- Haskell-Cafe - Haskell.org

Problem on exception and laziness
by Magicloud Magiclouds 23 Apr '19

23 Apr '19

Hi, I am aware that laziness sometimes may cause exception handling in unexpected ways. Recently I have such a problem that I cannot figure out how to fix (simply). I have a function `watch` that parse the content of /proc/. And I use `catch` to wrap `readFile` call, since the data in /proc/ could be gone any time. ```Haskell wrap2Maybe :: IO a -> IO (Maybe a) wrap2Maybe f = catch ((<$>) Just $! f) (\(_ :: IOException) -> return Nothing) watch args = do mStat <- wrap2Maybe (readFile ("/proc/" </> fp </> "stat")) >>= return . flip (>>=) parseStat print mStat watch args ``` But occasionally, my tool exits with message: GoM: /proc/1484/stat: hGetContents: does not exist (No such process) Since above code is the only `readFile` call which leads to hGetContents, I could not figure out what else should I do to catch the error. -- 竹密岂妨流水过山高哪阻野云飞

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how do I intercalate for Data.Text.Lazy.Builder
by Javran Cheng 22 Apr '19

22 Apr '19

Hi Cafe, Just a quick question, the title says it all: how do I intercalate for the Builder? Given what it does, it makes sense that things like `intercalate` should be out of the box, yet I feel the export list of Builder is very limited. -- Javran (Fang) Cheng

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When does ICFP open registration?
by Alexey Raga 22 Apr '19

22 Apr '19

Hello, For bureaucracy reasons, it may be a time in our company to start sorting out ICFP trips/tickets. Does anyone have any information about when the registration may be opening? I couldn't find anything on their website, and I couldn't find any contact details on that website either, so asking here just in case if anyone knows :) Best Regards, Alexey Raga

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Re: [Haskell-cafe] Is there already a lib to parse a line of string with a pattern and generate key-value map or a record?
by Olaf Klinke 19 Apr '19

19 Apr '19

You could emulate this with some custom parser combinators on top of any monadic parser combinator library such as attoparsec or megaparsec. keyed :: (Ord key, Functor parser) => key -> parser a -> parser (Map key a) keyed key = fmap (singleton key) Then, assuming 'char' and 'digitChar' exists in the parser library, you may write integer = some digitChar logline = fmap mconcat $ sequence [ keyed "year" (integer <* char '/'), keyed "month" (integer <* char '/'), keyed "day" (integer <* char ' '), -- etc. ] Olaf

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-fdefer-type-errors
by Bill Hallahan 18 Apr '19

18 Apr '19

Hi, I'm trying to use the -fdefer-type-error flag, discussed in an ICFP paper from 2012: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.366.841&rep=rep1&t… <http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.366.841&rep=rep1&t…> On page 2, the paper has an example, showing that fdefer-type-errors will allow a the first element to be extracted fro a tuple, even if the second element does not correctly typecheck: ghci> let foo = (True, ’a’ && False) Warning: Couldn’t match ‘Bool’ with ‘Char’ ghci> :type foo (Bool, Bool) ghci> fst foo True However, with GHC 8.2.2, I get an error when trying to do this: GHCi, version 8.2.2: http://www.haskell.org/ghc/ :? for help Prelude> let foo = (True, 'a' && False) <interactive>:1:18: warning: [-Wdeferred-type-errors] • Couldn't match expected type ‘Bool’ with actual type ‘Char’ • In the first argument of ‘(&&)’, namely ‘'a'’ In the expression: 'a' && False In the expression: (True, 'a' && False) Prelude> :type foo foo :: (Bool, Bool) Prelude> fst foo *** Exception: <interactive>:1:18: error: • Couldn't match expected type ‘Bool’ with actual type ‘Char’ • In the first argument of ‘(&&)’, namely ‘'a'’ In the expression: 'a' && False In the expression: (True, 'a' && False) (deferred type error) Does anyone know the reason for the change in the behavior? Is there any way to get back to the old behavior, i.e. allow the first element of the tuple to still be extracted? Ideally, I would like to get the errors given by defer-type-errors to be as fine-grained/close-to-the-source as possible. Thanks, Bill Hallahan

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Is there already a lib to parse a line of string with a pattern and generate key-value map or a record?
by Magicloud Magiclouds 18 Apr '19

18 Apr '19

Hi, I saw this function in some web tools, pretty useful when parsing log or so. So I wonder if Haskell has similar libs already. Sample: Input line: "2019/04/17 17:27 User magicloud runs command ls." Pattern: "${year}/${month}/${day} ${hour}:${minute} User ${username} runs command ${command}." Output: toList [ ("year", "2019"), ("month", "04") , etc ] -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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monad example
by Damien Mattei 17 Apr '19

17 Apr '19

hello, i still seaching a good monad example and after passed the classic tutorial about Maybe monad or State monad which is found in almost 90% of the litterature i try to learn in deep the example of monad that talk about probability ( http://learnyouahaskell.com/for-a-few-monads-more ) for this i try to trace the calls of the inner monad definition like in the file, i give the file in attachment (monade.hs) and the output has a lot of garbage data, not the nicer way to display it,sorry. What i can not figure out is at what stage the program construct the [Tails,Heads] , [Heads,Heads] ... etc i do not know, if someone could explain me in detail the action of a <- coin b <- loadedCoin return [a,b] step by step it would be of great help... Regards, Damien note: here is the ouput: *Main> :load monade.hs [1 of 1] Compiling Main ( monade.hs, interpreted ) Ok, one module loaded. *Main> flipTwoVal Prob {getProb = (Monad Prob >>= flatten (fmap f m) Functor Prob Functor Prob (f x,1 % 2) flatten multAll map ... p= 1 % 2 (Monad Prob >>= flatten (fmap f m) Functor Prob Functor Prob (f x,1 % 10) flatten multAll map ... p= 1 % 10 Monad Prob return) [ flatten multAll (1 % 10*1 % 1) flatten multAll (1 % 2*1 % 10) ([Heads,Heads],1 % 20) Functor Prob (f x,9 % 10) flatten multAll map ... p= 9 % 10 Monad Prob return) , flatten multAll (9 % 10*1 % 1) flatten multAll (1 % 2*9 % 10) ([Heads,Tails],9 % 20) Functor Prob (f x,1 % 2) flatten multAll map ... p= 1 % 2 (Monad Prob >>= flatten (fmap f m) Functor Prob Functor Prob (f x,1 % 10) flatten multAll map ... p= 1 % 10 Monad Prob return) , flatten multAll (1 % 10*1 % 1) flatten multAll (1 % 2*1 % 10) ([Tails,Heads],1 % 20) Functor Prob (f x,9 % 10) flatten multAll map ... p= 9 % 10 Monad Prob return) , flatten multAll (9 % 10*1 % 1) flatten multAll (1 % 2*9 % 10) ([Tails,Tails],9 % 20)]} *Main> flipTwoVal Prob {getProb = [([Heads,Heads],1 % 20),([Heads,Tails],9 % 20),([Tails,Heads],1 % 20),([Tails,Tails],9 % 20)]}

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shake and cmd argument dependency
by PICCA Frederic-Emmanuel 17 Apr '19

17 Apr '19

Hello, I am writting a program which execute some scientific task via shake. 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 mr mo rdir images -- upload the result into the Databse toFilePath uploaded %> \_out -> actionXml xml b c uploaded processXdsMe :: Path Abs Dir -> Maybe Cell -> Maybe SpaceGroup -> Maybe Resolution -> Maybe Optimize -> Path Rel File -> String -> Action () processXdsMe cwd' mcell msg mr mo rdir images = cmd opts args where opts :: [CmdOption] opts = px1Opts ++ [Cwd . fromAbsDir $ cwd', AutoDeps] args = xdsMePath : catMaybes params params :: [Maybe String] params = [ Just "--brute" , Just "--weak" , Just "--xml" , Just ("-p" ++ fromRelFile rdir) , fmap (\(Cell a b c alpha beta gamma) -> printf "-c%f,%f,%f,%f,%f,%f" a b c alpha beta gamma) mcell , fmap (\sg -> "-s" ++ unpack sg) msg , fmap (\(Resolution r) -> printf "--resolution %f" r) mr , fmap (\o -> printf "--optimize %d" (fromEnum o)) mo , Just images ] actionXml :: Path Abs File -> Beamline -> SomeDataCollection -> Path Abs File -> Action () actionXml xml b c uploaded = do need [toFilePath xml] container <- liftIO . fromFile . toFilePath $ xml -- post processing let attachment = _autoProcProgramAttachment . _autoProcProgramContainer $ container attachment' <- liftIO $ runReaderT (toRuchePath attachment) b _ <- copyAttachment' attachment attachment' let container' = (autoProcProgramContainer . autoProcProgramAttachment .~ attachment') container -- replace attachement -- upload into ISPYB liftIO $ storeAutoProcIntoISPyB c NoAnomalous container' cmd_ ("touch" :: String) (toFilePath uploaded) My users want to change the arguments in the processXdsMe cmd args. How can I teach shake to rebuild a rules when the argument of the cmd change ? thanks for your help. Cheers

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GHC 8.8 and MonadFail proposal
by Kazu Yamamoto 17 Apr '19

17 Apr '19

Hello, I built the GHC 8.8 branch and tried to use it. I was very surprised because many packages cannot be built due to the removal of the "fail" method from the Monad typeclass. GHC 8.6 (with -Wcompat) does not warn even if "fail" is defined. I had no chance to notice this change. Isn't this change too radical? The same thing happened in GHC 8.4 due to Semigroup. I could not notice this change beforehand because I did not know the "-Wcompat" option. In my opinion, we should have included "-Wcampat" in "-Wall". Anyway, if I understand correctly, "-Wcompat" does not help in the MonadFail case. I'm a fan of both proposal. But I feel that no period of warnings is not a good approach. --Kazu

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Compose Conference tickets are now available
by MightyByte 16 Apr '19

16 Apr '19

I'm pleased to announce that ticket sales are now open for both the New York Compose Conference and Unconference happening June 22 - 25. See our website for more details: http://www.composeconference.org/ There is one week left until the talk submission deadline. So, if you would like to submit a talk, make sure you get it in before April 23 at 11:59PM EDT. Part of our mission is to be as open and inclusive as possible. To support this, we have a limited number of volunteer tickets available if you would like to attend but the ticket price is an obstacle for you. Email us at nyc(a)composeconference.org with some information about you, your situation, and what you would like to get out of the conference and we will work with you. Sent via Superhuman ( https://sprh.mn/?vip=mightybyte@gmail.com )

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