September 2012 - Beginners

firering event in netwire
by Nathan Hüsken 29 Oct '12

29 Oct '12

Hey, I have a few question about the usage of netwire: How do I fire events? For example my main wire should take keyboard events as input and output some state (which is than somehow outputed). I see that there is an event type, which has the some type as the identiy wire. I am just not sure how to use it? Or asked differently, how is the "keyDown" event defined, that was talked of in an earlier mail? Thanks! Nathan

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Re: [Haskell-beginners] FRP
by Christopher Howard 16 Oct '12

16 Oct '12

On 09/17/2012 12:09 PM, Kim-Ee Yeoh wrote: > Ever wired up some simple electronic circuits? Take a flip-flop for > instance. Suppose each electronic component was a Haskell function. How > would you design their types in order for the whole program be > functionally equivalent? > > FRP is a theory for the design and construction of programs along such > lines. > > > -- Kim-Ee > > Which FRP framework (i.e., Haskell package) is the best one to play with for someone still trying to grasp the basics of FRP? -- frigidcode.com indicium.us

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Using my first map instance
by Darren Grant 30 Sep '12

30 Sep '12

Hi all, I'm working on project euler problem 14, whose solution is the maximum Collatz chain length for all natural numbers less than 1-million. The naive approach takes too long to execute: collatz 1 = [1] collatz n = let next x | even x = x `div` 2 | otherwise = 3*x+1 in n:collatz (next n) result = maximum [length (collatz x) | x <- [1..999999]] I know there are a handful of approaches to take to reduce computation time, but in this case I am focusing on exploiting fast recollection of previously computed sub-chain lengths in a map. So I wrote the following instead: import qualified Data.Map as M type CollatzSubMap = M.Map Int [Int] collatz :: (Int, CollatzSubMap) -> ([Int], CollatzSubMap) collatz (1,m) = ([1], m) collatz (n,m) = let next x | even x = x `div` 2 | otherwise = 3*x+1 in case M.lookup n m of Nothing -> let (ns,m') = collatz (next n, m) in (n:ns, M.insert n (n:ns) m') Just ns -> (ns,m) result = maximum [length $ fst $ collatz (x, M.empty) | x <- [1..999999] :: [Int]] Where I'm currently stumped is in feeding the resulting map from one call to collatz into the next iteration in the list comprehension; that M.empty should carry the end result of previous iterations. Can anyone point me in the right direction? Other criticisms of the code are also welcome. Cheers, Darren

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Making sense of currying in the context of the Hask Category
by Lino Rosa 29 Sep '12

29 Sep '12

Hi, I'm a Haskell newbie and I couldn't quite make sense of how currying maps to the the Hask Category. How would I map, for instance (+) to a Hask 'arrow'? If objects are types on Hask, then would a -> a -> a be the first object on this chain? In that case, for the first arrow, would I have as many arrows as there are possible partial applications on this type? In other words, would I have (+) 1, (+) 2, (+) 3 ... all those transitioning to the second object ( a -> a ) Or, do I have ONE arrow only, like (+) a ? In either case, what happens after 'm left with the object a -> a? What function (arrow) mutates it to the final value 'a'? That's the function resulting from the previous partial application of (+), but that fuction only exists at run time, after you apply the first one. I guess the question is, if you'd have to write a diagram for this, what would you write beside each object and beside each arrow?

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confused by <-
by Bryce Verdier 28 Sep '12

28 Sep '12

I'm trying to play with a Yesod application. I had working code before, but tried stretching myself a bit and adding in some error checking. This code worked: getGoogR = do body <- simpleHttp "http://www.google.com" jsonToRepJson $ object ["response" .= (showDigest $ sha1 body)] This is the new code that is trying to use try (imported from Control.Exception) getGoogR = do body <- try (simpleHttp "http://www.google.com") :: IO (Either SomeException Data.ByteString.Lazy.Internal.ByteString) case body of Left _ -> jsonToRepJson $ object ["response" .= ( show "ERROR: NO DATA FOR ONE REASON OR ANOTHER")] Right val -> jsonToRepJson $ object ["response" .= (showDigest $ sha1 val)] and I get this error: Couldn't match expected type `IO b0' with actual type `GHandler sub0 master0 RepJson' In the expression: jsonToRepJson $ object ["response" .= (show "ERROR: NO DATA FOR ONE REASON OR ANOTHER")] In a case alternative: Left _ -> jsonToRepJson $ object ["response" .= (show "ERROR: NO DATA FOR ONE REASON OR ANOTHER")] In a stmt of a 'do' block: case body of { Left _ -> jsonToRepJson $ object ["response" .= (show "ERROR: NO DATA FOR ONE REASON OR ANOTHER")] Right val -> jsonToRepJson $ object ["response" .= (showDigest $ sha1 val)] I'm sure that my problem has a simple fix, but I just don't know what it is. I will be happy to share any other code if people ask, I'm just giving the short version for brevities sake. Thanks in advance, Bryce

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FRP
by Christopher Howard 27 Sep '12

27 Sep '12

Hi. I'm trying to understand what Functional Reactive Programming is, or, more properly, what distinguishes it from "regular" functional programming, and what it would look like if I were to program in a "FRP Style", i.e., without some mysterious FRP module hiding the details. After reading a bunch of links and even looking at some source code I'm still not clear on this. If I program in an event-based style, and have data structures updating over time, with some data structures dependent on others for their values, and I use a functional language, is that FRP? Or is there some essential element I'm missing here? -- frigidcode.com indicium.us

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Lazy variant of sequence (or other way to approach problem)
by Nathan Hüsken 27 Sep '12

27 Sep '12

Hey, In my (SDL based) haskell program, I do: events <- liftM ( takeWhile (/= NoEvent)) $ sequence $ repeat pollEvent The execution of this never returns, I am guessing that is because sequence evaluation never stops. But if sequence would be lazy (and assuming pollEvent returns NoEvent at some point) this should stop, should it not? Is there a lazy variant of sequence? Or am I missing something here completely? Thanks! Nathan

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Picking a random element from a Map
by Harald Bögeholz 26 Sep '12

26 Sep '12

Hi, how do I (quickly) pick a random element from a `Data.Map.Map`? Right now I am using `head $ Map.keys p` to get the first element, but I'd like to replace this by a random element. I know I have to drag around the state of the random generator somewhere, but if I have one I am wondering what's the best way to use one to take a pick from the map. Thanks -- Harald Bögeholz <bo(a)ct.de> (PGP key available from servers) Redaktion c't Tel.: +49 511 5352-300 Fax: +49 511 5352-417 http://www.ct.de/ int f[9814],b,c=9814,g,i;long a=1e4,d,e,h; main(){for(;b=c,c-=14;i=printf("%04d",e+d/a),e=d%a) while(g=--b*2)d=h*b+a*(i?f[b]:a/5),h=d/--g,f[b]=d%g;} (Arndt/Haenel) Affe Apfel Vergaser /* Heise Zeitschriften Verlag GmbH & Co. KG * Karl-Wiechert-Allee 10 * 30625 Hannover * Registergericht: Amtsgericht Hannover HRA 26709 * Persönlich haftende Gesellschafterin: Heise Zeitschriften Verlag * Geschäftsführung GmbH * Registergericht: Amtsgericht Hannover, HRB 60405 * Geschäftsführer: Ansgar Heise, Dr. Alfons Schräder */

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Mapping list over datatype using Traversable and State monad.
by Dmitriy Matrosov 25 Sep '12

25 Sep '12

Hi. I have a data type data Line a = Line [a] [a] which groups elements into "ordered" (first list) and "other" (second list) ones. And i want a functions representing Line, which actually has two heads (one from 1st list and one from 2nd), as single-headed. In other words, i want to map a list over Line (preserving Line structure), i.e. implement a function with type f :: [a -> b] -> Line a I have done this using Traversable import Data.Monoid import qualified Data.Foldable as F import qualified Data.Traversable as T import Control.Applicative import Control.Monad.State instance Functor Line where fmap f (Line xs ys) = Line (map f xs) (map f ys) instance F.Foldable Line where foldMap f (Line xs ys) = (F.foldMap f xs) `mappend` (F.foldMap f ys) instance T.Traversable Line where traverse f (Line xs ys) = Line <$> (T.traverse f xs) <*> (T.traverse f ys) and my function (which actually used for adding separators (sp ++) into (Line String), and i don't want to add separator before the first element, regardless of whether first element is "ordered" or "other") looks like inlineSeps :: (a -> a) -> Line a -> Line a inlineSeps g = fst . flip runState (id : repeat g) . T.mapM f where f x = do (f : fs) <- get put fs return (f x) It works, but i'm not sure whether using state monad here is good? And whether this is good solution for such problem at all?

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alut initialization
by Christopher Howard 23 Sep '12

23 Sep '12

Hi. I'm trying to learn ALUT/OpenAL modules so I can have sound in my app. I'm a bit confused by the alut module interface, because it uses this alutRun function that is different than what is in the actual alut library documentation. I thought I figured it out, but apparently not because this simple program doesn't work: code: -------- import Sound.ALUT main = do putStrLn $ "alutAPIVersion: " ++ alutAPIVersion runALUT "" [] (\x y -> return ()) mimes <- get bufferMIMETypes mapM putStrLn mimes *Main> main alutAPIVersion: 1.1 *** Exception: user error (bufferMIMETypes: The operation was invalid in the current ALUT state) -------- Is there anyone familiar with the module that can point me in the right direction? -- frigidcode.com indicium.us

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