June 2014 - Beginners

Data.Stream interleave implementation question
by Bryan Brady 18 Aug '14

18 Aug '14

I'm in the process of learning Haskell and am implementing my own Stream type. ( I'm working my way through homework problems I found at http://www.seas.upenn.edu/~cis194/hw/06-laziness.pdf ) One of the questions is: "Deﬁne the stream ruler :: Stream Integer which corresponds to the ruler function 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 4, . . . where the nth element in the stream (assuming the ﬁrst element corresponds to n = 1) is the largest power of 2 which evenly divides n. " I got pretty close, but ended up looking at Data.Stream to nail down my mistake. My original solution was: ----- data Stream a = Cons a (Stream a) deriving (Eq, Ord) streamRepeat :: a -> Stream a streamRepeat a = Cons a (streamRepeat a) interleaveStreams :: Stream a -> Stream a -> Stream a interleaveStreams (Cons a as) (Cons b bs) = Cons a (Cons b (interleaveStreams as bs)) ruler :: Stream Integer ruler = foldr1 interleaveStreams (map streamRepeat [0..]) ----- Attempting to evaluate 'ruler' in ghci does not finish. I knew something was wrong with my implementation of interleaveStreams because it worked when I expanded it out manually for a few steps. The way Data.Stream implements interleave is: interleaveStreams (Cons a as) bs = Cons a (interleaveStreams bs as) I don't understand why this works and why my original implementation doesn't. I figure if the latter works, the former should as well. Here is my reasoning: In the latter definition, Cons a (interleaveStreams bs as), (interleaveStreams bs as) is a thunk. The thunk should only be evaluated when it is needed. In my original definition, (interleaveStreams as bs) is a thunk. The difference is an extra Cons (e.g., Cons b). It seems like the additional Cons is causing problems, I just don't understand why. Can anyone point out what I'm missing? thanks, bb

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Functors in Haskell
by Dimitri DeFigueiredo 06 Jul '14

06 Jul '14

Hi All, I'm trying to merge in my head the concepts of: - functors from category theory; with, - the Functor typeclass in haskell. Here's what I have come up with: A Functor F in haskell is a type constructor that provides a functor map (fmap) and that maps Hask (the category of haskell types) into a category generated by the Functor. For example, Maybe maps the category of all haskell types "a" (i.e. Hask) into the category of all types of the form "Maybe a" by providing an implementation of fmap. Neglecting the fact that apparently Hask isn't a category, does this sound reasonable? I think the insight for me here is that functors in haskell are type constructors mapping Hask into another category. Is that a good way to think about it? Thanks! Dimitri

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Most important Functional Methods
by Richard Seldon 03 Jul '14

03 Jul '14

Hello, I have a general question, not specific to Haskell although I am learning Haskell as I ask this question.. Please can someone provide consensus on the most important functional methods in their view. I read somewhere that having map, reduce, filter, mergeAll, and zip pretty much means everything else can be derived. Further, what I specifically want to know is whether flatMap should be included as an implementation in a functional library, or whether it should be excluded because it is derivable using “building block” functions. Hope this makes sense. In languages like JS, there is a lot of resistance to include flatMap in prominent libraries such as underscore and lodash. Am curious to know what the Haskell community view is here, or whether in this community too opinions are widely divided. Alternatives I have seen are: 1). Chaining functions together chain(map(), map() 2). Using reduce() - which seems so versatile like a for loop for anything if really want 3). Using flatten(), or chaining pluck and flatten 4). Using map flatten compact - this gets rid of null, undefined etc in a language like JS too Recently, Reactive extensions have got really popular and it seems increasingly that flatMap is ideal for these situations. Would welcome some general advice and thoughts. If this is off-topic of course, then sorry to have caused any inconvenience. My thoughts are that the Haskell community have the necessary experience regarding functional programming in general to offer opinion. Best regards,

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Functions in "where" block.
by Venu Chakravorty 24 Jun '14

24 Jun '14

Hello everyone, I can't figure out why the following function does not work: myFun :: Integer -> Integer myFun x = op x + ep x where op x = 99 ep x = 1 -- squawks here Yeah it's a stupid function, but I was just trying to declare two functions in the "where" block. I expected this to work and always return 100. This is the error I get while compiling: ================== Prelude> :l fun.hs [1 of 1] Compiling Main ( fun.hs, interpreted ) fun.hs:4:15: parse error on input `=' Failed, modules loaded: none. ================== Could someone please tell me where I am going wrong? Thanks in advance. Regards, Venu Chakravorty.

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attoparsec: How to use the many1 combinator
by Roland Senn 21 Jun '14

21 Jun '14

I'm trying to do a first very simple example with attoparsec. The file "test.txt" contains the line: START 111 2222 333 END The following code works and gives the result: Done "\n" ["111","2222","333"] {-# LANGUAGE OverloadedStrings #-} import qualified Data.ByteString as BS import Control.Applicative import Data.Attoparsec.ByteString as P main :: IO() main = do bs <- BS.readFile "test.txt" parseTest pTest bs pTest :: Parser [BS.ByteString] pTest = do string "START" n1 <- pNumber n2 <- pNumber n3 <- pNumber string "END" return [n1, n2, n3] pNumber :: Parser BS.ByteString pNumber = do pSkipSpaces term <- P.takeWhile (\c -> c >= 0x31 && c <= 0x39) pSkipSpaces return term pSkipSpaces :: Parser () pSkipSpaces = do P.takeWhile (\c -> c == 0x20) return () Unfortunately I must have exactly three numbers between START and END. To make this more flexible, I changed pTest: pTest :: Parser [BS.ByteString] pTest = do string "START" ns <- P.many1 $ pNumber <* (string "END") return ns Now the program fails with: Fail "2222 333 END\n" [] "Failed reading: takeWith" Why ?? Many thanks for your help!

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Lions, Wolves and Goats
by Elric 16 Jun '14

16 Jun '14

Hi, / Disclaimer: I have been learning Haskell for a month and there are still several things about this wonderful language I know nothing of, so please bear with me. Also, I apologize for this (somewhat) long mail./ I came across this article: http://unriskinsight.blogspot.co.at/2014/06/fast-functional-goats-lions-and… a couple of days ago. This compares performance of solving a problem (which I will get to) using the functional constructs alone in languages like C++11 and Java 8. Since, Haskell is my first foray into FP, I thought I should try solving this in Haskell. So the problem at hand is this: There is a magical forest which has only Lions, Wolves and Goats. Lions are stronger than Wolves which are in turn stronger than Goats. Each strong animal is capable of eating a weaker animal, which also in turn transforms the '/eater/' into an animal which was not involved. /i.e./ If a Lion eats a Wolf it gets transformed into a Goat. If a Wolf eats a Sheep it gets transformed into a Lion. Below are the two versions of the code I came up with to solve this. Neither of them converge to the 'endStates' even after about 15 minutes. So there is definitely something wrong with what I have done. But after banging my head on the keyboard for more then a day with this, I would appreciate some pointers or help. -- version 1 import Data.List data Animal = Lion Int | Wolf Int | Goat Int deriving (Show, Eq) type Forest = [Animal] -- lions f = count 0 f -- where -- count acc [] = acc -- count acc ((Lion a):as) = acc + a + (count acc as) -- count acc (_:as) = acc + (count acc as) -- wolfs f = count 0 f -- where -- count acc [] = acc -- count acc ((Wolf a):as) = acc + a + (count acc as) -- count acc (_:as) = acc + (count acc as) -- goats f = count 0 f -- where -- count acc [] = acc -- count acc ((Goat a):as) = acc + a + (count acc as) -- count acc (_:as) = acc + (count acc as) lions [Lion l, Wolf w, Goat g] = l wolfs [Lion l, Wolf w, Goat g] = w goats [Lion l, Wolf w, Goat g] = g --Invalid eat calls are returned with [], to denote termination eat :: Forest -> Animal -> Animal -> Forest eat f (Lion _) (Goat le) = if (l >= le && g >= le) then [Lion (l-le), Wolf (w+le), Goat (g-le)] else [] where l = lions f w = wolfs f g = goats f eat f (Lion _) (Wolf le) = if (l >= le && w >= le) then [Lion (l-le), Wolf (w-le), Goat (g+le)] else [] where l = lions f w = wolfs f g = goats f eat f (Wolf _) (Goat we) = if (w >= we && g >= we) then [Lion (l+we), Wolf (w-we), Goat (g-we)] else [] where l = lions f w = wolfs f g = goats f eat _ _ _ = [] lionEatGoat :: Forest -> Forest lionEatGoat f = eat f (Lion 0) (Goat 1) lionEatWolf :: Forest -> Forest lionEatWolf f = eat f (Lion 0) (Wolf 1) wolfEatGoat :: Forest -> Forest wolfEatGoat f = eat f (Wolf 0) (Goat 1) meal :: Forest -> [Forest] meal [] = [] meal f@[Lion l, Wolf w, Goat g] | endState f = [] | l == 0 = [f] ++ weg | w == 0 = [f] ++ leg | g == 0 = [f] ++ lew | (l /= 0) && (w /= 0) && (g /= 0) = [f] ++ leg ++ lew ++ weg | otherwise = [] where leg = meal $ lionEatGoat f lew = meal $ lionEatWolf f weg = meal $ wolfEatGoat f endState :: Forest -> Bool endState f = if ((l == 0 && g == 0) || (l == 0 && w == 0) || (w == 0 && g == 0)) then True else False where l = lions f w = wolfs f g = goats f endStates = filter endState main = do putStrLn $ show $ endStates $ meal [Lion 6, Wolf 55, Goat 17] I thought using the ADT was causing the performance issue and reverted to using a plain 3-termed list which holds [Lion count, Wolf Count, Sheep Count] :: [Int] -- version 2 import Data.List lionEatGoat :: [Int] -> [Int] lionEatGoat [l,w,g] = [l-1,w+1,g-1] lionEatWolf :: [Int] -> [Int] lionEatWolf [l,w,g] = [l-1,w-1,g+1] wolfEatGoat :: [Int] -> [Int] wolfEatGoat [l,w,g] = [l+1,w-1,g-1] meal :: [Int] -> [[Int]] meal [] = [] meal f@[l, w, g] | endState f = [] | l == 0 = (f:weg:(meal weg)) | w == 0 = (f:leg:(meal leg)) | g == 0 = (f:lew:(meal lew)) | (l /= 0) && (w /= 0) && (g /= 0) = (f:leg:lew:weg:(meal leg ++ meal lew ++ meal weg)) | otherwise = [] where leg = lionEatGoat f lew = lionEatWolf f weg = wolfEatGoat f endState :: [Int] -> Bool endState [l,w,g] = if ((l == 0 && g == 0) || (l == 0 && w == 0) || (w == 0 && g == 0)) then True else False endStates = filter endState main = do putStrLn $ show $ endStates $ meal [Lion 6, Wolf 55, Goat 17] This is still extremely slow, without the program ever terminating. Can someone please tell me what I am doing wrong. -Elric

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Cabal and proxy on Windows 8
by Christian Sperandio 10 Jun '14

10 Jun '14

Hi, I’d like to try Haskell at my work and if the result is good I want to push this language in the company. But I’ve got an issue because of our proxy configuration. The developers work on Windows 8 and our proxy is configured with a PAC file. The issue is we can’t use cabal to fetch package. When we run the cabal update command, nothing happen. We understand the problem comes from the proxy is not used inside a command prompt. I tried to use the Eclipse plugin but unfortunately, it uses the cabal command too. Somebody knows how we can have a well configured cabal ? Thanks. Chris Ps1. I try the netssh winhttp command. But I don’t succeed to define the PAC file. It seems, we can only define a proxy address. But it’s not enough. Ps2. The internet settings on Windows is like that: http://proxyserve:3810/fpac_file

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Made a video sharing my Haskell workflow and thought process while hacking on a library I wrote
by Christopher Allen 08 Jun '14

08 Jun '14

Not a straw-project, real thing I've been working on. Have gotten good feedback so far and was told I should post it here. More feedback, improvement suggestions desired. This is my first video. Made it because I thought there's a bit of a gap between the beginner and Kmett-grade expert material. This might help explain some Aeson usage patterns as well. https://www.youtube.com/watch?v=Li6oaO8x2VY I'll make more videos covering a broader range of topics if people seem interested. Thanks for your time everybody. --- Chris

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About ! before a type signature
by Song Zhang 07 Jun '14

07 Jun '14

This is about syntax of Haskell. I am reading the source code of Yampa. I find that a definition like the following data SF' a b where SFArr :: !(DTime -> a -> Transition a b) -> !(FunDesc a b) -> SF' a b in Yampa.hs.

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Project euler question
by martin 06 Jun '14

06 Jun '14

Hello all, I tried to solve Problem 24 (https://projecteuler.net/problem=24) and came up with the following solution: import Data.List.Ordered import Data.Char elems = [0,1,2,3,4,5,6,7,8,9] :: [Int] x = do a <- elems b <- elems `without` [a] c <- elems `without` [a,b] d <- elems `without` [a,b,c] e <- elems `without` [a,b,c,d] f <- elems `without` [a,b,c,d,e] g <- elems `without` [a,b,c,d,e,f] h <- elems `without` [a,b,c,d,e,f,g] i <- elems `without` [a,b,c,d,e,f,g,h] j <- elems `without` [a,b,c,d,e,f,g,h,i] return [a,b,c,d,e,f,g,h,i,j] without a b = minus a ( sort b) solution = filter isDigit $ show $ (x !! 1000001) -- "2783915640" PE tells me that this is wrong, and I peeked the correct answer, which is 2783915460 (the 4 and 6 are swapped). So I tried to find out where the correct answer is in my list x and added y = filter (\(x,y) -> x == "2783915460") $ zip (map (filter isDigit . show) x) [1..] -- [("2783915460",1000000)] How can that be? "solution" tells me that the millionth element is "2783915640" but "y" tells me that "2783915460" is at the millionth position? I just cannot see it.

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