- Beginners - Haskell.org

I am having trouble with the type declaration for creating an identity matrix.
by KC 24 Apr '12

24 Apr '12

initIdentityMat :: Int -> ST s (STUArray s (Int,Int) ((Int, Int), Double)) initIdentityMat m = newListArray ((1,m),(1,m)) ([((i,j), if i == j then 1.0 else 0.0) | i <- [1..m], j <- [1..m]] :: [((Int,Int), Double)]) Doesn't seem to compile, nor do minor variations of the type declaration. -- -- Regards, KC

1 0

A strange expression in Quick Hull example
by Zhi-Qiang Lei 24 Apr '12

24 Apr '12

I copy the Quick Hull snippet from http://darcs.haskell.org/packages/dph/examples/quickhull/QuickHull.hs The "packed" expression in "hsplitList" is much confusing to me. Why does it have to zip points with cross? Is [ p | (c, p) <- cross, c > 0.0 ] not enough? Thanks. data Point = Point !Double !Double data Line = Line Point Point instance Show Point where show (Point x y) = show (x, y) distance :: Point -> Line -> Double distance (Point xo yo) (Line (Point x1 y1) (Point x2 y2)) = (x1 - xo) * (y2 - yo) - (y1 - yo) * (x2 - xo) upper :: (a -> a -> Bool) -> [(a, b)] -> b upper above = snd . foldl1 pick where pick left@(kl, _) right@(kr, _) | kl `above` kr = left | otherwise = right hsplitList :: [Point] -> Line -> [Point] hsplitList points line@(Line p1 p2) | length packed < 2 = p1:packed | otherwise = hsplitList packed (Line p1 pm) ++ hsplitList packed (Line pm p2) where cross = [ (distance p line, p) | p <- points ] packed = [ p | (p, (c, _)) <- zip points cross, c > 0.0 ] pm = upper (>) cross quickHullList :: [Point] -> [Point] quickHullList [] = [] quickHullList points = hsplitList points (Line minx maxx) ++ hsplitList points (Line maxx minx) where xs = [ (x, p) | p@(Point x y) <- points ] minx = upper (<) xs maxx = upper (>) xs Best regards, Zhi-Qiang Lei zhiqiang.lei(a)gmail.com

1 0

Re: [Haskell-beginners] [Haskell-cafe] The last statement in a 'do' construct must be an expression: a <- readArray arr (1, 1)
by KC 23 Apr '12

23 Apr '12

Thank you, that was it. I was mixing up and tabs and spaces. I expected new versions of NotePad++ to keep my old settings. On Mon, Apr 23, 2012 at 1:54 PM, Ryan Yates <fryguybob(a)gmail.com> wrote: > Perhaps you are mixing tabs and spaces? > > On Mon, Apr 23, 2012 at 4:52 PM, KC <kc1956(a)gmail.com> wrote: > >> I'm getting the above error message and I cannot figure out why? >> >> >> buildPair = >> do >> arr <- newArray ((1,1),(1,10)) 37 :: ST s (STArray s (Int,Int) Int) >> a <- readArray arr (1,1) >> writeArray arr (1,1) 64 >> b <- readArray arr (1,1) >> return (a,b) >> >> >> main = print $ runST buildPair >> >> >> -- >> -- >> Regards, >> KC >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> > -- -- Regards, KC

1 0

The last statement in a 'do' construct must be an expression: a <- readArray arr (1, 1)
by KC 23 Apr '12

23 Apr '12

I'm getting the above error message and I cannot figure out why? buildPair = do arr <- newArray ((1,1),(1,10)) 37 :: ST s (STArray s (Int,Int) Int) a <- readArray arr (1,1) writeArray arr (1,1) 64 b <- readArray arr (1,1) return (a,b) main = print $ runST buildPair -- -- Regards, KC

1 0

(no subject)
by KC 23 Apr '12

23 Apr '12

buildPair = do arr <- newArray ((1,1),(1,10)) 37 :: ST s (STArray s (Int,Int) Int) a <- readArray arr (1,1) writeArray arr (1,1) 64 b <- readArray arr (1,1) return (a,b) main = print $ runST buildPair -- -- Regards, KC

1 0

Is it only one data structure per ST monad?
by KC 23 Apr '12

23 Apr '12

Is it only one data structure per ST monad? -- -- Regards, KC

2 1

Fwd: Implementing a spellchecker - problem with Data.HashTable performance
by Radosław Szymczyszyn 23 Apr '12

23 Apr '12

Hello fellow Haskellers, It's my first post to this list, so at first: Greetings to everyone! I've wanted to pick up Haskell for some time and I've run in to a perfect (at least it seemed so) oocasion at my Natural Language Processing course. At first I tried using Python, but it turned out that some extra performance wouldn't hurt and I hoped Haskell would give me some boost without forcing me to sacrifice high-level expressiveness at the same time. The first lesson was to use ByteStrings instead of ordinary Strings for IO. Then I needed to build a dictionary of words found in the source text. That's the problematic part - I've tried using a few structures, namely Data.Map and Data.HashTable from base package and Data.HashMap from unordered-containers. In all cases I couldn't reach acceptable performance. As of that, I'd like to ask about some advice and comments about the following examples. First example [1] just builds a HashTable of ~1 500 000 ByteStrings. The program takes about 11 seconds to execute. Second example [2] does roughly the same, but taking the HashTable data from a file (just a dictionary of valid Polish forms). Reading data itself is not a problem, as I've measured the execution time of a program which just reads the file (and prints its last line to actually force the read, hence "handle" Haskell's laziness) and it takes no more than a second. Coupled together reading and building, however, takes unbearably long. I didn't wait till the program stopped and just terminated it, but it had been running for ~10 minutes already. The most disappointing fact is that a roughly equivalent (and rather effortless to write in contrast to the Haskell example - I'm just learning though) Python test which just reads everything into a dict takes about 10 seconds to execute (reading and updating the dict of course). Writing the spellchecker in Python seemed pretty straightforward for me (but its certainly not the only language I know; I've had some Java (who didn't nowadays), system level C and am daily working in Erlang). I understand of course, that the most convenient tool is generally the one you know the best, but my experience writing the Haskell equivalent reached both extremes - effortless expression of the logic, but frustration because of the unexplainable (for me at least) performance behaviour. The question is why is it so slow? (or AM I RILY DAT DUMB? ;) The link to examples: [1] https://gist.github.com/2411699 TestHashTableArtificial [2] https://gist.github.com/2411699 TestReadIntoHashTable I'm looking forward to somebody shedding some light on this. Thanks in advance! Best regards, Radek Szymczyszyn

7 10

Two questions for a production project...
by Mike Meyer 23 Apr '12

23 Apr '12

I've got a project coming up that I could use haskell for, providing I can convince myself that it's appropriate. The critical question is: Anyone using Haskell in a long-running production application? I'm talking about something where the program would run for weeks or months non-stop, with either multiple threads or multiple copies. <mike -- Mike Meyer <mwm(a)mired.org> http://www.mired.org/ Independent Software developer/SCM consultant, email for more information. O< ascii ribbon campaign - stop html mail - www.asciiribbon.org

5 5

state monad
by Paul Higham 22 Apr '12

22 Apr '12

I am trying to come to grips with the state monad and as a starting point I copied the some code directly from the book Learn You a Haskell for Great Good then added the type synonym for Stack import Control.Monad.State type Stack = [Int] pop :: State Stack Int pop = State $ \(x:xs) -> (x,xs) push :: Int -> State Stack () push a = State $ \xs -> ((),a:xs) However, attempting to load this into ghci produces the errors stackPlay.hs:6:7: Not in scope: data constructor `State' stackPlay.hs:9:10: Not in scope: data constructor `State' Please excuse the cognitive deficit, but I cannot figure out why this should not work as advertised. ::paul

3 2

Embed haskell
by Rosario Borda 22 Apr '12

22 Apr '12

Hi All, There is a simple method for embed haskell in other languages, like latex? An example in quasiquotation notation: \documentclass{article} \title{Hello World} \begin{document} \maketitle Hello world! and reversed: [haskell: putStrLn reverse "Hello, World!"] \end{document} Many thanks, :) Rosario

1 0