- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] ANN: leancheck-v0.7.4 with providers for Tasty, Hspec and test-framework
by waldmann 12 Sep '18

12 Sep '18

Hi. I do prefer the enumerative approach for test case generation over the randomized one (quickcheck) because I have use cases (testing student code) where I want to avoid IO, and I just want a pure list of (small) counterexamples. Among the enumerative testing libraries, the "competition" is between leancheck and smallcheck. For, me the important differences are Enumeration of algebraic data types (ADTs), via built-in serial combinators, e.g., cons0 Leaf \/ cons2 Branch * leancheck enumerate by size (number of nodes in tree) * smallcheck enumerates by depth (longest path in tree) I (much!) prefer "by size" because this delays combinatorial explosion somewhat. Automated Listable/Serial instances for ADTs * leancheck uses template-haskell * smallcheck uses DeriveGeneric Here I would actually prefer DeriveGeneric. But this certainly could be added to leancheck? Oh, and leancheck does not use fancy typeclassery (smallcheck needs Multiparamtypeclasses and Monads just to declare a Serial instance) and it is also easier to get at the list of counterexamples. - J.W.

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Is it possible to change the environment (reader) in applicative style?
by Olaf Klinke 12 Sep '18

12 Sep '18

Neither >>= nor 'return' nor 'join' for the reader monad change the environment, hence the name "reader". Thus I'd chime in and claim it is not possible using the Applicative or Monad combinators alone. After all, the reader monad is ignorant of what type it is reading. In other words, if the type class combinators alone could do this, then you'd have a function that worked for _every_ type Env. But updating a list of (variable,value) pairs is a rather specific task. Fixing the syntax for this post: type Reader a = Env -> a Since your 'let' function modifies the environment, it yields a function of type Env -> Env = Reader Env Say that your 'let' has the type let :: Def -> Reader Env for a suitable type Def, e.g. Def = (String,Int). Suppose you have implemented 'let'. We know the Applicative and Monad type class functions won't help here. But observe that (.) can be specialized to (.) :: Reader a -> Reader Env -> Reader a whence you can write eval term . let newBinding where term :: Term eval :: Term -> Reader Int let :: Def -> Reader Env newBinding :: Def Cheers, Olaf

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Is it possible to change the environment (reader) in applicative style?
by Rodrigo Stevaux 12 Sep '18

12 Sep '18

It is easy to read an environment in applicative style, ie: type Env = [(String, Int)] data Term = Add Term Term | Number Int | Var String deriving Show eval :: Term -> Env -> Int eval (Add a b) = (+) <$> eval a <*> eval b eval (Var name) = fetch name eval (Number i) = pure i fetch :: String -> Env -> Int fetch name = fromJust . lookup name But can the eval function change the Env being passed, as to implement a "let" operation, without using monads? I tried I lot but ultimately I resorted to (>>=) in the function monad: bind f k = \r -> k (f r) r I do not think so, because in applicative style each operand can have an effect (reading the environment) but can not affect other operands (including the next ones), i.e., there is no notion of sequencing in applicatives Is this reasoning right?

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Adding a custom lib to stack project
by Yotam Ohad 11 Sep '18

11 Sep '18

Hi, I made a lib from a cpp project with one function: BOOL Foo(LPCSTR bar) In the stack project I added the .lib file's folder to the extra-lib-dirs/extra-include-dirs and then, in main: {-# LANGUAGE ForeignFunctionInterface #-} module Main where import System.Win32.Types import Foreign.C.String main :: IO () main = do withCString "bar" c_Foo putStrLn "success" foreign import ccall "bindings.lib Foo" c_Foo :: LPCSTR -> IO BOOL When building I get the following error Building all executables for `tape' once. After a successful build of all of them, only specified executables will be rebuilt. tape-0.1.0.0: build (lib + exe) Preprocessing library for tape-0.1.0.0.. Building library for tape-0.1.0.0.. ignoring (possibly broken) abi-depends field for packages Preprocessing executable 'tape-exe' for tape-0.1.0.0.. Building executable 'tape-exe' for tape-0.1.0.0.. Linking .stack-work\dist\7d103d30\build\tape-exe\tape-exe.exe ... .stack-work\dist\7d103d30\build\tape-exe\tape-exe-tmp\Main.o:fake:(.text+0x102): undefined reference to `CreateDebuggedProcess' collect2.exe: error: ld returned 1 exit status `gcc.exe' failed in phase `Linker'. (Exit code: 1) -- While building custom Setup.hs for package tape-0.1.0.0 using: C:\sr\setup-exe-cache\x86_64-windows\Cabal-simple_Z6RU0evB_2.2.0.1_ghc-8.4.3.exe --builddir=.stack-work\dist\7d103d30 build lib:tape exe:tape-exe --ghc-options " -ddump-hi -ddump-to-file -fdiagnostics-color=always" Process exited with code: ExitFailure What am I doing wrong? Yotam

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Call for Papers: Postproceedings for ThEdu'18 by EPTCS
by Achim D. Brucker 11 Sep '18

11 Sep '18

(Apologies for duplicates) Open Call for Papers ************************************************************************** Postproceedings for ThEdu'18 by EPTCS Theorem proving components for Educational software http://www.uc.pt/en/congressos/thedu/thedu18 ************************************************************************** Workshop ThEdu at FLoC Federated Logic Conference 2018 http://www.floc2018.org/ ************************************************************************** THedu'18 Postproceedings: ThEdu's programme comprised seven contributions, presented also in the webpages. Now postproceedings are planned to collect the contributions upgraded to full papers. The contributions' topics are diverse according to ThEdu's scope, and this is a call open for everyone, also those who did not participate in the workshop. All papers will undergo review according to EPTCS standards. THedu'18 Scope: Computer Theorem Proving is becoming a paradigm as well as a technological base for a new generation of educational software in science, technology, engineering and mathematics. The workshop brings together experts in automated deduction with experts in education in order to further clarify the shape of the new software generation and to discuss existing systems. Topics of interest include: * methods of automated deduction applied to checking students' input; * methods of automated deduction applied to prove post-conditions for particular problem solutions; * combinations of deduction and computation enabling systems to propose next steps; * automated provers specific for dynamic geometry systems; * proof and proving in mathematics education. Important Dates * 2nd call for papers: 10 Sep 2018 * Submission (full papers): 18 Nov 2018 * Notification of acceptance: 17 Dec 2018 * Revised papers due: 21 Jan 2019 Submission We welcome submission of papers presenting original unpublished work which is not been submitted for publication elsewhere. The authors should comply with the "instructions for authors", LaTeX style files and accept the "Non-exclusive license to distribute" of EPTCS: Instructions for authors (http://info.eptcs.org/) LaTeX style file and formatting instructions (http://style.eptcs.org/) Copyright (http://copyright.eptcs.org/) Papers should be submitted via easychair, https://easychair.org/conferences/?conf=thedu18. In case the contributions finally do not reach the standards of EPTCS in number, there will be an alternative to publish as a techreport at CISUC https://www.cisuc.uc.pt/publications. Program Committee Francisco Botana, University of Vigo at Pontevedra, Spain Roman Hašek, University of South Bohemia, Czech Republic Filip Maric, University of Belgrade, Serbia Walther Neuper, Graz University of Technology, Austria (co-chair) Pavel Pech, University of South Bohemia, Czech Republic Pedro Quaresma, University of Coimbra, Portugal (co-chair) Vanda Santos, CISUC, Portugal Wolfgang Schreiner, Johannes Kepler University, Austria -- Dr. Achim D. Brucker | Software Assurance & Security | University of Sheffield https://www.brucker.ch | https://logicalhacking.com/blog @adbrucker | @logicalhacking

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modules as implicit data structures
by Petr Pudlák 11 Sep '18

11 Sep '18

Hi Cafe, Let's say we have a module like module Foo where foo, bar :: String -> Something ... and use it elsewhere as import qualified Foo as F ... F.foo ... This is almost just if we had a data type defined as data FooModule = FooModule { foo :: String -> Something, bar :: String -> Something } together with a singleton `f : :FooModule` and referenced it as f.foo, f.bar etc. Has some language explored this idea of making modules explicit as language-level objects? It seems that there could be some interesting possibilities, such as: - Abstract modules (just a definition of the data type). Then - Being able to replace a module with a different one (like a fake one for testing). - Polymorphic modules that could be instantiated for specific types. Thanks, Petr

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Difference between `type` and `newtype` in type checking
by Rodrigo Stevaux 11 Sep '18

11 Sep '18

Hi, I was studying this post ( http://www.haskellforall.com/2012/12/the-continuation-monad.html) on CPS and I tried the following code: module Main where newtype Cont r a = Cont { runCont :: (a -> r) -> r } onInput :: Cont (IO ()) String onInput f = do s <- getLine f s onInput f main :: IO () main = onInput print I fails to compile: "Couldn't match expected type ‘Cont (IO ()) String’ with actual type ‘(String -> IO a0) -> IO b0’ • The equation(s) for ‘onInput’ have one argument, but its type ‘Cont (IO ()) String’ has none" But I thought Cont a b would be expanded to (b -> a) -> a so that Cont (IO ()) String became (String -> IO ()) -> IO (), and if I give that type using `type` instead of `newtype`, it does type-check: type Cont r a = (a -> r) -> r What am I missing here about Haskell? thanks folks!

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ANN: proto-lens-0.4.0.0
by Judah Jacobson 10 Sep '18

10 Sep '18

I'm pleased to announce the release of proto-lens-0.4.0.0. The library provides an API for protocol buffers, a language-independent binary file format. Cabal and Stack projects can use proto-lens to automatically generate Haskell source bindings from the original protocol buffer specifications. Some significant changes in this new release include: - Simplifying the overloaded lens instances to improve readability and type error messages (along with the associated release of lens-labels-0.3.0.0) - Switching to a custom class for default messages - Hiding the internals of generated message types - Making the Show instances more readable and concise - Splitting the Cabal support into finer-grained packages, for better integration with Nix and Bazel For general library documentation and tutorials: https://google.github.io/proto-lens/ For assistance in migrating from earlier versions: https://github.com/google/proto-lens/wiki/Migration-Guide For detailed changelogs: https://hackage.haskell.org/package/proto-lens/changelog https://hackage.haskell.org/package/proto-lens-protoc/changelog Best, -Judah

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Mathematics of Program Construction (MPC), Portugal, 2019
by Graham Hutton 10 Sep '18

10 Sep '18

Dear all, The next Mathematics of Program Construction (MPC) conference will be held in the historic city of Porto, Portugal in October 2019, co-located with the Symposium on Formal Methods (FM). Please share, and submit your best papers! Best wishes, Graham Hutton Program Chair, MPC 2019 ====================================================================== 13th International Conference on Mathematics of Program Construction 7-9 October 2019, Porto, Portugal Co-located with Formal Methods 2019 https://tinyurl.com/MPC-Porto ====================================================================== BACKGROUND: The International Conference on Mathematics of Program Construction (MPC) aims to promote the development of mathematical principles and techniques that are demonstrably practical and effective in the process of constructing computer programs. MPC 2019 will be held in Porto, Portugal from 7-9 October 2019, and is co-located with the International Symposium on Formal Methods, FM 2019. Previous conferences were held in Königswinter, Germany (2015); Madrid, Spain (2012); Québec City, Canada (2010); Marseille, France (2008); Kuressaare, Estonia (2006); Stirling, UK (2004); Dagstuhl, Germany (2002); Ponte de Lima, Portugal (2000); Marstrand, Sweden (1998); Kloster Irsee, Germany (1995); Oxford, UK (1992); Twente, The Netherlands (1989). SCOPE: MPC seeks original papers on mathematical methods and tools put to use in program construction. Topics of interest range from algorithmics to support for program construction in programming languages and systems. Typical areas include type systems, program analysis and transformation, programming language semantics, security, and program logics. The notion of a 'program' is interpreted broadly, ranging from algorithms to hardware. Theoretical contributions are welcome, provided that their relevance to program construction is clear. Reports on applications are welcome, provided that their mathematical basis is evident. We also encourage the submission of 'programming pearls' that present elegant and instructive examples of the mathematics of program construction. IMPORTANT DATES: Abstract submission 26th April 2019 Paper submission 3rd May 2019 Author notification 14th June 2019 Camera ready copy 12th July 2019 Conference 7-9 October 2019 SUBMISSION: Submission is in two stages. Abstracts (plain text, maximum 250 words) must be submitted by 26th April 2019. Full papers (pdf, formatted using the llncs.sty style file for LaTex) must be submitted by 3rd May 2019. There is no prescribed page limit, but authors should strive for brevity. Both abstracts and papers will be submitted using EasyChair. Papers must present previously unpublished work, and not be submitted concurrently to any other publication venue. Submissions will be evaluated by the program committee according to their relevance, correctness, significance, originality, and clarity. Each submission should explain its contributions in both general and technical terms, clearly identifying what has been accomplished, explaining why it is significant, and comparing it with previous work. Accepted papers must be presented in person at the conference by one of the authors. The proceedings of MPC 2019 will be published in the Lecture Notes in Computer Science (LNCS) series, as with all previous instances of the conference. Authors of accepted papers will be expected to transfer copyright to Springer for this purpose. After the conference, authors of the best papers from MPC 2019 and MPC 2015 will be invited to submit revised versions to a special issue of Science of Computer Programming (SCP). For any queries about submission please contact the program chair, Graham Hutton <graham.hutton(a)nottingham.ac.uk>. PROGRAM COMMITTEE: Patrick Bahr IT University of Copenhagen, Denmark Richard Bird University of Oxford, UK Corina Cîrstea University of Southampton, UK Brijesh Dongol University of Surrey, UK João F. Ferreira University of Lisbon, Portugal Jennifer Hackett University of Nottingham, UK William Harrison University of Missouri, USA Ralf Hinze University of Kaiserslautern, Germany Zhenjiang Hu National Institute of Informatics, Japan Graham Hutton (chair) University of Nottingham, UK Cezar Ionescu University of Oxford, UK Mauro Jaskelioff National University of Rosario, Argentina Ranjit Jhala University of California, San Diego, USA Ekaterina Komendantskaya Heriot-Watt University, UK Bernhard Möller University of Augsburg, Germany Shin-Cheng Mu Academia Sinica, Taiwan Mary Sheeran Chalmers University of Technology, Sweden Alexandra Silva University College London, UK Georg Struth University of Sheffield, UK VENUE: The conference will be held at the Alfândega Porto Congress Centre, a 150 year old former custom's house located in the historic centre of Porto on the bank of the river Douro. The venue was renovated by a Pritzer prize winning architect and has received many awards. LOCAL ORGANISERS José Nuno Oliveira University of Minho, Portugal For any queries about local issues please contact the local organiser, José Nuno Oliveira <jno(a)di.uminho.pt>. ====================================================================== This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please contact the sender and delete the email and attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. Email communications with the University of Nottingham may be monitored where permitted by law.

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Re: [Haskell-cafe] ANN: leancheck-v0.7.2, enumerative QuickCheck-like testing
by Rodrigo Stevaux 09 Sep '18

09 Sep '18

Hi, nice to see a fellow Brazilian. How does your work differ from QuickCheck?

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Re: [Haskell-cafe] ANN: leancheck-v0.7.4 with providers for Tasty, Hspec and test-framework
by Rudy Matela 08 Sep '18

08 Sep '18

Hello Haskell-Café, A new version of LeanCheck is out: v0.7.4. LeanCheck is a property testing library (like QuickCheck) that tests values enumeratively rather than at random (unlike QuickCheck). _Whats new?_ I have created providers to make it easy to incorporate LeanCheck properties in Tasty, Hspec and test-framework test suites. This is optional and provided in separate packages: LeanCheck can still be used alone. LeanCheck and its framework providers are available on Hackage. You can install them with: $ cabal install leancheck $ cabal install tasty-leancheck $ cabal install hspec-leancheck $ cabal install test-framework-leancheck Check out the README files for examples of use: * https://github.com/rudymatela/leancheck * https://github.com/rudymatela/tasty-leancheck * https://github.com/rudymatela/hspec-leancheck * https://github.com/rudymatela/test-framework-leancheck Here's a sneak peek of LeanCheck+Tasty: ## Test program import Test.Tasty import Test.Tasty.LeanCheck as LC import Data.List main :: IO () main = defaultMain tests tests :: TestTree tests = testGroup "Test properties checked by LeanCheck" [ LC.testProperty "sort == sort . reverse" $ \list -> sort (list :: [Int]) == sort (reverse list) , LC.testProperty "Fermat's little theorem" $ \x -> ((x :: Integer)^7 - x) `mod` 7 == 0 -- the following property do not hold , LC.testProperty "Fermat's last theorem" $ \x y z n -> (n :: Integer) >= 3 LC.==> x^n + y^n /= (z^n :: Integer) ] ## Output for the test program $ ./test Test properties checked by LeanCheck sort == sort . reverse: OK +++ OK, passed 200 tests. Fermat's little theorem: OK +++ OK, passed 200 tests. Fermat's last theorem: FAIL *** Failed! Falsifiable (after 71 tests): 0 0 0 3 1 out of 3 tests failed (0.00s) You can't see it here, but Tasty's output is highlighted in colours: "FAIL" appears in red so you can quickly see which properties failed. Best Regards, Rudy

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common class for Set (and Map, resp.) implementations with different constraints on the keys
by Johannes Waldmann 07 Sep '18

07 Sep '18

Dear Cafe, we have Data.Set, Data.IntSet, Data.HashSet, and they all have similar API, where the only difference is the constraint on the elements. (Same thing for maps.) Can we unify this as follows: {-# language ConstraintKinds, TypeFamilies #-} class SetC s where type Con s :: * -> Constraint singleton :: (Con s a) => a -> s a foldMap :: (Con s a, Monoid m) => (a -> m) -> s a -> m ... Then for Data.Set, we write instance SetC S.Set where type Con S.Set = Ord ; ... It seems to work, and it allows me to write polymorphic code, and switch implementations from the top. Full source: https://gitlab.imn.htwk-leipzig.de/waldmann/pure-matchbox/tree/master/src/D… Example use case (switch implementation): https://gitlab.imn.htwk-leipzig.de/waldmann/pure-matchbox/blob/master/src/M… Still, there are some clumsy corners in this code, perhaps you can help: * for instance SetC HashSet, there are two constraints. I want to write type Con HashSet = \ e -> (Hashable e, Eq, e) but this does not work (there is no "type lambda"?) * for maps, I want to write class (forall k . Foldable m k) => MapC m but this seems impossible now (This is would work with -XQuantifiedConstraints ?) * in some other code using the same idea (the class exports the constraint), I had an instance where the constraint was empty. Again, I cannot write type Con Foo = \ s -> () - J.W.

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Passing creation flags to CreateProcess on windows
by Yotam Ohad 07 Sep '18

07 Sep '18

Hello cafe, I'm working on a small debugger so I would like to use the winapi CreateProcess function with the appropriate flags for debugging. Unfortunately, it seems that in Haskell <http://hackage.haskell.org/package/process-1.6.4.0/docs/System-Process.html…>there is no way to pass the flags. I would like to not use LoadLibrary to get every function I need. Thanks Yotam

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Stack project, dependency with build flags
by John Z. 06 Sep '18

06 Sep '18

Hi everyone, I'll try with tl:dr; first: I have a project with a dependency that -on my system- needs a build flag, in order to correctly link against its native dependencies. What is a proper way to set up a project like this? A longer explanation: My project uses `ncurses` library and I'm running Arch. There seems to be some religious opinion going around about the layout of ncurses headers in /usr/include, and -naturally- Arch and `ncurses` disagree. Fortunately, author of `ncurses` included a build flag to allow (easy) build on systems which follow the 'wrong' layout. Thing is, I couldn't figure out how to add this configuration to the .cabal of my own project, so for now, I'm stuck with a Makefile that invokes `stack` commands with `--flag ncurses:force-narrow-library`. I looked in the manual and there doesn't seem to be a way to handle this aside the commandline argument. So this got me thinking - is there a different way projects like these are supposed to be organized? For example, am I supposed to check out `ncurses` as a submodule of my project, then replicate this build flag on my own project and 'pass it down' somehow? Any tips are highly appreciated; I'm really not proficient with organizing stack projects properly. Cheers! -- "That gum you like is going to come back in style."

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how to Enumerate without lists?
by Johannes Waldmann 06 Sep '18

06 Sep '18

Dear Cafe (again), I was trying to write sum $ map (^ 2) $ [ 1 :: Int .. 10^8 ] in a list-free style getSum $ foldMap (Sum . (^ 2)) $ [ 1 :: Int .. 10^8 ] This avoids building an intermediate list (of squares) but it will allocate, since foldMap uses foldr (by default, and it's not overridden for lists) The conclusion would be: not to use lists at all. Which will be the point of my talk anyway. But here, we get a list from enumFromTo. Can we avoid that? Let's try: we just reify enumFromTo data Enumerator a = Enumerator {from :: a, to :: a} we have this for decades, it is called (a,a) in Data.Ix, and then instance Foldable Enumerator where ... Oh no, foldMap (and others) would need an Enum constraint! - J

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small example for space-efficiency via lazy evaluation?
by Johannes Waldmann 05 Sep '18

05 Sep '18

Dear Cafe, I wanted to demonstrate that main = print $ sum $ map (^ 2) $ [ 1 :: Int .. 10^8 ] without any optimisations, still runs in constant space because garbage is collected immediately. Except that it does not: ghc -O0 space.hs -rtsopts -ddump-simpl ./space +RTS -M80k -A10k gives me unoptimized Core as expected, but exhausts the heap. After some experimentation, I found that replacing Prelude.sum with Data.Foldable.foldl' (+) 0 achieves what I want. I think the reason is that instance Foldable [] implements sum by foldl (non-strict). Both versions will run without any allocation as soon as we compile with -O1 . So, my question is, what do you use as a (teaching) example for space-efficiency via lazy evaluation? Preferrably a one-liner, using only standard libraries, and such that the effect is not rendered moot by -O2. - J PS: It is magic that foldl and foldl' produce identical core here? $wgo_s5we (w_s5w8 :: GHC.Prim.Int#) (ww1_s5wc :: GHC.Prim.Int#) = case GHC.Prim.==# w_s5w8 ww_s5w5 of { __DEFAULT -> jump $wgo_s5we (GHC.Prim.+# w_s5w8 1#) (GHC.Prim.+# ww1_s5wc (GHC.Prim.*# w_s5w8 w_s5w8));

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Fwd: Setting RTS thread names useful?
by Michal J Gajda 05 Sep '18

05 Sep '18

It seems `GHC.Conc.labelThread` does not have corresponding `getThreadLabel`. It would be useful when redirecting all exceptions to `journald` etc. The problem might be, that this function is documented to function in debugging builds only :-(. Just like stack traces which need special options.

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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 05 Sep '18

05 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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Re: [Haskell-cafe] if-then without -else?
by Rodrigo Stevaux 31 Aug '18

31 Aug '18

> > which cannot be prohibited statically > if we want both the language to be Turing complete, > and type inference to be decidable. > > Could you be so kind as to give a hint on literature that could help me understand this statement?

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Re: [Haskell-cafe] Alternative instance for non-backtracking parsers
by Olaf Klinke 31 Aug '18

31 Aug '18

>Hello, Olaf. I have some distrust of elegant solutions (one of them are >C.P. libs). I am a mathematician, I trust a lot in elegance and beauty. As long as the performance penalty is not prohibitively large, I'd prefer a beautiful implementation over an aggressively optimized one. At least when the code does not live in some real-time system. Certainly there are cases where you absolutely need speed or low memory footprint. > For example, I read that they have problems with memory >consuming (famous article about arrow-based parsers), also your example >(my own experience includes a lot of tricks which I don't like)... So >(but this is my IMHO) for big and serious project with parsing of >something complex I will use bindings to good known parser libraries... I consider Parsec and it descendants good and known. I could not write something like that. Currently I rely on megaparsec because of its informative error messages, knowing that I can switch to one of the siblings without much effort when I need more speed. >Maybe something like this: https://github.com/markwright/antlrc. Btw, >Happy looks still live (https://github.com/simonmar/happy) Have you used any of the above? > >I use "Read"-style parsers for small constructions only, so fail of such >small parser is only case where error is producing and I do it >explicitly (as biggest example: header of Git patch). > >Olaf, what do you think about > 1. http://hackage.haskell.org/package/frisby Frisby looks quite old, as it has idiosyncratic combinators for stuff now in Data.Functor. I've never used it, so I can't tell what its strengths are. Can you? > 2. http://tanakh.github.io/Peggy Notice that in the examples of bothy frisby and peggy, parsing a number only parses a string of digits, then passes it to 'read', i.e. another parser. Is that efficient, elegant parsing? I'm nevertheless intrigued by PEG parsers, because I once read about the connection between grammars and compression: A set of production rules can be quite a bit smaller than the actual text. This turns parsing upside down: The text is constant, the grammar unknown. Olaf

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 180, Issue 32
by Michal J Gajda 31 Aug '18

31 Aug '18

I got 4.7s for similar amount of data in 2013. However I was pretty sure that fully inlined implementation could potentially go 5x faster. http://hackage.haskell.org/package/hPDB Please check xeno XML parser benchmarks for another example. https://hackage.haskell.org/package/xeno On Fri, 31 Aug 2018 at 14:41, <haskell-cafe-request(a)haskell.org> wrote: > Send Haskell-Cafe mailing list submissions to > haskell-cafe(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > or, via email, send a message with subject or body 'help' to > haskell-cafe-request(a)haskell.org > > You can reach the person managing the list at > haskell-cafe-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Haskell-Cafe digest..." > Today's Topics: > > 1. Re: HDBC packages looking for maintainer (Tobias Dammers) > 2. Re: Alternative instance for non-backtracking parsers > (Olaf Klinke) > 3. Re: Alternative instance for non-backtracking parsers > (Bardur Arantsson) > > > > ---------- Forwarded message ---------- > From: Tobias Dammers <tdammers(a)gmail.com> > To: haskell-cafe(a)haskell.org > Cc: > Bcc: > Date: Thu, 30 Aug 2018 15:24:04 +0200 > Subject: Re: [Haskell-cafe] HDBC packages looking for maintainer > Hi, > > I'd be interested. I've used HDBC on a few projects, and my yeshql > library was originally built with HDBC as the only backend. It would be > a terrible shame to see this bitrot. > > Cheers, > > Tobias (tdammers on github etc.) > > On Mon, Aug 13, 2018 at 12:07:38PM +0200, Erik Hesselink wrote: > > 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 > > > _______________________________________________ > > Haskell-Cafe mailing list > > To (un)subscribe, modify options or view archives go to: > > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > > Only members subscribed via the mailman list are allowed to post. > > > -- > Tobias Dammers - tdammers(a)gmail.com > > > > > ---------- Forwarded message ---------- > From: Olaf Klinke <olf(a)aatal-apotheke.de> > To: PY <aquagnu(a)gmail.com> > Cc: haskell-cafe <haskell-cafe(a)haskell.org> > Bcc: > Date: Thu, 30 Aug 2018 20:21:07 +0200 > Subject: Re: [Haskell-cafe] Alternative instance for non-backtracking > parsers > > 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? > > > > ---------- Forwarded message ---------- > From: Bardur Arantsson <spam(a)scientician.net> > To: haskell-cafe(a)haskell.org > Cc: > Bcc: > Date: Thu, 30 Aug 2018 21:43:55 +0200 > Subject: Re: [Haskell-cafe] Alternative instance for non-backtracking > parsers > On 30/08/2018 20.21, Olaf Klinke wrote: > >> Hello, Olaf. I have some distrust of elegant solutions (one of them are > >> C.P. libs). > > > > [*] 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? > > Not an expert, but for something as (relatively!) standard as CSV, I'd > probably go for a specialized solution like 'cassava', which seems like > it does quite well according to https://github.com/haskell-perf/csv > > Based purely the lines/second numbers on that page and the number you've > given, I'd guesstimate that your parsing could potentially be as fast as > (3.185ms / 1000 lines) * 130000 lines = 414.05ms = 0.4 s. > > (Of coure that still doesn't account for extracting the Int, Double, > etc., but there are also specialized solutions for that which should be > pretty hard to beat, see e.g. bytestring-lexing.) > > It's also probably a bit less elegant than a generic parsec-like thing, > but that's to be expected for a more special-case solution. > > Regards, > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >

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Installing hsdev on windows
by Yotam Ohad 31 Aug '18

31 Aug '18

Hi cafe, I installed haskell platform for windows 10 and then ran `cabal install hsdev` I got the following error: Resolving dependencies... cabal: Could not resolve dependencies: [__0] trying: hsdev-0.3.1.4 (user goal) [__1] trying: hlint-2.1.10 (dependency of hsdev) [__2] trying: haskell-src-exts-1.20.2 (dependency of hsdev) [__3] next goal: ghc-syb-utils (dependency of hsdev) [__3] rejecting: ghc-syb-utils-0.3.0.0 (conflict: hsdev => ghc-syb-utils>=0.2.3 && <0.3) [__3] trying: ghc-syb-utils-0.2.3.3 [__4] next goal: ghc (dependency of hsdev) [__4] rejecting: ghc-8.4.3/installed-8.4..., ghc-8.4.3, ghc-8.4.1 (conflict: ghc-syb-utils => ghc>=7.0 && <8.4) [__4] rejecting: ghc-8.2.2, ghc-8.2.1 (conflict: hsdev => ghc==8.4.*) After searching the rest of the dependency tree exhaustively, these were the goals I've had most trouble fulfilling: hsdev, haskell-src-exts, ghc-mod, hlint, base, ghc, ghc-syb-utils Any Ideas on how to solve this? I'm clueless thanks

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Parsing Haskell with Layout
by Benjamin Redelings 30 Aug '18

30 Aug '18

Hi, Section 10.3 of the Haskell 2010 report describes the layout rule for Haskell in terms of rules for inserting curly braces and semicolons into the token stream. I'm not sure of the theory behind implementing Note 5 in section 10.3. This says roughly that we insert a "}" token after token t[n] if the tokens t[1]...t[n+1] have no valid parses, and the sequence of tokens t[1] ... t[n] followed by "}" DO have a valid parse. I think this allows you to write f z = let x=2;y=3 in x+y+z However, except for Note 5, it seems like you could do a simple preprocessing step on the token stream to insert "{", ";", and "}" before parsing. Note 5 seems more complicated. (I feel like I read a note somewhere that said "read this and weep" and then failed to implement Note 5. But now I can't remember where I read this.) In simple explanations like Write you a haskell (http://dev.stephendiehl.com/fun/008_extended_parser.html) it seems that the approach is simply to ignore Note 5. Can anyone fill me in on (a) what concepts are assumed by the report that would make it "obvious" how Note 5 is supposed to be implemented? (b) a simple (hopefully) implementation strategy for implementing it? I'm looking for a language-independent approach for parsing. I did see some mention of error-correcting parsers in the Parsec paper, which might allow adding the "}" at a parse error. But I see there is a paper called " An error correcting parser for context free grammars that takes less than cubic time". The "less than cubic time" seems rather worrisome, so I don't know if this is the right approach. Also, when using parsec, it seems that you might get an idea how many tokens are a valid prefix of the grammer when the token stream fails to parse. Hence, you could try inserting a "}" into the token stream at the point of a parse error. However, a naive approach would then require re-parsing the whole stream from the beginning until each parse error, which seems very expensive. Thanks for any help! -BenRI

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Dependent types, weak references, and stable names
by David Feuer 29 Aug '18

29 Aug '18

I just realized that dependent types should let us do some things with weak references and stable names less awkwardly. In particular, there are two invariants that are not (and cannot be) expressed in the type system at present: 1. If two stable names are equal, then their underlying objects are equal. 2. If a weak reference was built using a key and a value, and the key is alive, then the value is also alive. I hope we'll be able to express these with Dependent Haskell, using whatever the real syntax will be. data Weak :: k -> Type -> Type mkWeak :: (key :: k) -> v -> IO () -> IO (Weak key v) deRefWeakSurely :: (key :: k) -> Weak key v -> IO v -- Intentionally omitted: finalize data StableName :: k -> Type makeStableName :: (key :: k) -> IO (StableName key) eqStableName :: StableName x -> StableName y -> Maybe (x :~: y) The idea is that we can calculate the StableName of a key, look for that that in a hash table of weak pointers, and if we find it, we get evidence that we can use to dereference the weak pointer (deRefWeakSurely would use touch# under the hood, but that's not our concern).

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Setting RTS thread names useful?
by Viktor Dukhovni 28 Aug '18

28 Aug '18

The RTS sets thread names on Linux, but not other POSIX platforms. FreeBSD has the requesite functionality, and the patch below can enable it. Not sure whether this is in fact useful, since all threads would get the same name ("ghc_worker"): diff --git a/rts/posix/OSThreads.c b/rts/posix/OSThreads.c index 7dcf0eed15..9d2d9a897e 100644 --- a/rts/posix/OSThreads.c +++ b/rts/posix/OSThreads.c @@ -139,6 +139,9 @@ createOSThread (OSThreadId* pId, char *name STG_UNUSED, pthread_detach(*pId); #if defined(HAVE_PTHREAD_SETNAME_NP) pthread_setname_np(*pId, name); +#elif defined(freebsd_HOST_OS) + /* Since FreeBSD 5.1 */ + pthread_set_name_np(*pId, name); #endif } return result; On Linux the thread name is taken from the static variable "program_invocation_short_name", which is again the same for all threads of each program, but is not globally the same. FreeBSD could get the same result with getprogname(3). The question still remains as to whether setting such thread names is useful or pointless... -- Viktor.

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Re: [Haskell-cafe] Ghc error messages on Windows
by Olaf Klinke 28 Aug '18

28 Aug '18

Kees, the strange symbols are escape codes for coloring the output. Newer GHCs color the error messages, and what you see is when GHC thinks it prints to a color terminal but the terminal can not handle colors. I don't know how to switch that off, though. Olaf

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Mapping of async tasks to threads?
by Viktor Dukhovni 28 Aug '18

28 Aug '18

I have an application that is largely latency-bound, it operates on hundreds of concurrent DNS lookup tasks, to process workloads of millions to tens of millions such tasks. Each task is spawned via Control.Concurrent.Async.async, and the code is compiled with "-threaded", and runs with the "capability" count set to 4: $ ps -o command= -p $(pgrep -x danescan) danescan ... +RTS -N4 ... The main task collects batches of outputs from the worker tasks and stores the results in a database. With the worker count set to 350, looking at per-thread "ps" output on FreeBSD, I see 24 threads of which 3 look idle, 20 seem to be doing the DNS work (with a few noticeably less than the rest), and one is the main thread doing the database updates: $ ps -o pid,lwp,time,mwchan,msgrcv,msgsnd,state -H -p $(pgrep -x danescan) | sort -k5n PID LWP TIME MWCHAN MSGRCV MSGSND STAT 38083 100763 0:00.00 uwait 0 0 I+ 38083 101152 0:00.00 uwait 0 0 I+ 38083 101922 0:00.00 uwait 0 0 I+ 38083 101343 0:02.29 uwait 27194 27627 I+ 38083 101335 0:04.30 uwait 51689 52517 I+ 38083 101391 0:13.33 uwait 173618 177160 S+ 38083 101917 0:22.63 kqread 295420 300894 S+ 38083 101362 0:28.73 uwait 371419 378350 I+ 38083 101349 0:46.11 uwait 594307 604816 I+ 38083 101352 0:49.65 uwait 641703 653557 S+ 38083 101370 0:53.93 select 699057 712217 S+ 38083 101358 0:58.15 uwait 757636 771437 S+ 38083 101376 1:00.84 kqread 794335 809328 S+ 38083 101375 1:02.31 uwait 806276 821284 S+ 38083 101342 1:02.34 uwait 809367 823963 S+ 38083 101244 1:03.26 select 812395 827329 S+ 38083 101360 1:03.47 uwait 815955 831710 S+ 38083 101356 1:03.03 uwait 818215 833649 I+ 38083 101364 1:03.54 uwait 825807 841300 S+ 38083 101347 1:03.89 uwait 832981 848893 S+ 38083 101334 1:04.42 kqread 836132 851386 S+ 38083 101357 1:06.04 uwait 859308 875754 S+ 38083 101134 1:06.67 kqread 859410 875176 S+ 38083 100655 5:36.98 uwait 11780649 11780685 S+ The number of open UDP DNS query sockets matches what I expect (each active query uses a fresh socket, closing it when the results arrive): $ lsof -n -P -i -a -p $(pgrep -x danescan) | egrep -c '(TCP|UDP).*:53$' 349 How is the actual thread count related to the RTS "-N" option? I did not expect to see 20 threads... -- Viktor.

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Re: [Haskell-cafe] Alternative instance for non-backtracking parsers
by Olaf Klinke 28 Aug '18

28 Aug '18

Thanks to Doaitse for the example. I yet have to wrap my head around it (since I don't know how pChainr is related to <|>) but playing with q parsing the empty string is a good start. When you said > One might expect <|> to be associative, but even that is in no way guaranteed. I suppose you meant that type class laws can not be enforced, because the documentation of Alternative states that <|> should be associative. It would be quite horrible if the Alternative instances of parsers weren't. @Paul: I think that parser combinator libraries are one of the most wonderful things functional programming has given us. How else would you parse your input data? Anything cobbled together manually with the help of some Read instances is bound to be unmaintainable and does not yield good error messages. What would you suggest as a replacement? How would you write composable parsers for a complicated file type? Today I added a 'try' method to my MonadParse type class. This lets me write (try p <|> q) generically. An instance for Attoparsec would declare try=id, since it always backtracks. We'll see how this goes. What I learned form this thread is: * arrange your usages of <|> so that both sides can never share a common prefix, then no backtracking will be necessary * for that reason, some parsers like Megaparsec define their Alternative instances the way they do, without backtracking. Please allow one more question: Is the backtracking performance penalty linear in the amount of backtracking? That is, suppose that p = char '-' >> someHugeParser q = char '-' >> anotherHugeParser Could (try p <|> q) decide after seeing two characters that p fails and commit to the second branch, freeing memory? Thanks, Olaf

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Ghc error messages on Windows
by Kees Bleijenberg 28 Aug '18

28 Aug '18

I've installed stack on Windows 7 64 bits. When I do "stack build" I get the output below. It looks like a character encoding problem. The generated executable is okay. This problems exists with all projects. Stack uses ghc version 8.4.3? Kees d:\haskell\Sudoku>stack build [0mSudoku-0.1.0.0: unregistering (local file changes: src\CalcSudoku.hs)[0m [0mSudoku-0.1.0.0: build (lib + exe)[0m [0mPreprocessing library for Sudoku-0.1.0.0..[0m [0mBuilding library for Sudoku-0.1.0.0..[0m [0m[2 of 3] Compiling SudokuTypes ( src\SudokuTypes.hs, .stack-work\dist\7 d103d30\build\SudokuTypes.o )[0m [0m[3 of 3] Compiling CalcSudoku ( src\CalcSudoku.hs, .stack-work\dist\7d 103d30\build\CalcSudoku.o )[0m [0m[0m [0m[;1msrc\CalcSudoku.hs:181:1: [;1m[31merror:[0m[0m[;1m[0m[0m[;1m[0m [0m parse error (possibly incorrect indentation or mismatched brackets)[0m [0m[0m [0m[;1m[34m |[0m[0m[0m [0m[;1m[34m181 |[0m[0m [;1m[31mg[0m[0metMin :: [a] -> [a -> Int] -> a [0m[;1m[34m |[0m[0m[;1m[31m ^[0m[0m[0m [0m[0m[0m[0m[0m

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Does GADTs imply ExistentialQuantification?
by Cosmia Fu 27 Aug '18

27 Aug '18

Hey everyone, I found that it seems that GADTs implies ExistentialQuantification, but not mentioned in the manual. Is it a bug? https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_ext… GHCi, version 8.4.3: http://www.haskell.org/ghc/ :? for help Prelude> :set -XGADTs -XExplicitForAll Prelude> data Q = forall a. Q a Prelude> ---- Cosmia Fu

4 4

Munich Haskell Meeting, 2018-08-30 @ 19:30
by Leah Neukirchen 27 Aug '18

27 Aug '18

Dear all, This week, our monthly Munich Haskell Meeting will take place again on Thursday, August 30 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-aug-2018/ Everybody is welcome! cu, -- Leah Neukirchen <leah(a)vuxu.org> http://leah.zone

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OT: compiler writing like company from EU like country having made more than 3e6 sales?
by Marc Weber 26 Aug '18

26 Aug '18

Eventually its possible to get 150K of funding from EU to move 'knowledge' into Kosovo/Albania. I've been missing a simple compiler with some features for a long time. This might be a chance to get it done. By features I mean - discus like region typing - ur like mixing of server/client lanugage - quick and dirty way to implement type level type deducations. Eg matrix * matrix => new type so that you also can type databases and the like. I know that Haskell is touring complete in this regard, but it also should be fast. - macro programming Reply to this thread or private message if you think you own a company which have been working in hosting or package management or the like and is located within Europe including Switzerland. Marc Weber

1 0

Blog post about modelling state machines with free categories
by Marcin Szamotulski 25 Aug '18

25 Aug '18

Hello, I wrote a blog post about modelling state machines with free categories: https://marcinszamotulski.me/posts/finite-state-machines.html Best regards, Marcin Sent with ProtonMail Secure Email.

1 0

Alternative instance for non-backtracking parsers
by Olaf Klinke 24 Aug '18

24 Aug '18

Dear Cafe, can anyone explain the use of an Alternative instance for a parser monad where the parser (p <|> q) does not backtrack automatically when p fails? Consider megaparsec for example. When p is a parser that does not backtrack automatically, then (p <|> q) never succeeds with q. While I do understand that fine-grained control of backtracking is desirable for performance reasons in general, it escapes me why such an Alternative instance could be useful. See also "A note on backtracking" in the parser-combinators package documentation [1]. Minimal code example and context below. Thanks, Olaf [1] http://hackage.haskell.org/package/parser-combinators-1.0.0/docs/Control-Ap… import Text.Megaparsec import Text.Megaparsec.Char import Data.Void p :: Parsec Void String Bool p = do char '-' string "True" return True q :: Parsec Void String Bool q = do char '-' string "False" return False >>> parseTest (p <|> q) "-True" True >>> parseTest (p <|> q) "-False" 1:2: unexpected "Fals" expecting "True" -- Since q can parse "-False", I would expect (p <|> q) to parse "-False", too. Context: Of course the example above can be mitigated by moving the char '-' outside the alternative. But we might not have control over whether the two parsers share a common prefix. I am trying to write some code generic in the parser monad and I am having difficulties making my code work for both backtracking and non-backtracking parsers. If possible, I'd like to keep my parser type class devoid of a 'try' combinator, which would be the identity function for parser monads that always backtrack.

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Cobertura from .tix/.mix
by Paul 24 Aug '18

24 Aug '18

Hello, Cafe! Is any tool to generate Cobertura from .tix/.mix files? As I understand they are serialized Mix and Tix types from HPC related libraries, but better to avoid coding and to use ready tool :)

1 0

GHC gets simpler!?
by Jeffrey Brown 24 Aug '18

24 Aug '18

I just flipped through the list of accepted GHC proposals[1]. I feel like a kid in a candy shop. I am particularly excited to see the large fraction of proposals that look like either simplifications or homogenizations (making the language more uniform): * Make Constraint not apart from Type * Allow signatures on pattern synonym constructors * Explicit foralls ("Permit ``forall ...`` to bind type variables after the word ``instance`` ...") * Remove the * kind syntax * Allow ScopedTypeVariables to refer to types * Treat kind and type vars identically with `forall` * Embrace (Type :: Type) ("This proposal expands the meaning of -XPolyKinds to include -XTypeInType, deprecates -XTypeInType, and advocates for using Type in place of *.") At various times, I have tried to explore the GHC extensions thoroughly -- well enough to know which ones will let you do what, so that I can learn the right one in detail when I need to. I have always turned back; the space seems too big to cover, and only growing. But now I see that GHC hosts competing forces of simplification and complication. This might be a squishy question, but I'm interested in your feelings regarding how navigable the space of extensions is, and whether you think balkanization is a problem, and how you expect those qualities to evolve. [1] https://github.com/ghc-proposals/ghc-proposals/pulls?page=2&q=label%3AAccep… -- Jeff Brown | Jeffrey Benjamin Brown Website <https://msu.edu/~brown202/> | Facebook <https://www.facebook.com/mejeff.younotjeff> | LinkedIn <https://www.linkedin.com/in/jeffreybenjaminbrown>(spammy, so I often miss messages here) | Github <https://github.com/jeffreybenjaminbrown>

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How to express constraint succeeded?
by Paul 23 Aug '18

23 Aug '18

Hello, Dear Cafe! I have expression 'True ~ 'True in right hand side of type level function. Is some "constant" for the same?

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CFP: 4th International Workshop on Real World Domain Specific Languages
by Simon Thompson 23 Aug '18

23 Aug '18

Apologies for any duplicates! CALL FOR PAPERS 4th International Workshop on Real World Domain Specific Languages https://sites.google.com/site/realworlddsl In conjunction with The International Symposium on Code Generation and Optimisation 2019. http://cgo.org/cgo2019/ Washington DC, USA, 16th February, 2019 As the use of computers proliferates, the complexity and variety of systems continues to grow. As a result, it is becoming increasingly inflexible to "hard wire” behaviours into software. Software developers can enable more control over their software configurations by exploiting Domain Specific Languages (DSLs). Such DSLs provide a systematic way to structure the underlying computational components: to coin a phrase, a DSL is a library with syntax. There is an enormous variety of DSLs for a very wide range of domains. Most DSLs are highly idiosyncratic, reflecting both the specific natures of their application domains and their designers’ own preferences. This workshop will bring together constructors of DSLs for “real world” domains; that is, DSLs intended primarily to aid in building software to solve real world problems rather than to explore the more theoretical aspects of language design and implementation. We are looking for submissions that present the motivation, design, implementation, use and evaluation of such DSLs. Previous workshops were RWSDL’16 (Barcelona), RWDSL’17 (Austin) and RWDSL’18 (Vienna), all collocated with CGO and published in the ACM Digital Library. Key Dates: Paper submission deadline: 9th November 2018 Author notification: 7th December 2018 Final manuscript due: 11th January 2019 Workshop: 16th February 2019 Submission Instructions: The EasyChair submission page for this workshop is: https://easychair.org/conferences/?conf=rwdsl19 Accepted submissions will be published in the ACM Digital Library within its International Conference Proceedings Series. Submissions should be 8-10 pages in ACM double-column format. Authors should follow the information for formatting ACM SIGPLAN conference papers, which can be found at http://www.acm.org/publications/proceedings-template. Full submission details are on the workshop web page. Program Chairs Rob Stewart, Heriot-Watt University, UK Greg Michaelson, Heriot-Watt University, UK PC Members Allin Cottrell, Wake Forest University, USA Nina Dethlefs, University of Hull, UK Andy Gill, X – The Moonshot Factory, USA Kevin Hammond, University of St Andrews, UK Patrick Maier, University of Glasgow, UK Mathijs Schuts, Phillips Healthcare, Netherlands Simon Thompson, University of Kent, UK Phil Trinder, University of Glasgow, UK Contact Please email inquiries concerning the workshop to: R.Stewart(a)hw.ac.uk

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Access violation when stack haddock haskell-src-exts since LTS 12.0
by Yuji Yamamoto 23 Aug '18

23 Aug '18

I always get an " Access violation in generated code when reading 0x28900000" error when executing > stack haddock haskell-src-exts since LTS 12.0. I'm not sure to which project I should submit a bug report. GHC? Haddock? Or Stack? And how can I make a smaller case to reproduce the same error? I attached a log when executing with --verbose --cabal-verbose for your info. -- 山本悠滋 twitter: @igrep GitHub: https://github.com/igrep GitLab: https://gitlab.com/igrep Facebook: http://www.facebook.com/igrep Google+: https://plus.google.com/u/0/+YujiYamamoto_igrep

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ANN: leancheck-v0.7.2, enumerative QuickCheck-like testing
by Rudy Matela 22 Aug '18

22 Aug '18

Hello Haskell-Café, LeanCheck is a simple enumerative property-based testing library for Haskell. It is highly portable, and even works on Hugs98 (v2006-09). It does not pull any dependencies other than what's bundled with GHC. Check it out on Hackage and GitHub: https://hackage.haskell.org/package/leancheck https://github.com/rudymatela/leancheck A simple example follows: > import Test.LeanCheck > import Data.List (sort, union) > check $ \xs -> sort (sort xs) == sort (xs::[Int]) +++ OK, passed 200 tests. > check $ \xs ys -> xs `union` ys == ys `union` (xs::[Int]) *** Failed! Falsifiable (after 4 tests): [] [0,0] LeanCheck works on all types that are instances of the Listable typeclass. A TH automatic deriver is provided with LeanCheck so you don't need to learn LeanCheck's DSL to test with your types. I recently released 0.7.2, there are several additions since last announcement here. Notable changes include: * minor fixes for loading on Hugs98; * functions to compute statistics about generated values; and * a family of functions to test invariants of typeclass instances: > data MyTy = ... > instance ... MyTy where ... > import Test.LeanCheck.Utils > check $ (okEqOrd :: MyTy -> MyTy -> MyTy -> Bool) +++ OK, passed 200 tests. > check $ (okNum :: MyTy -> MyTy -> MyTy -> Bool) +++ OK, passed 200 tests. LeanCheck is available on Hackage or Stackage, and can be installed using: $ cabal install leancheck As far as I am aware, there are no bindings to use LeanCheck with tasty, test-framework or hspec yet, but contributions are welcome. I may create bindings for those if I ever get the time. Best Regards, Rudy

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how do FFI calls show up in profiling info?
by Johannes Waldmann 22 Aug '18

22 Aug '18

Dear Cafe, how would FFI calls show up in profiles? I am using hmatrix-glpk. The relevant FFI call is in https://hackage.haskell.org/package/hmatrix-glpk-0.19.0.0/docs/src/Numeric.… I am compiling with profiling (for executables and libraries), and I am running with +RTS -P . The .prof file does not contain c_simplex_sparse (the C function) It does mention simplexSparse (the Haskell function that calls it) but with a suspiciously low time. - J.W.

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