- Beginners - Haskell.org

Is there a name for the following?
by Gregory Malecha 02 Jun '14

02 Jun '14

Hello -- I'm wondering if there is any "name" for a functor "F" that has a function with the following type: (a -> F b) -> F (a -> b) or (F a -> F b) -> F (a -> b) If there is, I would imagine it being related to Applicative in some way, though clearly there are many Applicatives that do not have an implementation of this. Thanks. -- gregory malecha

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installing cabal 1.20 on linux mint debian edition (LMDE)
by Ovidiu Deac 31 May '14

31 May '14

I'm trying to install cabal 1.20 like this: I installed ghc and ghc-prof from apt repository so I got the following: ii ghc 7.6.3-5 amd64 The Glasgow Haskell Compilation s ii ghc-prof 7.6.3-5 amd64 Profiling libraries for the Glasg Then I git cloned cabal and checked out Cabal-v1.20.0.0 Then I ran bootstrap.sh in cabal-install dir and I get the following error: Using gcc for C compiler. If this is not what you want, set CC. Using /usr/lib/gcc/x86_64-linux-gnu/4.8/collect2 instead. Checking installed packages for ghc-7.6.3... deepseq is already installed and the version is ok. time is already installed and the version is ok. Cabal is already installed and the version is ok. transformers is already installed and the version is ok. mtl is already installed and the version is ok. text is already installed and the version is ok. parsec is already installed and the version is ok. network is already installed and the version is ok. HTTP is already installed and the version is ok. zlib is already installed and the version is ok. random is already installed and the version is ok. stm is already installed and the version is ok. [...skipping many lines] package cabal-install-1.20.0.0 requires network-2.4.1.2 Building cabal-install-1.20.0.0... Preprocessing executable 'cabal' for cabal-install-1.20.0.0... <command line>: cannot satisfy -package-id network-2.4.1.2-fc99093587d92370c7febe034504fb40: network-2.4.1.2-fc99093587d92370c7febe034504fb40 is shadowed by package network-2.4.1.2-040cee5ece44014a8574cb3f87b1eec4 (use -v for more information) Error during cabal-install bootstrap: Building the cabal-install package failed. What am I doing wrong here? Thanks, Ovidiu

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Best way to have a fresh&updated Haskell installation
by Christian Espinoza L. 31 May '14

31 May '14

Hi guys, like subject says , how can I get it? I'm using Debian Linux, and I need a guide to have a updated haskell platform, easy to update when will necessary. Haskell Platform is totally out of date, I know is required compile all, but I need a guide in order to get a standard way. Thanks in advance. Christian.

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Using "cabal test" and getting "cabal: Prelude.read: no parse"
by Daniel King 30 May '14

30 May '14

Hi all, I created a cabal project with the attached cabal file. I run: cabal configure --enable-tests cabal build cabal test and then I get the error: cabal: Prelude.read: no parse I'm not sure how to debug this any further. If I execute: ./dist/build/scientific-pl-testsStub/scientific-pl-testsStub with any input I've tried (numbers, array notation, string notation, etc.) and I get that same error. I can post the whole project if that is helpful. Thanks for the debugging tips! The full log is: danking@spock # cabal configure --enable-tests Resolving dependencies... Configuring scientific-pl-0.1.0.0... danking@spock # cabal build Building scientific-pl-0.1.0.0... Preprocessing test suite 'scientific-pl-tests' for scientific-pl-0.1.0.0... [1 of 3] Compiling SPLData ( SPLData.hs, dist/build/SPLData.o ) [2 of 3] Compiling SPLEval ( SPLEval.hs, dist/build/SPLEval.o ) [3 of 3] Compiling Tests ( tests/Tests.hs, dist/build/Tests.o ) In-place registering scientific-pl-tests-0.1.0.0... [1 of 1] Compiling Main ( dist/build/scientific-pl-testsStub/scientific-pl-testsStub-tmp/scientific-pl-testsStub.hs, dist/build/scientific-pl-testsStub/scientific-pl-testsStub-tmp/Main.o ) Linking dist/build/scientific-pl-testsStub/scientific-pl-testsStub ... Preprocessing executable 'scientific-pl' for scientific-pl-0.1.0.0... [1 of 3] Compiling SPLData ( SPLData.hs, dist/build/scientific-pl/scientific-pl-tmp/SPLData.o ) [2 of 3] Compiling SPLEval ( SPLEval.hs, dist/build/scientific-pl/scientific-pl-tmp/SPLEval.o ) [3 of 3] Compiling Main ( Main.hs, dist/build/scientific-pl/scientific-pl-tmp/Main.o ) Linking dist/build/scientific-pl/scientific-pl ... danking@spock # cabal test Running 1 test suites... Test suite scientific-pl-tests: RUNNING... cabal: Prelude.read: no parse 1 danking@spock # -- Dan King

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compilation joys
by Elise Huard 29 May '14

29 May '14

Hi, I've been compiling this project https://github.com/mlesniak/game.git It uses Hipmunk (2D collision detection/physics framework) and OpenGL. Now the problem is that Hipmunk uses Data.StateVar, and OpenGL/GLUT use Graphics.Rendering.OpenGL.GL.StateVar Both have pretty much the same code and functionality, but using both together gives problems with conflicting types. My current solution was to locally fork Hipmunk and make it use Graphics.Rendering.OpenGL.GL.StateVar (because the other way round - making OpenGL use Data.StateVar, is a cascading nightmare, lots of libs to patch), and this works. My question: is this the only possible solution? Is there some way that doesn't involve patching one library or the other? The other thing I had to locally patch is to force cabal to use freeglut instead of the local Mac OS X GLUT. I did this by again, locally forking GLUT (long live cabal sandbox add-source!) and adding extra-lib-dirs: /usr/local/Cellar/freeglut/2.8.1/lib which is the local homebrew freeglut dir. Is there a better, more reproducible way to do this? Thank you, Elise

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Network.HTTP.Conduit - body in REST/POST request
by Miro Karpis 29 May '14

29 May '14

Hi, I'm having difficulties with sending body parameters/values in a POST request. It seems that the server does not receive in body. ideas/comments very welcome ;-) cheers, m. {-# LANGUAGE OverloadedStrings #-} import Network.HTTP.Conduit import Data.Word(Word8) import Data.ByteString.Lazy(pack) import qualified Data.ByteString.Char8 as B x :: B.ByteString x = "?value=10" post = do r <- parseUrl "http://postcatcher.in/catchers/5385d4e0b6887c0200000071" putStrLn $show r let request = r { secure = True , method = "POST" , requestBody = RequestBodyBS s , requestHeaders = (requestHeaders r) ++ [("Content-Type", "application/json")]} putStrLn $ show request response <- withManager $ httpLbs request print $ responseBody response

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Re: [Haskell-beginners] apply a function to all params of another function
by Brent Yorgey 28 May '14

28 May '14

On Mon, May 26, 2014 at 04:46:29PM +0200, Kees Bleijenberg wrote: > In have a lot of functions in a program with params that are alle Int's. > f :: Int -> Int -> Int > g :: Int -> Int -> Int > h :: Int -> Int -> Int -> Int > ... > and convertParam :: Int -> Int -- say (+1) > I want to call functions like f, g and h, but apply convertParam to all > params first. > f2 a b c = f (convertParam a) (convertParam b) (convertParam c) > > I tried: > run1p f conv = \a -> f (conv a) -- for functions with one > param > run2p f conv = \a b -> f (conv a) (conv b) -- for functions with two > params > . > Can you do this in a more generalized way (using currying?) > Any ideas? class Convertible t where convert :: t -> t instance Convertible Int where ... instance Convertible t => Convertible (Int -> t) where ... I'll let you fill in the ... ! =) Note this only works well because the base case is Int. You could also add some extra base cases for other concrete return types. However, it is quite difficult to give a Convertible instance which applies to "all types which are not functions", so this will only work if you are willing to make one instance for each concrete result type your functions use. -Brent

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Re: [Haskell-beginners] Haskell type Array a b
by Kevin Van Horn 27 May '14

27 May '14

On May 26, Brandon Allbery <allbery.b(a)gmail.com> wrote: > On Mon, May 26, 2014 at 5:12 PM, Kevin Van Horn <kevin(a)ksvanhorn.com> wrote: > I'm looking on the Haskell 98 Report, and the definition of (==) for values of type Array a b looks wrong to me. Here is the definition given: > > a == a' = (assocs a == assocs a') > > But this fails to account for the array length in each dimension; hence, if > > Have you tested this? Yes. For the arrays I defined I had assocs a = [ ], because the length in one dimension was equal to 0. So I had x == y but bounds x != bounds y. > I think if you're looking for anything more precise, you need to switch to something that can't be represented in H'98 or even H'2010, namely arrays with the bounds in the type. Why would you need that? Array equality could have been defined to be a1 == a2 = (bounds a1 == bounds a2) && (assocs a1 == assocs a2) and I don't understand why this is not the definition.

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Haskell type Array a b
by Kevin Van Horn 27 May '14

27 May '14

I'm looking on the Haskell 98 Report, and the definition of (==) for values of type Array a b looks wrong to me. Here is the definition given: a == a' = (assocs a == assocs a') But this fails to account for the array length in each dimension; hence, if x is a 0 x 2 array, y is a 2 x 0, and z is a 0 x 0 array, then x == y x == z, and y == z are all True, even though bounds x != bounds y, bounds x != bounds z, and bounds y != bounds z. This violates the most fundamental property of equality: that if a == b, then you can substitute a for b in an expression without changing the value of the expression. I can't possibly be the first person to have noticed this. So why is equality for Arrays defined this way?

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Evaluate function
by Derek McLoughlin 27 May '14

27 May '14

Reading "Parallel and Concurrent Programming in Haskell", at the bottom of page 27 the author wants to force the evaluation of a list of Strings: evaluate ( length puzzles ) Why not just evaluate puzzles ? https://github.com/simonmar/parconc-examples/blob/master/sudoku4.hs

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