September 2007 - Haskell-Cafe

Graphics.SOE
by Peter Padawitz 21 Sep '07

21 Sep '07

Can scrollbars be attached to windows created with openWindow?

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Slightly Off-topic
by PR Stanley 21 Sep '07

21 Sep '07

Hi Does anyone know of a good active list for students and experts in discrete mathematics, in particular, logic? Thanks, Paul

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Accumulator value for "and" and "or"
by PR Stanley 21 Sep '07

21 Sep '07

Hi or = foldl (||) False and = foldl (&&) True I can understand the rationale for the accumulator value - True && [] where [] = True and True || [] where [] = False Other than the practical convenience is there a reason for having the empty list in and and or equating to True and False? Thanks, Paul

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How can I stop GHCi from calling "show" for IO actions?
by Ryan Ingram 21 Sep '07

21 Sep '07

Prelude> let inf = repeat 1 Prelude> inf [1,1,(lots of output until I press ctrl-c),Interrupted. (I expect this to happen) Prelude> let x = inf (no output here!) Prelude> :t x x :: [Integer] Prelude> return inf [1,1,(lots of output until I press ctrl-c),Interrupted. (I also expect this to happen) Prelude> y <- return inf [1,1,(lots of output until I press ctrl-c),Interrupted. (I do not expect this to happen here!) Prelude> :t y <interactive>:1:0: Not in scope: 'y' Is this a bug? Why does "y <- return exp" have different behavior than "let y = exp"? Is there a way to make GHCi not print the result of an action but still make my variables get bound? -- ryan

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interaction between OS processes
by Andrea Rossato 21 Sep '07

21 Sep '07

Hi, there's something I don't get about interaction among OS processes and Haskell handles/channels. Suppose I have a very small program that takes a line and prints it till you write "quit": main = do s <- getLine case s of "quit" -> putStrLn "quitting" >> return () _ -> loop s where loop s = do putStrLn s main This is a small interactive process I would like to talk to from another Haskell program, like the following one which, indeed, is just a wrapper around the first. Now, if I write a single line with "quit", I get "quitting" back, otherwise the program doesn't work. I think I need some direction in order to understand how handles work. The same with channels, I'm afraid. Could you please point me in the right direction? Thanks for your kind attention. Andrea The not working code: import Control.Concurrent import System.Process import System.IO main = do c <- runInteractiveCommand "./main2" loop c loop c@(i,o,e,p) = do s <- getLine hPutStrLn i s hFlush i -- now "i" is closed, right? s' <- hGetLine o putStrLn s' loop c

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reverse with foldl
by PR Stanley 21 Sep '07

21 Sep '07

Hi reverse = foldl (xs x . x:xs) [] (xs ++ ) = foldl (\ys y -> ys ++ [y]) xs If I were to define reverse with foldl I would do it this way: reverse = foldl (\x -> \xs -> xs:x) [] Any idea what the first code frag is suposed to achieve? Thanks, Paul

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Re: [Haskell-cafe] What on earth
by PR Stanley 21 Sep '07

21 Sep '07

but ebook usually means a PDF which would be all right if the text didn't contain specialist symbols. The publishers weren't very helpful when I contacted them. According to the "lady" on the phone they didn't even have the latex source code. I'll probably try again. Hopefully this time I'll get someone more reasonable to deal with. Cheers, Paul At 00:51 21/09/2007, you wrote: >According to >http://www.cambridge.org/catalogue/catalogue.asp?isbn=9780511292187, >it is available as an ebook. Perhaps if you contact them and explain >that you have the paper version, they will send you an electronic copy >of it? > >Rodrigo > >On 20/09/2007, PR Stanley <prstanley(a)ntlworld.com> wrote: > > Hi > > tfoldr (-) 1 [4,3] > > = 4-3-(-1) > > = 2 > > A similar thing happens with foldr (/) 2 [8,2] = 8.0 > > I thought foldr \oplus v [] = v > > Any ideas? > > Thanks, Paul > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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Re: [Haskell-cafe] What on earth
by Miguel Mitrofanov 20 Sep '07

20 Sep '07

> tfoldr (-) 1 [4,3] > = 4-3-(-1) > = 2 Erm? You mean "foldr"? foldr (-) a [x,y,z,t,w] == x - (y - (z - (t - (w - a)))) So, foldr (-) 1 [4,3] == 4 - (3 - 1). > A similar thing happens with foldr (/) 2 [8,2] = 8.0 8/(2/2) = 8/1 = 8 > I thought foldr \oplus v [] = v Yeah, that's right.

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Haskell guards
by Victor Nazarov 20 Sep '07

20 Sep '07

I still can't remember how guards are treated in Haskell. Here is the code snippet in question: foo a | a == 1 = 6 foo a | a == 2 = 7 foo a = 8 Would Haskell fall through to the third alternative if a is not equal to 1 or 2. I know that this can be rewritten more sanely, but I just consider guard behavior. What would be the value of (foo 3)? -- vir http://vir.comtv.ru/

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length defined with foldr
by PR Stanley 20 Sep '07

20 Sep '07

Hi length = foldr (. n . 1 + n) 0 Any idea how to define length with foldr. The above definition doesn't make much sense. Many thanks, Paul

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