September 2018 - Haskell-Cafe

Senior Haskell Developer opening at Mpowered
by Emil Axelsson 05 Sep '18

05 Sep '18

We are looking for a Haskeller to help us move our application forward using Haskell. We're a remote-only team based in South Africa and Sweden. Bonus if you've used some Nix or Ruby before! https://mpowered.co.za/jobs/ / Emil

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ANN: network 2.8.0.0
by evan＠evan-borden.com 04 Sep '18

04 Sep '18

Announcing the release of network 2.8.0.0 http://hackage.haskell.org/package/network-2.8.0.0 Version 2.8.0.0 * Breaking change: PortNumber originally contained Word32 in network byte order and used "deriving Ord". This results in strange behavior on the Ord instance. Now PortNumber holds Word32 in host byte order. [#347](https://github.com/haskell/network/pull/347) * Breaking change: stopping the export of the PortNum constructor in PortNumber. * Use bytestring == 0.10.* only. * Use base >= 4.7 && < 5. -- Evan Borden

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[Haskell, FP] Anduril Industries is Hiring
by Travis Whitaker 04 Sep '18

04 Sep '18

Anduril Industries (https://www.anduril.com/careers) is hiring. Come write Haskell, Nix, and a bit of C++ to solve problems in detection, tracking, hardware interfaces, sensor fusion, and computer vision. Here are just a few of the things we're hacking on in Haskell: - Nix workflow tools for cross-compilation, CI, deployment, and upgrades over heterogeneous, unreliable infrastructure. - Radar signal processing (with CUDA via *Accelerate*), target detection, and real-time visualization. - sUAS controls and mission planning. - Comprehensively tested (i.e. thoroughly QuickCheck'd/SmallCheck'd) implementations of industrial hardware interface protocols like CAN, CANopen, MAVLink, etc., as well as internally developed protocols. - High-reliability systems for performing health checks and over-the-air firmware upgrades of embedded systems deployed in remote environments. - A library of low-latency, high-throughput video processing components, used for performing image stabilization, object detection, and transcoding in real time on streaming video. - TUI debugging tools built with *brick*. We're looking for junior and senior devs who are able to relocate to our lab in Orange County, California, USA. If this sounds interesting, shoot me an email at travis(a)anduril.com Thanks for reading! Travis Whitaker

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Building stack projects as docker images
by Daniel Rolls 04 Sep '18

04 Sep '18

I am building Haskell web services using docker. I do this utilising the stack tool within the official Haskell Docker image. Doing this has a number of advantages - It's predictable - I can control bringing in new versions of stack and ghc. - It's easy to share - no stack or Haskell installation is required to build the image - These services can interact with other services in a controlled environment using Docker compose. One downside is that I doing this naively means waiting for a modules to download every time docker build is run since the stack build command will start from scratch each time. I worked around this by adding two build steps. The first runs a hello world example with the stack.yml and package.yml from my project. The second build builds the intended code. This trick separated the downloading of dependent libraries from building the code and means day to day code changes are quick to build since the first stack build is cached as a docker image layer. Still though, making a small change to a stack yml file means a long wait re-downloading all dependent libraries. Firstly, is there a better way of doing this? I know stack supports building within docker but when I worked this way I still found projects depended on the system stack and would commonly fail due to stack bugs. Secondly, my two "stack build" calls where the first builds a hello world program feels a bit hacky. is there a better way to achieve this? Thanks, Dan

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Diagrams
by Roger Mason 04 Sep '18

04 Sep '18

Hello, Does anyone know the current status of the Diagrams package? I'm having some trouble and wonder if anyone can help, given the last release was in 2016. Thanks, Roger

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HDBC packages looking for maintainer
by Erik Hesselink 04 Sep '18

04 Sep '18

Hi all, I've been the maintainer for some of the HDBC packages for a while now. Sadly, I've mostly neglected them due to lack of time and usage. While the packages mostly work, there are occasional pull requests and updates for new compiler versions. Because of this I'm looking for someone who wants to take over HDBC and related packages [1]. If you use HDBC and would like to take over maintainership, please let me know and we can get things set up. Regards, Erik [1] https://github.com/hdbc

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Re: [Haskell-cafe] Alternative instance for non-backtracking parsers
by Olaf Klinke 03 Sep '18

03 Sep '18

> Hello, Olaf. I have some distrust of elegant solutions (one of them are > C.P. libs). I have a program that parses several CSV files, one of them 50MB in size, and writes its result as HTML. When I started optimizing, the execution time was 144 seconds. Profiling (thanks to Jasper Van der Jeugt for writing profiteur!) revealed that most of the time was spent parsing and postprocessing the 50MB CSV file. Changing the data structure of the postprocessing stage cut down the execution time to 32 seconds, but still the majority is spent on parsing. Then I realized that (StateT String Maybe) is a parser which conveniently has all the class instances one needs, most notably its Alternative instance make it a backtracking parser. After defining a few combinators I was able to swap out my megaparsec parser against the new parser, which slashed execution time in half. Now most of the parsing time is dedicated to transforming text to numbers and dates. I doubt that parsing time can be reduced much further [*]. The new parser was identical to the old parser, only the combinators now come from another module. That is the elegant thing about monadic parser libraries. I will now use the fast parser by default, and if it returns a Nothing, the program will suggest a command line flag that switches to the original megaparsec parser, exactly telling the user where the parse failed and why. I am not sure whether there is another family of parsers that have interfaces so similar that switching from one package to another is as effortless as monadic parsers. Cheers Olaf [*] To the parser experts on this list: How much time should a parser take that processes a 50MB, 130000-line text file, extracting 5 values (String, UTCTime, Int, Double) from each line?

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Testing of GHC extensions & optimizations
by Rodrigo Stevaux 03 Sep '18

03 Sep '18

Hi, For those familiar with GHC source code & internals, how are extensions & optimizations tested? And what are the quality policies for accepting new code into GHC? I am interested in testing compilers in general using random testing. Is it used on GHC?

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Applying typed hole proposal leads to compilation failure
by Vlatko Basic 03 Sep '18

03 Sep '18

Hello Cafe, I encountered a strange issue regarding typed hole. If I ask GHC (8.4.3) for a typed hole proposal, it says 'use c', but if I use 'c', it complains that it can't unify 'c' and 'c1'. Why does GHC think those two are different and how to tell to GHC they are the same? Best regards, vlatko *Typed hole proposal:* • Found type wildcard ‘_c’ standing for ‘c’ Where: ‘c’ is a rigid type variable bound by the type signature for: f1 :: forall (m :: * -> *) c. MonadIO m => c -> m Bool at Test.hs:15:1-32 • In the type signature: run :: MS _c Int a -> (Either String a, Int) *Error when typed hole used:* • Couldn't match type ‘c1’ with ‘c’ ‘c1’ is a rigid type variable bound by the type signature for: run :: forall c1 a. MS c1 Int a -> (Either String a, Int) at Test.hs:21:5-47 ‘c’ is a rigid type variable bound by the type signature for: f1 :: forall (m :: * -> *) c. MonadIO m => c -> m Bool at Test.hs:15:1-32 Expected type: MS c1 Int a -> (Either String a, Int) Actual type: MS c Int a -> (Either String a, Int) * * *Minimal runnable example:* {-# LANGUAGE ScopedTypeVariables, PartialTypeSignatures #-} module Test where import Prelude import Control.Monad.IO.Class (MonadIO) import Control.Monad.Trans.Except (ExceptT, runExceptT) import Control.Monad.Trans.Reader (Reader, runReader) import Control.Monad.Trans.State (StateT, runStateT) type MS c s = ExceptT String (StateT s (Reader c)) runMS :: c -> s -> MS c s a -> (Either String a, s) runMS c s f = runReader (runStateT (runExceptT f) s) c f1 :: (MonadIO m) => c -> m () f1 c = do let _x1 = run f2 let _x2 = run f3 return () where run :: MS *_c*Int a -> (Either String a, Int) --- failure or success, 'c' or '_' run = runMS c 0 f2 :: MS c s Bool f2 = pure False f3 :: MS c s [Int] f3 = pure []

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Re: [Haskell-cafe] Alternative instance for non-backtracking parsers
by Olaf Klinke 02 Sep '18

02 Sep '18

Thanks Bardur for the pointer to bytesting-lexing and cassava. The problem is that my csv files are a little bit idiosyncratic. That is, they have BOM, semicolons as separators, The fractional numbers are in the format 1.234,567 and there are dates to parse, too. I tried parseTimeM from the time package, but that is slower than my own parser. That said, my megaparsec parser seems to spend quite some time skipping over text with regex ';"[^"]*"', that is, fields whose content does not concern the application at hand. Hence using a CSV library for tokenizing might be a good idea. Thanks to Peter Simons for pointing out that cassava indeed has attoparsec as a dependency. Maybe CSV is a red herring, after all. It's just a some text-based syntax where the count of semicolons indicate where in the line I can find the data I'm interested in. I'm more curious about how to make number conversion fast. I've looked at the source of megaparsec-6.5.0, attoparsec-0.13.2.2, bytestring-lexing-0.5.0.2 and base-4.11.1.0. They all do the same thing: Convert digits to numbers individually, then fold the list of digits as follows: f x digit = x * 10 + value digit number = foldl' f 0 digits For 'value' above, Megaparsec uses Data.Char.digitToInt while Attoparsec uses Data.Char.ord. I also rolled my own Double parser for locale reasons. Are there any libraries that handle all the formats 1,234.567 1234.567 1.234,567 1234,567 maybe by specifying a locale up front? It can't be done without, since 123.456 on its own is ambigous. Maybe the locale can be guessed from the context, maybe not, but that is certainly an expensive operation. MS Excel guesses eagerly, with occasional amusing consequences. Thanks to all who contributed to this thread so far! Olaf

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