. I just started getting an error on compilation with the system GC: clang: error unknown argument: -no-pie Does I just need to update the system GHC or is this a symptom of something deeper? D

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Overlapping instances via Nat-kind
by Dannyu NDos 02 Jan '19

02 Jan '19

Hi everyone. I'm not so skillful with GADTs and DataKinds, so I assume this mailing list is appropriate to post on. Basically, I wanted to do GADT like this (involving GHC.TypeNats): data Symmetric (n :: Nat) where S1 :: Symmetric 1 (:.) :: (KnownNat n, 2 <= n) => Cyclic n -> Symmetric (n-1) -> Symmetric n But I had some issues with this. The full question is here (on Stack Overflow): https://stackoverflow.com/q/54000646/4540658 Sincerely, Dannyu NDos

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fclabels-2.0.3.3 build hangs on ghc-8.6.3/cabal-2.4.1.0 under Windows 10
by Graham Gill 27 Dec '18

27 Dec '18

While trying to build the haskell-ide-engine project (https://github.com/haskell/haskell-ide-engine) on Windows 10, using my system global install of ghc-8.6.3, the process seems to hang while building fclabels-2.0.3.3. (I followed the project build instructions, using command > .\build-all.ps1 8.6.3 in the project root folder, which uses stack resolver lts-13.0 and ghc-8.6.3.) I get the same result (process never finishes) trying to build fclabels by itself: > stack install fclabels --stack-yaml .\stack-8.6.3.yaml in the haskell-ide-engine root folder. To simplify, I tried to build fclabels with cabal install (cabal-2.4.1.0) in my home folder (where there are no .cabal files present): > cabal -v -j1 v2-install fclabels Output is attached. The process hangs at compiling Data.Label.Base source to object file. Windows task manager shows ghc working merrily away (overnight, on my first attempt) without finishing. On the other hand, if I run > stack install fclabels --stack-yaml .\stack-8.4.4.yaml in the haskell-ide-engine root folder, which uses stack resolver lts-12.20 and ghc-8.4.4, fclabels-2.0.3.3 builds successfully. Because of the complexity of cabal and stack package management I don't know where to pin the blame, perhaps it's on my own system setup, hence don't know where/whether to file a bug report. Regards, Graham

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MonadThrow, MonadReader and shake
by PICCA Frederic-Emmanuel 19 Dec '18

19 Dec '18

Hello, I try to write this sort of code xdsme' :: SomeDataCollection -> Maybe Cell -> Maybe SpaceGroup -> GZiped -> [Path Abs File] -> ReaderT Beamline IO () xdsme' c@(SomeDataCollection SCollect SHdf5 _) cell sg z is = do -- xdsme compute the output path by himself. cwd' <- toProcessDataPath c rdir <- resultsPrefixFile xdsMePrefix c dir <- resultsPrefixDir ("xdsme_" ++ xdsMePrefix) c dir' <- resultsPrefixFile "" c xmlPath <- parseRelFile $ toFilePath dir' ++ "_xdsme.xml" xml <- parseAbsFile $ toFilePath cwd' </> toFilePath dir </> toFilePath xmlPath uploadedPath <- parseRelFile $ toFilePath dir' ++ "_xdsme.uploaded" uploaded <- parseAbsFile $ toFilePath cwd' </> toFilePath dir </> toFilePath uploadedPath let shakeFiles' = toFilePath cwd' </> toFilePath dir </> ".shake/" let images = getImages c z liftIO $ shake shakeOptions{ shakeFiles=shakeFiles' , shakeReport=["/tmp/shake.html"] , shakeVerbosity=Diagnostic} $ do want [toFilePath uploaded] -- execute xdsme and deal with input dependencies toFilePath xml %> \_out -> do need (map toFilePath is) processXdsMe cwd' cell sg rdir images toFilePath uploaded %> \_out -> do need [toFilePath xml] container <- liftIO . fromFile . toFilePath $ xml -- post processing let attachment = _autoProcProgramAttachment . _autoProcProgramContainer $ container attachment' <- toRuchePath attachment <- HERE PROBLEM _ <- copyAttachment' attachment attachment' let container' = (autoProcProgramContainer . autoProcProgramAttachment .~ attachment') container -- replace attachement -- upload into ISPYB liftIO $ storeAutoProcIntoISPyB c NoAnomalous container' cmd_ ("touch" :: String) (toFilePath uploaded) where toRuchePath :: (MonadReader Beamline m, MonadThrow m) => [AutoProcProgramAttachment WithPrefix] -> m [AutoProcProgramAttachment ISPyB] toRuchePath = mapM go where go :: (MonadReader Beamline m, MonadThrow m) => AutoProcProgramAttachment WithPrefix -> m (AutoProcProgramAttachment ISPyB) go a = do (d, _) <- toPath a b <- ask newd <- mkText255 . pack . toRuchePath' b . fromAbsDir $ d return a {filePath = newd} but when I try to compile this I get this error. How can I teach ghc how to solve this issue ? thanks for your help Frederic src/XdsMe.hs:211:22-43: error: • Could not deduce (MonadThrow Action) arising from a use of ‘toRuchePath’ from the context: t ~ 'Collect bound by a pattern with constructor: SCollect :: SCollectType 'Collect, in an equation for ‘xdsme'’ at src/XdsMe.hs:180:30-37 or from: f ~ 'ISPyB.DataCollection.Hdf5 bound by a pattern with constructor: SHdf5 :: SCollectSourceFormat 'ISPyB.DataCollection.Hdf5, in an equation for ‘xdsme'’ at src/XdsMe.hs:180:39-43 • In a stmt of a 'do' block: attachment' <- toRuchePath attachment In the expression: do { need [toFilePath xml]; container <- liftIO . fromFile . toFilePath $ xml; let attachment = _autoProcProgramAttachment . _autoProcProgramContainer $ container; attachment' <- toRuchePath attachment; .... } In the second argument of ‘(%>)’, namely ‘\ _out -> do { need [...]; container <- liftIO . fromFile . toFilePath $ xml; .... }’

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Regex
by mike h 12 Dec '18

12 Dec '18

Hi, Which package is the most popular and/or easy to used for dealing with regular expressions? My regex is quite simple and I just need to get 2 or 3 capture groups from the result. Many thanks Mike

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monad and variable result
by Damien Mattei 11 Dec '18

11 Dec '18

have some code that works but want to put it in a simple function without sucess: getBD :: Connection -> String -> Float getBD conn name = noBDfp where qry_head = "select `N° BD` from sidonie.Coordonnées where Nom = ?" :: Query bd_rows = do local_bd_rows <- query conn qry_head (Only (name::String)) return local_bd_rows i want the variable local_bd_rows accessible in the 'where' clause how can i do that? note the goal is to do the same thing of my main function that works : main :: IO () --main :: Int main = do conn <- connect defaultConnectInfo { connectHost = "moita", connectUser = "mattei", connectPassword = "******", connectDatabase = "sidonie" } -- we get all the Double Stars that have angular distance superior to a threshold of 1 second = 0.000278 degree rows <- query_ conn "SELECT Nom,distance FROM AngularDistance WHERE distance > 0.000278" forM_ rows $ \(name,distance) -> putStrLn $ Text.unpack name ++ " " ++ show (distance :: Double) -- we will get the Durchmusterung Number BD from Sidonie and WDS and compare them for a given name -- Warning: there could be multiple result in WDS for a given entry name (due to components) -- first we get the N°BD from sidonie let name = "A 20" -- let qry = "select `N° BD` from Coordonnées where Nom = " ++ name let qry_head = "select `N° BD` from sidonie.Coordonnées where Nom = ?" :: Query -- bd_rows <- query_ conn "select `N° BD` from sidonie.Coordonnées where Nom = 'A 20'" bd_rows <- query conn qry_head (Only (name::String)) putStrLn $ show bd_rows putStrLn $ show name let resLst = Prelude.map fromOnly bd_rows let noBDtxt = fromOnly (Prelude.head bd_rows) :: Text -- let noBD2 = _ (Prelude.head bd_rows) putStrLn $ show resLst putStrLn $ show noBDtxt forM_ bd_rows $ \(Only a) -> putStrLn $ Text.unpack a let noBDstr = Text.unpack noBDtxt :: String let noBDfp = read $ noBDstr :: Float putStr "noBDfp =" (putStrLn (show noBDfp)) close conn print "Exit." for now i have errors in the function: Prelude> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:47:28: error: • Ambiguous type variable ‘r0’ arising from a use of ‘query’ prevents the constraint ‘(QueryResults r0)’ from being solved. Relevant bindings include bd_rows :: IO [r0] (bound at UpdateSidonie.hs:46:9) Probable fix: use a type annotation to specify what ‘r0’ should be. These potential instances exist: instance Result a => QueryResults (Only a) -- Defined in ‘Database.MySQL.Simple.QueryResults’ instance (Result a, Result b) => QueryResults (a, b) -- Defined in ‘Database.MySQL.Simple.QueryResults’ instance (Result a, Result b, Result c) => QueryResults (a, b, c) -- Defined in ‘Database.MySQL.Simple.QueryResults’ ...plus 21 others (use -fprint-potential-instances to see them all) • In a stmt of a 'do' block: local_bd_rows <- query conn qry_head (Only (name :: String)) In the expression: do local_bd_rows <- query conn qry_head (Only (name :: String)) return local_bd_rows In an equation for ‘bd_rows’: bd_rows = do local_bd_rows <- query conn qry_head (Only (name :: String)) return local_bd_rows | 47 | local_bd_rows <- query conn qry_head (Only (name::String)) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. -- Damien.Mattei(a)unice.fr, Damien.Mattei(a)oca.eu, UNS / OCA / CNRS

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database, list result, extraction and conversion
by Damien Mattei 07 Dec '18

07 Dec '18

again un haskell beginner question ;-) ... i have this: [Only {fromOnly = "-04.3982"}] as result of a database query, i understand it's a list and i can get the first element: *Main> Data.List.head [Only {fromOnly = "-04.3982"}] Only {fromOnly = "-04.3982"} *Main> let s = Data.List.head [Only {fromOnly = "-04.3982"}] *Main> s Only {fromOnly = "-04.3982"} *Main> show s "Only {fromOnly = \"-04.3982\"}" but how can i get the String "-04.3982" ? and after converting it to a Float -- Damien.Mattei(a)unice.fr, Damien.Mattei(a)oca.eu, UNS / OCA / CNRS

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