October 2011 - Beginners

Iterating a monadic action with memoization
by Tim Cowlishaw 29 Jan '12

29 Jan '12

Hi all, I was wondering if any of you knew whether the following was possible, (I discussed this a little on #haskell at the weekend but didn't quite get to the bottom of it): Say I have a monadic action: > f :: a -> m a and an initial value of type a. I'd like to iterate this action and collect the results of each iteration in a list, as with 'iterate' on normal functions: > iterate (>>= f) . return $ a However, I would also like to memoize the intermediate results of the iteration, such that the entire iteration runs in O(n) time rather than O(n^2). This also implies a slight semantic difference, as side-effecting or non-deterministic actions will not be repeated in each iteration, while maintaining the laziness of the iteration function. Is it possible to do this? Many thanks, Tim

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Stack space overflow: using strict accumulator still fails
by Hugo Ferreira 01 Nov '11

01 Nov '11

Hello, Have a stack overflow but cannot see why (read up on [1], may be missing something trivial). Once again using the http://nlpwp.org/ book code. If I call the following function, it blows its top: scoreRule :: TransformationRule -> Z.Zipper (Tag, Tag) -> Int scoreRule r z = nCorrect - nIncorrect where (nCorrect, nIncorrect) = scoreRule_ r z scoreRule_ :: TransformationRule -> Z.Zipper (Tag, Tag) -> (Int, Int) scoreRule_ r = Z.foldlz' (scoreElem r) (0, 0) where scoreElem r s@(nCorrect, nIncorrect) z = case ruleApplication r z of Just tag -> if tag == correct then (nCorrect + 1, nIncorrect) else (nCorrect, nIncorrect + 1) Nothing -> s where (correct, _) = Z.cursor z however I see that the eager version of foldlz is being used. I also though that maybe ruleApplication my not be executing immediately. But I cannot see why (added definition below for reference). Can anyone point out why this is not strict? TIA, Hugo F. [1] http://www.haskell.org/haskellwiki/Stack_overflow ruleApplication :: TransformationRule -> Z.Zipper (Tag, Tag) -> Maybe Tag ruleApplication (NextTagRule (Replacement old new) next) z = do (_, proposed) <- Z.safeCursor z (_, nextProposed) <- rightCursor z if proposed == old && nextProposed == next then Just new else Nothing ruleApplication (PrevTagRule (Replacement old new) prev) z = do (_, proposed) <- Z.safeCursor z (_, prevProposed) <- leftCursor z if proposed == old && prevProposed == prev then Just new else Nothing ruleApplication (SurroundTagRule (Replacement old new) prev next) z = do (_, proposed) <- Z.safeCursor z (_, nextProposed) <- rightCursor z (_, prevProposed) <- leftCursor z if proposed == old && prevProposed == prev && nextProposed == next then Just new else Nothing

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Fwd: Stuck in debug of loop detection
by Lev Broido 31 Oct '11

31 Oct '11

---------- Forwarded message ---------- From: Lev Broido <lev.broido(a)gmail.com> Date: Mon, Oct 31, 2011 at 2:31 PM Subject: Stuck in debug of loop detection To: beginners(a)haskell.org, haskell-cafe(a)haskell.org Hi I am playing around with building graph using 'tying the knote' technique , according to rule a is child of b iff predicate a b == true Additional step is to detect loops in graph to enable it's print . Then I am using graph traversal to detect loops prior to calling show . For now I am stuck debugging loop detection . I am using 'trace' for a first time and I am not sure I am fully understanding it's behavior In additional I tried to make all calculations strict for debug . Can you please provide tips / review code . My code is at http://pastebin.com/5RvQ42z2 Thanks, Lev

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State Monad - Not in scope: data constructor `State'
by Hugo Ferreira 31 Oct '11

31 Oct '11

Hello, I am trying to use some State Monad code in order to learn about this. However in GHCi I cannot use the "State" constructor. import qualified Control.Monad.State >:{ | let fromStoAandS c | c `mod` 5 == 0 = ("foo",c+1) | | otherwise = ("bar",c+1) | :} > :t fromStoAandS >fromStoAandS :: Integral t => t -> ([Char], t) >:{ | let stateIntString = State fromStoAandS >Prelude Control.Monad.State| :} > ><interactive>:1:22: Not in scope: data constructor `State' I have found a message stating that this is not possible because their is "no State data constructor", it is only "defined to be a type alias". The author of the message then says we should use: http://hackage.haskell.org/packages/archive/transformers/latest/doc/html/Co… instead. Still lost however. So, how do I create the "Sate" above? TIA, Hugo F.

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difference between . and $
by haxl＠nym.hush.com 30 Oct '11

30 Oct '11

Doesn't . and $ do the same thing? I always get confused about that, like when would I use one over the other.

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Getting started with repa on MacOS
by Konrad Hinsen 29 Oct '11

29 Oct '11

Hi everyone, I had looked at Haskell some years ago but dropped it because there wasn't much support for the kind of work I do, which is numerical computation. Now there's exciting new stuff such as repa, so I decided to give it another try. I installed the Haskell platform 2011.2.0.1 for MacOS X 10.6 and started working through the tutorial at http://www.haskell.org/haskellwiki/Numeric_Haskell:_A_Repa_Tutorial Unfortunately I didn't get very far, because I am stuck at installing repa: cabal install repa Resolving dependencies... cabal: cannot configure repa-2.2.0.1. It requires base ==4.4.* For the dependency on base ==4.4.* there are these packages: base-4.4.0.0. However none of them are available. base-4.4.0.0 was excluded because of the top level dependency base -any I don't understand what this means (what's so bad about "any"?) nor do I have the slightest clue what to do about it. There's a brief Stack Overflow discussion about this at http://stackoverflow.com/questions/7375719/cabal-install-dependency-error-b… but it doesn't provide a solution either. Any help would be greatly appreciated - I just recall that my goal is not understanding cabal, but working with repa. Konrad.

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Haskell reports package?
by Gregory Guthrie 28 Oct '11

28 Oct '11

Are there facilities for generating nicely formatted reports from Haskell, perhaps like the Java report generation tools? I am mostly interested in nice tabular output from a data analysis, like the things that Jasper does. I suppose one approach is to generate eth appropriate Jasper XML (JRXML) from Haskell, and then process that. Any good experiences or recommendations in this area appreciated. >From Python there are Excel interfaces to have Excel do the formatting, is there a similar API from Haskell? -------------------------------------------

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Need some advices about university
by Zhi-Qiang Lei 28 Oct '11

28 Oct '11

Hi, I'm making a plan to gain more computer knowledge (functional programming and Haskell especially) and a Master's degree in a university. Does anybody know which university in USA has good resources in this domain? Thanks. Best regards, Zhi-Qiang Lei zhiqiang.lei(a)gmail.com

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IO question
by Rustom Mody 28 Oct '11

28 Oct '11

I have this from Peyton Jones awkward squad paper getTwoChars :: IO (Char,Char) getTwoChars = do c1 <- getChar c2 <- getChar return (c1,c2) Can someone explain what is happening here? *Main> getTwoChars ab ('a','b') *Main> getTwoChars a ('\n','a')

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Help with getLine and getChar
by Avery Robinson 27 Oct '11

27 Oct '11

I'm writing a program that calculates the maximum height of a stomp rocket (or any free falling object) given the starting height of the object and the amount of time that it takes to hit the ground. Here's what I have written: http://pastebin.com/KBJdPEJd The behavior that I am expecting is something like this: Starting height (ft): [user enters something here] Total air time (seconds): [user enters something here] Maximum height (ft): [the program's answer] Another? (Y/N) Then everything starts over if the user presses y, and execution stops if the user presses n. What actually happens: 1. The program waits for user input for both of the two fields before anything else, then it prints the prompts along with the answer. 2. The user needs to press enter before the their response to "Another? (Y/N)" is reacted to. I'd rather that the response is instant after just the y or n key is pressed. I don't really understand exactly how lazy evaluation works in the context of IO, so any help will be much appreciated.

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