August 2015 - Haskell-Cafe

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/

4 3

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.

24 79

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 --

1 0

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

2 2

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.

1 0

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

1 0

Re: [Haskell-cafe] List of nitpicks
by Johannes Waldmann 27 Aug '15

27 Aug '15

>> quickCheck $ ( (\ xs -> xs == reverse xs ) :: Eq a => [a] -> Bool) > This can be solved, by the way: > https://ro-che.info/articles/2013-02-19-smallcheck#required-type-annotations Very good indeed. But (and here's another nit to pick - about ghc) - your > smallCheck 5 $ \x y -> x == (y :: Integer) would look much clearer that way: > smallCheck 5 $ \ (x::Integer) (y :: Integer) -> x == y but ghc complains about missing ScopedTypeVariables and that error message is fully incomprehensible (e.g., to beginning students) as there is no type variable anywhere. There was a PatternSignatures option but it seems gone (deprecated). I believe that (PatternSignatures and) ScopedTypeVariables should be on by default. It is currently off only to help those that are too lazy to write "forall t . " in (inner) declarations? Leaving out quantifiers should never be encouraged. And putting type declarations next to the name declaration (that is, on the "x" under the lambda) should be the normal thing to do. Everything else feels like a work-around, and it is a distraction when teaching. Putting language options at the top of the source file, or in .ghci, is also a distraction. - J.W.

4 3

Elementary math question: primality test for huge numbers
by Christopher Howard 25 Aug '15

25 Aug '15

Hi. I was working through a math chapter explaining the RSA algorithm, and I was implementing it in Haskell for learning sake. However, the chapter does not explain how you efficiently pick the two prime numbers. I scanned through the WP page on primality tests but it was somewhat overwhelming. What would be the standard primality test for this sort of application, and is it already built in to Base somewhere? -- Biblical creationism: http://tinyurl.com/qfyeg4a Free Bible software: http://xiphos.org Software freedom: http://tinyurl.com/qjnpnsm Free computer operating system: http://tinyurl.com/7wczchu Alternative to MS Office: http://tinyurl.com/aw9p6d4

5 4

'Scientific' loses significant digits
by Alex 25 Aug '15

25 Aug '15

Hi: Does anyone know why is this true? ((fromJust . readMaybe) "1.00000000" :: Scientific) == ((fromJust . readMaybe) "1" :: Scientific) -- Alex

2 1

Munich Haskell Meeting, 2015-08-27 @ 19:30 Augustiner-Keller Biergarten
by Christian Neukirchen 25 Aug '15

25 Aug '15

Dear all, This week, our monthly Munich Haskell Meeting will take place again on Thursday, August 27 at **Augustiner-Keller Biergarten** (Arnulfstr. 52) in the self-service area (rougly here: https://goo.gl/maps/QmnMc) at 19h30. For details see: http://www.haskell-munich.de/dates Please note the changed location this time! The weather will be nice. If you plan to join, please add yourself to this dudle. It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-aug-2015/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

1 0