March 2015 - Beginners

Using LTS et. al, a small thank you.
by emacstheviking 06 Mar '15

06 Mar '15

First, find yourself a Pharrel Williams youtube video of Happy! ;) It might seem crazy what I'm about to say Functional is here, you can take a break No more OO confusion that takes up too much space Punch the air like you don't care it's cool as I say... Because I code Haskell Code along if you feel like a project without a boss Because I code Haskell Code along if you know that OO is just a bunch of dross Because I code Haskell Code along if you know what productivity is to you Because I code Haskell Clap along if you that's what you wanna do I am sure I could write more but that floated in out of nowhere and stuck as the song came on the radio!! LMAO In all honesty, I have now got OpenGL, http-client and a bunch of stuff all playing nicely. Thanks very much everybody for your help yesterday, it's noce to be hacking Haskell once more. I'm happy..........

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Function Composition/Map
by Animesh Saxena 06 Mar '15

06 Mar '15

I have a simple function fmove::Char->(Char,Char)->Char fmove s (x,y) | s == x = y | otherwise = s If I say fmove 'a' ('a','b') it will move to 'b' If I say fmove 'a' ('d','c') it will stay at 'a' Now I am trying to use this for graph, coz one move can result in multiple result nodes. For example in graphs you can have move from a to b and a to c. So I was trying something like this, :t (map fmove s) where s is the set of possible start nodes "abcd" (map fmove s) :: [(Char, Char) -> Char] Now I want to be able to pass the graph to this function and get all possible list of endings. For example my graph can be like [('a','b'),('b','c'),('c','a'),('c','d'),('e','a')]. You can see that both 'a' and 'd' are possible from node 'c'. I can see that above type signature is wrong for passing the whole graph coz it accepts [(Char, Char) -> Char]. I initially started with foldl but problem is recursion on top of foldl. The actual objective of the problem is to figure out if a node is reachable from a start node or not. Hope the problem is clear, can anyone suggest how to convert the map function to accept the graph? For each of the possible start nodes (accumulating at each step) it should apply it to the same graph. -Animesh

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<$> and <*> pronunciation
by Max Schneider 06 Mar '15

06 Mar '15

(I really hope 'no stupid questions' holds here) I'm not even going to try framing this into a weighty or practical question; how do you pronounce the <$> and <*> Applicative functions?

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cyclic dependency problem
by Konstantin Saveljev 05 Mar '15

05 Mar '15

Hello, I'm having some trouble with cyclic dependency. My simplified version of hierarchy: module A where import MyState data A a = A (StateT MyState IO a) deriving (...) Now there is a module MyState: module MyState where import SomeType data MyState = MyState { st :: SomeType, ... } Finally the module that introduces cyclic dependency: module SomeType where import A data SomeType = SomeType { f :: A (), ... } I have been suggested to move the types into one module and then import it wherever needed. But the problem is that even if I put them into one file those three data types still form a cyclic dependency and compiler throws errors saying "i don't know about this" So if I have a single module: data A a = A (StateT MyState IO a) deriving (...) data MyState = MyState { st :: SomeType, ... } data SomeType = SomeType { f :: A (), ... } With this setup the compiler is gonna tell us it doesn't know about MyState. And no matter how we shuffle these types within a file we are still getting some "unknowns" How would one approach such a problem? Best regards, Konstantin

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LTS is good but...
by emacstheviking 04 Mar '15

04 Mar '15

I started an OpenGL project within an LTS sandbox as guided on another post and, somewhat foolishly it would see, I did a "cabal install cabal-install" when prompted that a new version was available. The very next time I tried to work on my code.... I got this: bash-3.2$ pwd /Users/seancharles/Documents/Coding/haskell/lts1 bash-3.2$ cabal build cabal: You need to re-run the 'configure' command. The version of Cabal being used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0). bash-3.2$ cabal configure Resolving dependencies... Configuring lts1-0.1.0.0... cabal: At least the following dependencies are missing: base ==4.7.* bash-3.2$ cabal build cabal: You need to re-run the 'configure' command. The version of Cabal being used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0). bash-3.2$ ls -l total 112 -rw-r--r-- 1 seancharles staff 19 Mar 2 12:53 LICENCSE -rw-r--r-- 1 seancharles staff 552 Mar 3 20:09 Main.hs -rw-r--r-- 1 seancharles staff 477 Mar 3 20:07 Main.hs~ -rw-r--r-- 1 seancharles staff 46 Mar 2 12:52 Setup.hs -rw-r--r-- 1 seancharles staff 30198 Mar 2 12:51 cabal.config -rw-r--r-- 1 seancharles staff 1090 Mar 2 12:51 cabal.sandbox.config drwxr-xr-x 5 seancharles staff 170 Mar 2 12:53 dist -rw-r--r-- 1 seancharles staff 1932 Mar 2 12:52 lts1.cabal bash-3.2$ So, welcome to a different kind of hell, "cabal version hell" perhaps? Can anybody help me get it running again? Thanks. Sean.

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Alfred Programs in Haskell
by Richard Guay 04 Mar '15

04 Mar '15

Hi, I am using Haskell to write Alfred programs, but I have run into an issue I just can not fix. The routine for reading a data file in a application specific location for Alfred never detects the file as non-existant. It then dies on the read. Here is the code: getAlfredCacheFileContents :: String -> IO (String) getAlfredCacheFileContents fileName = do h <- getHomeDirectory fExist <- doesFileExist $ h ++ cacheDirBasic ++ getBundleID ++ "/" ++ fileName if fExist then do contents <- readFile $ h ++ cacheDirBasic ++ getBundleID ++ "/" ++ fileName return contents else return "" I just installed the latest version on my Mac Air. It always does the call to readFile, even when the file doesn't exist. -- Sent with Postbox <http://www.getpostbox.com>

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GHC 7.8.4 source build fails on OSX...
by emacstheviking 04 Mar '15

04 Mar '15

ld: couldn't dlopen() /usr/lib/libdtrace.dylib: dlopen(/usr/lib/libdtrace.dylib, 1): Symbol not found: _CGLGetCurrentContext Referenced from: /System/Library/Frameworks/CoreVideo.framework/Versions/A/CoreVideo Expected in: /System/Library/Frameworks/OpenGL.framework/Versions/A/OpenGL in /System/Library/Frameworks/CoreVideo.framework/Versions/A/CoreVideo for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation) make[1]: *** [rts/dist/build/libHSrts-ghc7.8.4.dylib] Error 1 make: *** [all] Error 2 iMac:ghc-7.8.4 vosabristol$ Perhaps I need to tweak the configure parameters and try again... stackoverflow has this page: http://stackoverflow.com/questions/19480099/compiling-macos-application-wit… There is no CGL framework in OS X (CGL is included in the OpenGL framework, you need only #include OpenGL/OpenGL.h), and you do not use -lXXX to link against a framework either. Now, depending on how your Makefile is structured, you may also have a property known as LD_FLAGS for a separate linking stage - you *also* need to include -framework OpenGL there. This directive does *both*, include path resolution and linker path resolution for frameworks. – Andon M. Coleman <http://stackoverflow.com/users/2423205/andon-m-coleman> Oct 20 '13 at 19:36 <http://stackoverflow.com/questions/19480099/compiling-macos-application-wit…>

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command-line interaction with running process
by Elise Huard 04 Mar '15

04 Mar '15

Hi, I'm trying to do the following: implementing a command-line interface to directly interact with the state or the parameters of a looping/running process (a game, to be specific). My first thought is to fork a thread, passing in a TChan/TVar or similar to enable communication, and to have the command-line interface on the thread (getLine or haskelline ...). Another option would be to have a bona fide server-client interface (with sockets or others), to fork a thread again to run the server and to connect using the client library - which would allow me to use ghci and have transparent serialization under the hood. Are there known use cases? Am I missing something, for instance is it possible to interact with a running process directly using ghci? Any tips? Thank you, Elise

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Google Summer of Code
by Michael Jones 04 Mar '15

04 Mar '15

If any students are interested in Google Summer of Code projects, there is a Minnow Board project here: http://elinux.org/Minnowboard:GSoC2015 There is a Haskell project listed in the SMBus/PMBus section. Mike

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fractal x = fractal [y - head x + z | y <- x, z <- x] with take doesnt end
by '2+ 04 Mar '15

04 Mar '15

hi! since my fractal :: [Int] -> [Int] fractal x = [y - head x + z | y <- x, z <- x] seems to be working fine with something like: take 300 $ cycle $ fractal $ fractal $ [11..17] ++ [55..77] wanted to twist the function to something totally recursive but: fractal x = fractal [y - head x + z | y <- x, z <- x] experimented with: take 300 $ fractal $ [11..17] ++ [55..77] doesnt end its process and eat up the RAM any hints to solve this issue? thank you ----- 2g --- http://sarigama.namaste.jp

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