- Beginners - Haskell.org

Re: \x -> x < 0.5 && x > -0.5
by Jordan Cooper 19 Oct '09

19 Oct '09

Whoa... how on earth does this work? How does it interpret the sections as Reader monads? > That's a job for the reader monad. > > > Lambda Fu, form 53 - silent reader of truth > > import Control.Monad > import Control.Monad.Reader > > filter (liftM2 (&&) (< 0.5) (> -0.5)) xs > > > > Regards, > apfelmus

3 3

edit-compile-test loop
by Tom Doris 19 Oct '09

19 Oct '09

Hi I'd like to know what the typical edit-compile-test loop looks like with the Haskell platform; that is, in C++ this would be edit, run make to compile everything in the project into libraries and executables, then run an executable test suite. I'm confused as to how people work on larger projects in Haskell - do you work on a single module and load it into ghci to test as you develop, then compile the entire package and run a test suite? Or do you generally only use ghci for prototyping and not when in the middle of proper development? Or do you compile the package and load that into ghci? I'd like to know as I'm starting to work on patches for some hackage packages which have proper cabal builds etc., and want to follow the correct (and efficient!) convention. Thanks Tom

6 7

map question
by Joost Kremers 18 Oct '09

18 Oct '09

Hi all, I've just started learning Haskell and while experimenting with map a bit, I ran into something I don't understand. The following commands do what I'd expect: Prelude> map (+ 1) [1,2,3,4] [2,3,4,5] Prelude> map (* 2) [1,2,3,4] [2,4,6,8] Prelude> map (/ 2) [1,2,3,4] [0.5,1.0,1.5,2.0] Prelude> map (2 /) [1,2,3,4] [2.0,1.0,0.6666666666666666,0.5] But I can't seem to find a way to get map to substract 1 from all members of the list. The following form is the only one that works, but it doesn't give the result I'd expect: Prelude> map ((-) 1) [1,2,3,4] [0,-1,-2,-3] I know I can use an anonymous function, but I'm just trying to understand the result here... I'd appreciate any hints to help me graps this. TIA Joost -- Joost Kremers, PhD University of Frankfurt Institute for Cognitive Linguistics Grüneburgplatz 1 60629 Frankfurt am Main, Germany

8 18

count function
by John Moore 18 Oct '09

18 Oct '09

Hi, Trying to get aprogram to count all the lines or words from a file. Not sure about the syntax. Probably missing some. Here's what I have so far any help in the right direction would be great. import System.IO main :: IO () main = do incom <- openFile "file.txt" ReadMode otcom <- openFile "prob.txt" WriteMode fCount incom otcom hClose incom hClose otcom fCount incomh otcomh = do eof <- hIsEof incomh if eof then return() else do c <- hGetCount incom it tells me parse error on = John

5 4

I/O
by John Moore 18 Oct '09

18 Oct '09

Hi, Could someone please tell me how to check this program. The problem is where do I find the output? I laod this into ghc and it comes up OK modules loaded: Main gchi> How do I now check if it works or see the result. Program below (taken from real world haskell) -- file: ch07/toupper-imp.hs import System.IO import Data.Char(toUpper) main :: IO () main = do inh <- openFile "quux.txt" ReadMode outh <- openFile "output.txt" WriteMode mainloop inh outh hClose inh hClose outh mainloop :: Handle -> Handle -> IO () mainloop inh outh = do ineof <- hIsEOF inh if ineof then return () else do inpStr <- hGetLine inh hPutStrLn outh (map toUpper inpStr) mainloop inh outh John

2 1

is there a best os to easily install external libraries ?
by david hodgetts 17 Oct '09

17 Oct '09

Hello everyone, to give some context: I am not a real developer. I am a motion designer who enjoys doing procedural animations and installations with tools such as processing, flash, openframeworks. This might explain some of the difficulties I describe below. Wanting to broaden my thinking skills and CS knowledge, I have started to learn Haskell, and I must say I am having a great time. I am at a point now where I would like to play around with some of the popular UI and graphical libraries (gtk+, freeGlut, sdl etc). But my experience up to now has been a bit painfull in the sense that most of these packages don't install nicely with caball ( on windows XP). I managed to get gtk, freeglut, and curl to install after long hours googling around and doing voodoo in msys and cygwin. Last night I tried to install the SDL bindings to follow a blog post about automata, but completely failed. This got me to wondering if the haskell experience with regards to installing external libraries might be any easier on Linux? in advance many thanks best regards david hodgetts

3 3

sorting by elements in a tuple
by Ryan Temple 16 Oct '09

16 Oct '09

I am wanting to sort a list of tuples by the first and then the second item in the tuple. So [(1,2),(4,2),(2,5)] would become [(1,2),(2,5),(4,2)] I am attempting to use the Sorty function but don't seem to be having much luck tuplesort :: [(a,b)] -> [(a,b)] tuplesort [(_,_)] = sortBy (comparing fst) -- this is an attempt to organise by the first element in the tuple Any help would be much appreciated

3 3

"sleep" in Haskell
by Michael Mossey 15 Oct '09

15 Oct '09

I can't find a "sleep" function in Haskell. This relates to my wish to use PortMidi and control the timing of actions. Maybe there is another way to do this sort of thing in Haskell. Can anyone help, either with PortMidi or just a general explanation of how IO operations could be placed at certain points in time? Thanks, Mike

6 9

Re: [Haskell-beginners] "sleep" in Haskell
by Michael Mossey 14 Oct '09

14 Oct '09

Hi Philippa, I think you have the right idea, and I want to try that too, but right now I'm using a kind of scattershot approach to try and learn everything I can, both about MIDI and Haskell in general. Since nothing is currently working, getting anything to work would be a learning experience. Thanks, Mike Philippa Cowderoy wrote: > Michael Mossey wrote: >> I can't find a "sleep" function in Haskell. This relates to my wish to >> use PortMidi and control the timing of actions. Maybe there is another >> way to do this sort of thing in Haskell. Can anyone help, either with >> PortMidi or just a general explanation of how IO operations could be >> placed at certain points in time? >> > > Y'know, I can't think of a single language where I'd consider using the > sleep function to time MIDI output. It's normally done with buffered > I/O. I don't know PortMidi, though. >

2 1

Problem to compile an example of Data Parallel Haskell
by Edgar GOMES DE ARAUJO 13 Oct '09

13 Oct '09

Hi, I have tried to compile an example of DPH (Data prallel Haskell) but I am not getting any success. The target is the concmop.hs (http://darcs.haskell.org/ghc-6.10/packages/dph/examples/concomp/) for calculate connected components in a graph. I am using GHC 6.10.4 compiled with extras libs (which also includes DPH 0.3.0). I've got this error. Seems that GHC can't find these functions/types. --------------------------------------------------------------------------------------------------------------------- edgar@edgar-ubuntu:~/Desktop/dph$ ghc --make -fdph-par -XTypeOperators ./concomp.hs [1 of 4] Compiling Graph ( Graph.hs, Graph.o ) Graph.hs:11:45: Not in scope: type constructor or class `:*:' Graph.hs:23:6: Not in scope: `hPutU' Graph.hs:33:15: Not in scope: `hGetU' --------------------------------------------------------------------------------------------------------------------- Using the example from Ben Jao Ming (http://overtag.dk/wordpress/2009/04/data-parallel-haskell/) I can successfully compile and run. Could anyone help me? -- Kind regards, ___________________________________________ *Ir. Edgar Gomes * Metallurgical Engineer /Ghent University/ Department Metallurgy and Materials Science B-9052 Zwijnaarde-Gent Belgium

2 1