April 2012 - Beginners

PutStrLn
by Pierre Michallet 10 Apr '12

10 Apr '12

I have the impression that putStrLn adjusts its intercharacter spacing according to the length of the string to print. Can someone tell me : 1 Is it right 2 if yes , is there a function which does not apply that kind of proportionality Thank you for your help Pierre Michallet

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self recursive lambda
by sarfraz 09 Apr '12

09 Apr '12

Hi, Can't seem to be able to debug the following: import System.IO nHead :: Int -> String -> String nHead n str = if (length str) < n then str else if n ==0 then str else (\x n str -> if n == 0 -- this lambda does not work then str else x (n - 1) (tail str)) main = interact (nHead 5) I see where the problem is. But surely it is possible to make a recursive lambda. (I want to use a lambda, i know how to do otherwise) -- Sarfraz K.

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install wxHaskell on osx 10.7 64bit
by Miguel Negrao 09 Apr '12

09 Apr '12

Hi list, I’m very Haskell newbie, and I’m trying to install wxHaskell in order to try out reactive-banana-wx. What I did so far: 1 ) Installed lastest Haskell platform 2) Installed wxWidgets from source, latest dev version from http://sourceforge.net/projects/wxwindows/files/2.9.3/wxWidgets-2.9.3.tar.b… compiled with ../configure --with-osx_cocoa 3 ) darcs get http://darcsden.com/kowey/wxhaskell-osx64-dave built this one But then doing 'cabal install reactive-banana-wx’ will start installing wxcore-0.12.1.7 when the step 3) already installed wxcore-0.13.1 and that is the repository that should have the wxHaskell which works in osx lion. So is it possible to tell reactive-banana-wx to use wxcore-0.13.1 instead of wxcore-0.12.1.7 ? best, Miguel Negrão

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Knights exercies in <Learn your good haskell>
by Haisheng Wu 06 Apr '12

06 Apr '12

Hi there, There is one trivial exercise in chapter 12 <List Monad> saying 1. As an exercise, you can change this function so that when you can reach one position from the other, it tells you which moves to take. 2. how to modify this function so that we also pass it the number of moves to take instead of that number being hard-coded. I have one answer (find the URL below) but I'm not happy with it. https://github.com/freizl/dive-into-haskell/blob/master/monad/knight-queue.… Do you have better ideas? thanks. -Haisheng

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list monad and filterM
by franco00＠gmx.com 06 Apr '12

06 Apr '12

I don't get this example ( from Learn you a Haskell http://learnyouahaskell.com/for-a-few-monads-more ). The question is about filterM and the List Monad > import Control.Monad ( filterM ) > powerset :: [a] -> [[a]] > powerset xs = filterM (\x -> [True, False]) xs Looking into Control.Monad, filterM is defined: filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a] filterM _ [] = return [] filterM p (x:xs) = do flg <- p x ys <- filterM p xs return (if flg then x:ys else ys) Ok, let's do it step by step. > test = powerset [1,2,3] 'p' (in the filterM function) is p = \x -> [True, False] so flg is [True, False] Let's not calculate ys for a second, but go directly to the return function. return (if [True, False] then x:ys else ys) I expect the above code to throw a "Couldn't match expected type `Bool' with actual type `[Bool]'" So I can't see how the trick works! Any hint?

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Combining the Rand and State monads
by Amy de Buitléir 05 Apr '12

05 Apr '12

I'm trying to develop a very simple simulation framework. A simulation consists of a list of models. Models generate output events based on an input event. Here is what I have currently (it works fine). -----8<----- import Data.List (foldl') import Control.Monad.State ( get, gets, modify, put, runState, State ) type Event = Char type Environment = String -- | Given an event, a model generates zero or more new events type Model = Event -> State Environment [Event] modelA :: Model modelA a = do s <- get put $ s ++ a:"A " return $ a:"hip " modelT :: Model modelT a = do s <- get put $ s ++ a:"T " return $ a:"hop " -- | Process a sequence of events using one model runModel :: Model -> [Event] -> State Environment [Event] runModel m [] = return [] runModel m (e:es) = do s <- get let (es', s') = runState (m e) s let (es'', s'') = runState (runModel m es) s' put s'' return $ es' ++ es'' -- | Process a sequence of events using multiple models runModels :: [Model] -> [Event] -> State Environment [Event] runModels [] es = return [] runModels (m:ms) es = do s <- get let (es', s') = runState (runModel m es) s let (es'', s'') = runState (runModels ms es) s' put s'' return $ es' ++ es'' ----->8----- Now what I want to do is to give models the ability to generate random numbers. So I thought I might change the definition of Model to: type Model g = Event -> StateT Environment (Rand g) [Event] And then I could alter one of the models to use random numbers like so: modelT :: Model g modelT _ = do s <- get put $ s ++ " R" n <- HOW DO I GET A RANDOM NUMBER???? return [n] I can't figure out how to get a random number from the inner monad. I assume I use lift, but I can't get it to work. Would it be better to use RandT instead of StateT? (And if so, how would I do the state operations?) Thank you in advance for any help!

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Overloading resolution with numbers
by j.romildo＠gmail.com 05 Apr '12

05 Apr '12

Hello. Consider the following ghci session: Prelude> :t read "2" + 1 read "2" + 1 :: (Num a, Read a) => a Prelude> :t read "2.3" + 1 read "2.3" + 1 :: (Num a, Read a) => a Prelude> read "2" + 1 3 Prelude> read "2.3" + 1 *** Exception: Prelude.read: no parse Why does (read "2" + 1) works, but (read "2.3" + 1) fail at runtime? Romildo

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Understanding yampas par and dpSwitch
by Nathan Hüsken 05 Apr '12

05 Apr '12

Hi, I am currently working my way through the paper "Function Reactive Programming Continued", which is about yampa. On page 55 (page 5 in the pdf) "par" with the signature: par :: Functor col => (forall sf . (a -> col sf -> col (b, sf))) -> col (SF b c) -> SF a (col c) is introduced. So the resulting signal function (of type SF a (col c)) takes its input (of type a) and puts it into the first argument of par together with its second argument (col (SF b c)) to receive a collection of input/signal pairs (col (b, sf)). Correct? My questions: 1. Shouldn't be the signature of the first argument be: a -> col (SF b c) -> col (b, (SF b c)) I mean it receives the second argument of par as its second argument?!? 2. When the collection of input signal/pairs is received (col (b, sf)), the b is applied to sf, correct? Why not directly return the result of this application? I hope I was able to state clear enough questions ... Thanks! Nathan

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Re: [Haskell-beginners] functional parser type error
by felipe zapata 05 Apr '12

05 Apr '12

Hi, The parser is defined *type Parser a = String → [(a, String)]* * * But for me it is not pretty clear, why i need to make Parser a newtype instead of working with this one. Can you give me some hints ?. Thanks Felipe

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computing multiple attributes in Happy
by kak dod 04 Apr '12

04 Apr '12

Hello, I am trying out the happy parser. Take the second example Binary to Decimal given on the happy site: http://www.haskell.org/happy/doc/html/sec-AttributeGrammarExample.html This example computes the decimal value equivalent to the given binary string. I want to modify it so that along with the decimal value, it should compute a list of data values, like MyLeft, MyRight. I have modified the grammar given on the site as shown below (in the end). What I want to do is generate a list of MyLeft, MyRight such that MyLeft denotes 0 and MyRight denotes 1. So, if my input is "1011\n" then I expect the list attribute to be: [MyRight, Myleft, MyRight, MyRight] The happy compiles this thing without any problem, ghci compiles and loads it correctly. But when I run the following command I always get only the decimal value and not the list: So my questions are: 1. *How shall one compute, access multiple attributes in happy first? **(use in Haskell code, write to file etc)* 2. *How shall one access any one or multiple of the computed attributes in GHCi then?* 3. *Am I computing the list correctly? * Thanks in advance. kak Here is my happy grammar code: ----------------------------- { module BitsParser (parse) where test = parse "1011\n" -- how to write the list attribute to a file here? data Dirs = MyLeft | MyRight deriving Show fun a b = a^b } %tokentype { Char } %token minus { '-' } %token plus { '+' } %token one { '1' } %token zero { '0' } %token newline { '\n' } %attributetype { Attrs } %attribute value { Integer } %attribute pos { Int } %attribute list { [Dirs] } %name parse start %% start : num newline { $$ = $1 } num : bits { $$ = $1 ; $1.pos = 0 ; $1.list = [] } | plus bits { $$ = $2 ; $2.pos = 0 ; $2.list = [] } | minus bits { $$ = negate $2; $2.pos = 0 ; $2.list = [] } bits : bit { $$ = $1 ; $1.pos = $$.pos ; $1.list = $$.list } | bits bit { $$ = $1 + $2 ; $$.list = $1.list ++ $2.list ; $1.pos = $$.pos + 1 ; $2.pos = $$.pos } bit : zero { $$ = 0 ; $$.list = [MyLeft] } | one { $$ = fun 2 ($$.pos) ; $$.list = [MyRight] } { happyError = error "parse error" }

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