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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 31 Oct '11

31 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 29 Oct '11

29 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 29 Oct '11

29 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 29 Oct '11

29 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 28 Oct '11

28 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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Generating Code To Embed in an Event Loop
by Darrin Thompson 27 Oct '11

27 Oct '11

I've been thinking about how I could use Haskell for programming just the "business logic" for games. So for a mobile game I might use an existing library such as libgdx or cocos2d for input handling and sprite batching. Where I'd like to use Haskell is for writing the big function to process the inputs and output a sprite list and audio triggers. Why Haskell? In even a simple 2d platformer there's a lot of polish needed to get the game to feel good to a player. For instance, if I was using box2d physics to take care of movement and collisions, I might still need to detect that inputs indicate applying a force vector opposite to the player's current velocity and the player is "on the floor". A naive game would show the player moon walking. A more sophisticated game would show a skidding player sprite or animation until the player's velocity reaches 0. I was hoping to use Haskell to capture a lot of little details like that more elegantly than I can in C or Java. However, my program needs to be compiled for a mobile device. I'm wary of trying to use GHC to generate arm machine code. I've had bad luck with cross compilers. It seemed to me that I could design a GADT to model my game actors and their relationships. I would then (misused term I'm sure) "lift" my behavior functions into some context where they can produce Java or C code for use in Xcode or Eclipse. That's my favored approach. Is there some background I could look at to save myself some dead ends pursuing this? -- Darrin

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Forcing output of show
by Hugo Ferreira 27 Oct '11

27 Oct '11

Hello, Apologies for this newbie question. I have a test function: testLearnRules train = do h <- IO.openFile train IO.ReadMode c <- IO.hGetContents h let proposedRules = instRules $ words c let rs = take 10 $ M.assocs proposedRules let _ = show rs -- let _ = length rs return () I realize that nothing is executed due to Haskell's lazy evaluation. How can I force show to print something to the screen? Appreciate any pointers, TIA, Hugo F.

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When to use ST
by Alex Rozenshteyn 26 Oct '11

26 Oct '11

Does anyone have any guidelines on when to use ST in an algorithm vs looking for a non-monadic, pure solution? The reason I ask is that I have just implemented an Earley parser for class (in Python) and would like to re-implement it in Haskell. The algorithm makes heavy use of mutable state, some of which can clearly be refactored out, but some of which can't be done so clearly. Thanks. -- Alex R

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