- Haskell-Cafe - Haskell.org

General Functor and Traversable
by M Farkas-Dyck 29 Aug '15

29 Aug '15

Consider this: class (Category c, Category d) => Functor c d f where map :: c a b -> d (f a) (f b) Clearly mapM of Traversable is simply map :: Kleisli m a b -> Kleisli m (f a) (f b). But what is traverse? can it be so defined? I am thinking no, as Kleisli composition is not defined in general, but I know very little category theory so I may easily be wrong.

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Re: [Haskell-cafe] List of nitpicks
by Johannes Waldmann 28 Aug '15

28 Aug '15

Re: http://repetae.net/recent/out/classalias.html Prelude> :set -XConstraintKinds Prelude> class C a; class D a; type E a = (C a, D a); instance E a => C (Maybe a) https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/constraint-… - J.W.

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[ANN] dejafu-0.1.0.0: Overloadable primitives for testable, potentially non-deterministic, concurrency.
by Michael Walker 28 Aug '15

28 Aug '15

Hi cafe, For the past few months I've been working on a library for concurrency testing in Haskell, which I'll be giving a talk on at the Haskell Symposium next week. Now it's on Hackage! --- Déjà Fu is a library for developing and testing concurrent Haskell programs, it provides a typeclass-abstraction over GHC’s regular concurrency API, allowing the concrete implementation to be swapped out. Why do we need this? Well, concurrency is really hard to get right. Empirical studies have found that many real-world concurrency bugs can be exhibited with small test cases using as few as two threads: so it’s not just big concurrent programs that are hard, small ones are too. We as programmers just don’t seem to have a very good intuition for traditional threads-and-shared-memory-style concurrency. The typical approach to testing concurrent programs is to just run them lots of times, but that doesn’t provide any hard coverage guarantees, and then we need to wonder: how many runs do we need? Fortunately, there has been a lot of research into testing concurrency in the past several years. Systematic concurrency testing is an approach where the source of nondeterminism, the actual scheduler, is swapped out for one under the control of the testing framework. This allows possible schedules to be systematically explored, giving real coverage guarantees for our tests. This is a library implementing systematic concurrency testing. It provides two typeclasses, MonadConc to abstract over much of Control.Concurrent and related modules, and MonadSTM, to similarly abstract over Control.Monad.STM. ## How to use it: If you’re not making use of any IO in your code other than for concurrency, the transition to using MonadConc and MonadSTM will probably just be a textual substitution: - IO is replaced with MonadConc m => m - STM with MonadSTM m => m - *IORef with *CRef - *MVar with *CVar - *TVar with *CTVar - Most other things have the same name, and so can be replaced by just swapping imports around. If you are using other IO, you will need a gentle sprinkling of MonadIO and liftIO in your code as well. ## Is this really just a drop-in replacement for IO/STM? That’s the idea, yes. More specifically, the IO instance of MonadConc and the STM instance of MonadSTM just use the regular IO and STM functions, and so should have no noticeable change in behaviour, except for CRef, the IORef equivalent, where modifyCRef behaves like atomicModifyIORef, not modifyIORef. There are some other differences which can lead to incorrect results when testing, but which should not affect code when used in an IO or STM context. Specifically: Control.Monad.Conc.Class.getNumCapabilities can lie to encourage more concurrency when testing; and Control.Exception.catch can catch exceptions from pure code, but Control.Monad.Conc.Class.catch can’t (except for the IO instance). --- There is a blog post with references to prior posts and related papers here: http://www.barrucadu.co.uk/posts/2015-08-27-announce-dejafu.html -- Michael Walker (http://www.barrucadu.co.uk)

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Re: [Haskell-cafe] Why Haskell is beautiful to the novice
by Nicola Gigante 28 Aug '15

28 Aug '15

> Il giorno 28/ago/2015, alle ore 15:37, Torsten Otto <torsten.otto(a)hamburg.de> ha scritto: > > What an interesting topic to discuss! > >> Am 28.08.2015 um 12:12 schrieb Nicola Gigante <nicola.gigante(a)gmail.com>: >> >> Using Haskell (specifically some kind of concepts, e.g. equational reasoning) >> will also make easier for students to see the connection between math >> and programming, making them more motivated to learn math as well. > > Can you elaborate on that? What would you do in class in terms of „equational reasoning“? > It depends on the age of the students and the math they already know. I don’t have specific examples of in-class exercises or things like that. However, the point is that writing Haskell code you can often come up with shorter or more elegant solutions by manipulating terms exactly as done in elementary algebra. Starting from a 10lines function and coming up with a simple and readable oneliner is fun and rewarding, and shows one of the possible connections to math, especially if you can make them reason about types, in which case the task becomes very similar to theorem proving than to algebraic manipulations alone. Another possible connection arise when you teach things such as the Monoid typeclass. Even if students have not been exposed to abstract mathematics, they can still appreciate the usefulness of abstracting similar operations (addition in this case) to being able to uniformly reason about apparently different concepts. Abstraction is the core of mathematics, but is also a fundamental concept in programming, of course, so you’re implicitly teaching two things at once. > Regards, > Torsten Greetings, Nicola

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LaTeX question: interpreter examples
by Christopher Howard 28 Aug '15

28 Aug '15

Hi. I'm doing a lot of Literate Haskell lately, with proofs and commentary in each module. I'm not very familiar with LaTeX, but I'm learning. (I'm using lhs2tex and texi2pdf.) However, in my commentary, I occasionally have blocks of example interpreter interaction, and I'm not sure what to do with it. Is there a certain command enclosure that would be appropriate? Ideally, it would treat the text like monospace, and also preserve line breaks. Example text would be: *GCD> extEuclidean 546 2022 (6,100,-27) *GCD> 546 * 100 + 2022 * (-27) == 6 True But this does not come out looking similar in the final output. -- http://justonemoremathproblem.wordpress.com/

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an idea for modifiyng data/newtype syntax: use `::=` instead of `=`
by Alexey Muranov 28 Aug '15

28 Aug '15

Hello, i would like to suggest an idea for modifying the basic data/newtype syntax in Haskell: replace the equality sign `=` with `::=`. When i started learning Haskell, the most confusing part of the syntax for me was the equality sign in `data` definition. I could not even guess what the `data` definition meant without reading a chapter or two about types in Haskell, and i think it was partially due to the equality sign. I still find this notation inconsistent with other uses of the equality sign in Haskell and in general. For example, in type Name = String data Date = Date Int Int Int data Anniversary = Birthday Name Date | Wedding Name Name Date the second line is particularly disorienting IMO because on two sides of the equality, `Date` denotes different things. As far as i understand, in all contexts except those of `data` and `newtype` definitions, the equality sign in Haskell denotes the actual equality for all purposes: if a line foo x y = bar y x is present in a program, `foo a b` and `bar b a` can be used more or less interchangeably elsewhere in the program. Similarly, if the line type Name = String is present, `Name` can be used as `String`. Clearly, the equality in data Date = Date Int Int Int does not have such property. I think that if `::=` was used instead of `=` in `data` and `newtype` definitions, this would suggest to a newcomer that the syntax of the two sides might be different, and would helpfully remind of the Backus–Naur Form for syntax rules. I think that a newcomer to Haskell, like myself, would have had a better chance of guessing the meaning of type Name = String data Date ::= Date Int Int Int data Anniversary ::= Birthday Name Date | Wedding Name Name Date IMO this would make the program easier to read in general and the difference between `type` and `newtype` more clear. Maybe the can even make the use of keywords redundant, by allowing to write simply Name = String Date ::= Date Int Int Int Anniversary ::= Birthday Name Date | Wedding Name Name Date What do you think? Alexey.

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ANN: wxHaskell 0.92
by Henk-Jan van Tuyl 27 Aug '15

27 Aug '15

L.S., I am happy to announce a new version of wxHaskell (0.92). What is it? ----------- wxHaskell[1] is a portable and native GUI library for Haskell. The goal of the project is to provide an industrial strength GUI library for Haskell, but without the burden of developing (and maintaining) one ourselves. wxHaskell is therefore built on top of wxWidgets – a comprehensive C++ library that is portable across all major GUI platforms; including GTK, Windows, X11, and MacOS X. Furthermore, it is a mature library (in development since 1992) that supports a wide range of widgets with the native look-and-feel. What's new? ----------- - wxc/setup.hs now stops searching for wxWidgets when a compatible version is found (this solves bug ticket 96) - Support for simple Wizards added - Calendar support added - GCC > 4.5 can now be used on Windows, which is a big improvement, as wxWidgets and wxHaskell must use the exact same GCC, to prevent compatibility problems - wxAui is added to wxc, wxAuiNotebook events are added to wxcore and wx - Missing GLAttributes added - Packet version limits adapted to the newest Haskell Platform - Bitness check on Windows no longer uses an external executable - wxHaskell can now be installed with MSYS2 - wxc/Setup.hs is modified to also link to the wx OpenGL libraries for wxGLCanvas - The "warning: Adding duplicate image handler for '... file'" messages are removed - Created a new class Updating with corresponding event "update". Provided instances for TextCtrl and ComboBox: update gets called when the text changes. - Support for wxSplashScreen in wxc and wxcore is added - Many warnings are solved Links ----- See the homepage of wxHaskell for more information: https://wiki.haskell.org/WxHaskell The packages are: - wxc https://hackage.haskell.org/package/wxc - wxdirect https://hackage.haskell.org/package/wxdirect - wxcore https://hackage.haskell.org/package/wxcore - wx https://hackage.haskell.org/package/wx Regards, Henk-Jan van Tuyl [0] https://www.wxwidgets.org [1] https://wiki.haskell.org/WxHaskell -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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Stack transitive dependencies
by Corentin Dupont 27 Aug '15

27 Aug '15

Hello, I have a package depending on another: Foo/stack.yaml: flags: {} packages: - '.' - location: ../Bar extra-dep: true extra-deps: - Bar-0.7.6 resolver: lts-2.19 When I compile (stack build), I obtain: -- While attempting to add dependency, Could not find package DebugTraceHelpers in known packages -- Failure when adding dependencies: DebugTraceHelpers: needed (==0.12.*), but not present in build plan, latest is 0.12 needed for package: Bar-0.7.6 I find it strange that it asks for DebugTraceHelpers because it's not a direct dependency of my package, but a dependency of a dependency. It is correctly mentioned in Bar/stack.yml: flags: {} packages: - '.' extra-deps: - DebugTraceHelpers-0.12 resolver: lts-2.19

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Haskell hacking internships at Los Alamos National Laboratory (Spring 2016, undergraduate)
by Kei Davis 27 Aug '15

27 Aug '15

Hello, We have an ongoing project developing an auto-parallelizing pure functional language implementation based loosely on Haskell/STG. If you are a United States citizen or permanent resident alien studying computer science or mathematics at the undergraduate level with strong interests in Haskell programming, compiler/runtime development, and pursuing a spring semester (2016) internship at a national laboratory this could be for you. We don't expect applicants to necessarily already be highly accomplished Haskell programmers--such an internship is expected to be a combination of (further) developing your programming/Haskell skills and putting them to good use. If you're already a strong C hacker we could use that too. The application process for spring semester internships is open here http://science.energy.gov/wdts/suli/. Note the deadline of Oct. 9. Email me if interested, and feel free to pass this along. -- Kei Davis Applied Computer Science Group CCS-7, Mail Stop B287 Los Alamos National Laboratory Los Alamos, NM 87545, U.S.A.

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Template Haskell and packages using Obj-C FFI
by Hiromi ISHII 27 Aug '15

27 Aug '15

Hi cafe, I'm currently writing objc specific marshaller for inline-c, and faced with strange behaviour of Template Haskell and FFI. It turns out that it's irrelevant to inline-c and matter of GHC. Here is minimal example package for failure: https://github.com/konn/objcth This tiny package is built up with two parts: 1. A wrapper library to call NSLog function with Haskell's String value. This uses FFI interface, 2. An executable which calls the above library to say hello. If 2. does not contain any Template Haskell macros, everything works fine. But if any TH macro presents, even only top level splice `$(return [])`, then compiler halts with following error: ``` <command line>: can't load .so/.DLL for: /Users/myname/Documents/Programming/Haskell/lab/ghcbugs/objcth/.stack-work/dist/x86_64-osx/Cabal-1.22.4.0/build/libHSobjcth-0.1.0.0-IBzJnZWD04lEmS0Ot4HRVO-ghc7.10.2.dylib (dlopen(/Users/myname/Documents/Programming/Haskell/lab/ghcbugs/objcth/.stack-work/dist/x86_64-osx/Cabal-1.22.4.0/build/libHSobjcth-0.1.0.0-IBzJnZWD04lEmS0Ot4HRVO-ghc7.10.2.dylib, 5): Symbol not found: _OBJC_CLASS_$_NSString Referenced from: /Users/myname/Documents/Programming/Haskell/lab/ghcbugs/objcth/.stack-work/dist/x86_64-osx/Cabal-1.22.4.0/build/libHSobjcth-0.1.0.0-IBzJnZWD04lEmS0Ot4HRVO-ghc7.10.2.dylib Expected in: flat namespace in /Users/myname/Documents/Programming/Haskell/lab/ghcbugs/objcth/.stack-work/dist/x86_64-osx/Cabal-1.22.4.0/build/libHSobjcth-0.1.0.0-IBzJnZWD04lEmS0Ot4HRVO-ghc7.10.2.dylib) ``` I used stack to build automatically, but cabal-install produces almost the same result (modulo paths to dist files). TH uses GHCi internally, so perhaps it's related to GHCi? If I replace the call for NSLog and [NSSring stringWith...] with puts c-function, everything gets fine. Is there any workaround, or it's the bug of GHC? Here is my environment: * OS X Yosemite 10.10.5 * Haskell Platform 7.10.2a * llvm-3.5 (installed via brew install llvm35) $ tail -n3 /Library/Frameworks/GHC.framework/Versions/7.10.2-x86_64/usr/lib/ghc-7.10.2/settings , ("LLVM llc command","llc-3.5") , ("LLVM opt command","opt-3.5") ] -- Hiromi ISHII konn.jinro(a)gmail.com

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