- Beginners - Haskell.org

Re: \x -> x < 0.5 && x > -0.5
by Stephen Tetley 20 Oct '09

20 Oct '09

Christian Maeder wrote: > Hoogle did not find a function of type: > > (b -> b -> b) -> (a -> b) -> (a -> b) -> a -> b > > or (b -> c -> d) -> (a -> b) -> (a -> c) -> a -> d > > But maybe such a function is worth being added to Data.Function. Hello Christian This is the big Phi or S' combinator. I would certainly second its addition to Data.Function. sprime p q r s = p (q s) (r s) Best wishes Stephen

4 3

new locale?
by Robert Wills 19 Oct '09

19 Oct '09

Hello, http://hackage.haskell.org/package/old-locale-1.0.0.1 says "This package provides the old locale library. For new code, the new locale library is recommended." Does anyone know where I would find the new library? Thanks, Rob

2 2

parsing upto n items with parsec
by Ashish Agarwal 19 Oct '09

19 Oct '09

Hi. I'm just learning Parsec and Haskell. It is a great library, and I see how "many" lets you parse 0 or more items, "many1" parses 1 or more items, and "count" parses exactly n items. However, there is no combinator that parses between m and n items. What would be the best implementation for this?

3 4

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