February 2020 - Haskell-Cafe

stack: pandoc fails to build as dependency
by Olaf Klinke 06 Aug '20

06 Aug '20

Dear cafe, I am baffled to learn that stack does not provide builds as reproducible as advertised. I have a project that depends on pandoc. It builds fine on my Debian 9 machine. Then I check out the project on a Debian 8 machine. Downloading and building pandoc-2.7.3 with the latest stack-2.1.3 on the Debian 8 machine runs fine. Now I create a new project, using the same stack.yaml as in the pandoc-2.7.3 package, except now pandoc-2.7.3 is an extra-dep. The project's only module contains nothing but: module Test where import Text.Pandoc Building this project with the same stack results in a linker error: pandoc > Preprocessing executable 'pandoc' for pandoc-2.7.3.. pandoc > Building executable 'pandoc' for pandoc-2.7.3.. pandoc > [1 of 2] Compiling Main pandoc > [2 of 2] Compiling Paths_pandoc pandoc > Linking .stack-work/dist/x86_64-linux/Cabal-2.4.0.1/build/pandoc/pandoc ... pandoc > collect2: error: ld returned 1 exit status pandoc > `gcc' failed in phase `Linker'. (Exit code: 1) This seems to be independent of the LTS version (the stack.yaml in pandoc-2.7.3 says lts-13.17 although lts-13.17 contains pandoc-2.5 and pandoc-2.7.3 is available only in 14.9). Can somebody hint what I am doing wrong? Should I file a bug report? Thanks Olaf

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Re: [Haskell-cafe] [ANNOUNCE] GHC 8.8.3 is now available
by Ben Gamari 29 Feb '20

29 Feb '20

Ben Gamari <ben(a)well-typed.com> writes: > --text follows this line-- > <#part sign=pgpmime> > > Hello everyone, > > The GHC team is proud to announce the release of GHC 8.8.3. The source > distribution, binary distributions, and documentation are available at > > https://downloads.haskell.org/~ghc/8.8.3 > > Release notes are also available [1]. > Unfortunately there has been a slight mix-up in the binary distributions associated with this release and I will need to do a re-spin. I have pulled down the binary distributions for the time being. They should be back in a few hours. Many apologies for the confusion! Cheers, - Ben

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How to deal with HTML input select option on a form using in a scotty and Text.Blaze.Html5
by Tebogo Sikwane 26 Feb '20

26 Feb '20

I am using scotty and Text.Blaze.Html5 Text.Digestive Text.Digestive.Blaze.Html5 What I want to achieve is a list of options as shown below. I have a Option sum type and will like to restrict the user to those options. data Options = Option1 | Option2 | Option3 | Option4 deriving Show Below is the Register record typeY data Register = Register { regName::Text , regOption::Options } deriving (Show,Eq) My problem is with Text.Digestive.choice. I don't know how to use it with Text.Digestive.Blaze.Html5 and will appreciate your help on the correct way to achieve this. I have tried to use the code below but battle to get it right. registerForm :: Monad m => Form Text m Register registerForm = Register <$> "name" .: text Nothing <*> "options" .: choice options Nothing where options= [(Option1,"Option1"),(Option2,"Option2"), (Option3, "Option3"),Option4,"Option4"] registerView:: View H.Html -> H.Html registerView view = do label "name" view "Name: " inputText "name" view H.br label "options" view "Options: " inputSelect "options" view H.br I get the following error when I compile my code: /root/work/src/Adapter/HTTP/Web/Auth.hs:107:29: error: • Couldn't match type ‘Text’ with ‘Char’ arising from the literal ‘"Option1"’ • In the first argument of ‘pack’, namely ‘"Option1"’ In the expression: pack "Option1" In the expression: (Option1, pack "Option1") | 107 | options= [(Option1,pack "Option1"),(Option2,pack "Option2"),(Option3,pack "Option3"),(Option4,pack "Option4")] Your help will be highly appreciated.

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GHC profiling overhead
by Kolář Dušan 25 Feb '20

25 Feb '20

Dear Café, I'm quite curious about overhead of the profiler... I've compiled my program with --make -threaded -rtsopts -O2 -prof options. It was started with +RTS -N5 -p Usually, the program evaluates like 6-7 hours of real time. What is the overhead of the profiling? After 8 hours of working, the program has not finished yet. I've looked over the Internet and it seems like the overhead may be 25 times slowdown and even worse. Is it true? If yes, it's not worth waiting then :-) Best regards, Dusan

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[ANNOUNCE] GHC 8.8.2 is now available
by Ben Gamari 25 Feb '20

25 Feb '20

Hello everyone, The GHC team is proud to announce the release of GHC 8.8.2. The source distribution, binary distributions, and documentation are available at https://downloads.haskell.org/~ghc/8.8.2 Release notes are also available [1]. This release fixes a handful of issues affecting 8.8.1: - A bug (#17088) in the compacting garbage collector resulting in segmentations faults under specific circumstances. Note that this may affect user programs even if they did not explicitly request the compacting GC (using the -c RTS flag) since GHC may fallback to compacting collection during times of high memory pressure. - A code generation bug (#17334) resulting in GHC panics has been fixed. - A bug in the `process` library causing builds using `hsc2hs` to fail non-deterministically on Windows has been fixed (Trac #17480) - A typechecker bug (#12088) resulting in programs being unexpectedly rejected has been fixed. - A bug in the implementation of compact normal forms resulting in segmentation faults in some uses (#17044) has been fixed. - A bug causing GHC to incorrectly complain about incompatible LLVM versions when using LLVM 7.0.1 has been fixed (#16990). As always, if anything looks amiss do let us know. Happy compiling! Cheers, - Ben [1] https://downloads.haskell.org/ghc/8.8.2/docs/html/users_guide/8.8.2-notes.h…

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Lecturer, Senior Lecturer or Associate Professor in Programming Languages: University of Bristol
by Steven Ramsay 23 Feb '20

23 Feb '20

The University of Bristol are seeking to appoint an assistant/associate professor in Programming Languages, those with expertise in functional programming especially welcome! (Postdoc and funded PhD positions also available). Position: Lecturer, Senior Lecturer or Associate Professor in Programming Languages Location: University of Bristol, UK Type: Full time, open-ended contract Salary: £38,017 -- £59,135 per annum Applications deadline: March 11 Website: http://www.bristol.ac.uk/jobs/find/details.html?nPostingId=65334&nPostingTa… The Department of Computer Science at the University of Bristol is seeking to appoint a full-time faculty member in Programming Languages at Lecturer, Senior Lecturer or Associate Professor level; comparable to Assistant / Associate Professorship. You will be expected to both deliver outstanding teaching and undertake internationally leading research in the area of Programming Languages. There is opportunity to play a significant role in shaping and leading a new Programming Languages research group. The Department of Computer Science is an international centre of excellence in the foundations and applications of computing, ranked 4th in the UK for research intensity by the 2014 REF. The Department is already home to expertise in functional programming, program analysis, algorithms and complexity; it benefits from a close association with neighbouring groups in Cryptography and Trustworthy Systems. The University of Bristol is a leading institution among the UK's Russell Group Universities and is regularly placed among the top-ranking institutions in global league tables. Bristol is a friendly, green, coastal city with a rich and lively cultural scene and has consistently been voted one of the best places to live in the UK. This post is being offered on a full-time, open-ended contract. A recruitment supplement scheme is available up to £5K. For informal discussion about the post you are welcome to contact: Seth Bullock (Head of Department, mailto:seth.bullock@bristol.ac.uk) or Steven Ramsay (Lecturer in Programming Languages, mailto:steven.ramsay@bristol.ac.uk). The closing date for all applications is 23:59 on Wednesday 11 March 2020. Interviews are expected to take place shortly after the closing date.

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A static analyzer
by Eirik Ravnskog 23 Feb '20

23 Feb '20

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Munich Haskell Meeting, 2020-02-25 @ 19:30
by Leah Neukirchen 22 Feb '20

22 Feb '20

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Tueday, February 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-feb-2020/ Everybody is welcome! cu, -- Leah Neukirchen <leah(a)vuxu.org> https://leahneukirchen.org/

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[Ann] base16, base16-lens, base32, base32-lens
by Emily Pillmore 18 Feb '20

18 Feb '20

Hi Everyone, It's been a productive weekend. I'm pleased to announce the following packages: * https://hackage.haskell.org/package/base16 * https://hackage.haskell.org/package/base16-lens * https://hackage.haskell.org/package/base32 * https://hackage.haskell.org/package/base32-lens These libraries are the conclusion of a small audit I've been doing of the existing base encoding implementations, focusing on performance and providing a common interface for both ByteString and Text values. As always, these libraries have a pretty obvious optical interface, and that has been provided as well. Cheers, and happy President's Day weekend! - Emily Pillmore

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Seeking help with polysemy interpreter ..
by Debasish Ghosh 17 Feb '20

17 Feb '20

Hello - I have a typeclass like this .. class (Monad m) => DB m where query :: Text -> m (Maybe Account) ... and an instance that uses persistent and sqlite .. instance (MonadIO m) => DB (SqlPersistT m) where query ano = get (AccountKey ano) .. Now I want to move this to using an effect system - Polysemy .. data DB m a where Query :: Text -> DB m (Maybe Account) ... makeSem ''DB which generates the functions .. I need some help figuring out the type of the function that interpretes the above .. An initial intuition is this .. runDB :: Member SqlPersistT r => Sem (DB ': r) a -> Sem r a runDB = interpret $ \case Query ano -> get (AccountKey ano) which doesn't compile .. errors out with .. Couldn't match type ‘Control.Monad.Trans.Reader.ReaderT backend0 m0 (Maybe Account)’ with ‘Sem r (Maybe Account)’ Expected type: Sem r x Actual type: Control.Monad.Trans.Reader.ReaderT backend0 m0 (Maybe Account) • In the expression: get (AccountKey accountNo) I think I am missing something here. The type of interpret is shown as interpret :: (forall x (m :: * -> *). DB m x -> Sem r x) -> Sem (DB : r) a -> Sem r a But how do I translate the SqlPersistT part ? Any help will be appreciated. regards. -- Debasish Ghosh http://manning.com/ghosh2 http://manning.com/ghosh Twttr: @debasishg Blog: http://debasishg.blogspot.com Code: http://github.com/debasishg

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