- Haskell-Cafe - Haskell.org

Why are && and || right-associative?
by Richard Eisenberg 24 Apr '19

24 Apr '19

Hi café, Why are && and || in the Prelude right-associative? This contradicts my expectation and the way these work in other languages. That said, I can't think of any harm in it. This came up from a question asked by a student, and I have no idea why the design is this way. Thanks, Richard

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[ANNOUNCE] GHC 8.6.5 is now available
by Ben Gamari 24 Apr '19

24 Apr '19

Hello everyone, The GHC team is proud to announce the release of GHC 8.6.5. The source distribution, binary distributions, and documentation are available at https://downloads.haskell.org/~ghc/8.6.5 Release notes are also available [1]. This release fixes a handful of issues with 8.6.4: - Binary distributions once again ship with Haddock documentation including syntax highlighted source of core libraries (#16445) - A build system issue where use of GCC with `-flto` broke `configure` was fixed (#16440) - An bug affecting Windows platforms wherein XMM register values could be mangled when entering STG has been fixed (#16514) - Several Windows packaging issues resulting from the recent CI rework. (#16408, #16398, #16516) Do note that due to linking issues (see #15934) profiling will be rather unreliable on Windows. This will be fixed in 8.8.1. As always, if anything looks amiss do let us know. Happy compiling! Cheers, - Ben [1] http://downloads.haskell.org/~ghc/8.6.5/docs/html/users_guide/8.6.5-notes.h…

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Munich Haskell Meeting, 2019-04-25 @ 19:30
by Leah Neukirchen 23 Apr '19

23 Apr '19

Dear all, This week, our monthly Munich Haskell Meeting will take place again on Thursday, April 25 at Cafe Puck at 19h30. For details see here: http://muenchen.haskell.bayern/dates.html If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-apr-2019/ Everybody is welcome! cu, -- Leah Neukirchen <leah(a)vuxu.org> http://leahneukirchen.org/

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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 23 Apr '19

23 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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