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I don't understand mapM in mapM id (Just 1, Nothing, Just 3)
by Kees Bleijenberg 06 Jun '11

06 Jun '11

I was playing with winghci and I tried: mapM id [Just 1, Just 2, Just 3] result: Just [1,2,3] I don't understand this answer. >From <http://members.chello.nl/hjgtuyl/tourdemonad.html#mapM> http://members.chello.nl/hjgtuyl/tourdemonad.html#mapM mapM mf xs takes a monadic function mf (having type Monad m => (a -> m b)) and applies it to each element in list xs; the result is a list inside a monad. A few things I've found: mapM :: (a -> m b) -> [a] -> m [b] So in this case: a = Maybe Int (second arg in mapM id [Just1, Just 2, Just 3] and b = Int and m = Maybe. So id is :: Maybe Int -> Maybe Int mapM id [Just 1, Nothing, Just 3] result: Nothing. My first guess for the result: Just [Just 1, Nothing, Just 3] when I do: mapM id [1,2,3] I get an error (id has wrong type, which makes sense) Can somebody explain what is going on here? Kees

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cabal sdist and Setup.hs
by Guy 06 Jun '11

06 Jun '11

I created a package with Build-Type: Simple, and when I run cabal sdist, I get Distribution quality errors: The package is missing a Setup.hs or Setup.lhs script. Is there some problem with the simple build type?

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Points-free style
by Alexander Shendi 06 Jun '11

06 Jun '11

Hi folks, I am working my way through "Learn You a Haskell for Great Good" and have reached page 84, where the book recommends that you define your functions in a "points-free style", using the "." and "$" operators. Now I have: sumProducts' :: Num a => [a] -> [a] -> a sumProducts' x y = sum (zipWith (*) x y) I would like to eliminate the "x" and the "y" in the definition, but all I have managed to contrive is: sumProducts :: Num a => [a] -> [a] -> a sumProducts x = sum . zipWith (*) x How do I proceed from here? Any advice is welcome :) Many thanks in advance, /Alexander

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Expected vs. Inferred Type
by Christopher Howard 06 Jun '11

06 Jun '11

Could someone explain the difference between an expected type and an inferred type as mentioned in a compiler error message? I've never heard definitions proper, and so I tend to get confused when the compiler says there is a difference between the type it expected and the type it inferred. -- frigidcode.com theologia.indicium.us

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Haskell Array Index out of range
by James 06 Jun '11

06 Jun '11

Dear all, Today I spend nearly one day's time to debug this exception, but still haven't got a solution. My code is pasted [here][1]. The program throws an exception when evaluating this line: points = assert (len > 0) $ A.listArray (1::Int, len) $ map (\(u,i,r) -> [u,i]) rts Below is my ghci debug session. I still don't understand why it has a range of (0, -193459561) when the 'len' binding is actually 90570. *Main> :break 125 Breakpoint 4 activated at SVMCF.hs:125:13-86 *Main> :trace main Stopped at SVMCF.hs:125:13-86 _result :: UA.Array Int [User] = _ len :: Int = 90570 rts :: [RTuple] = (1,1,5.0) : (1,2,3.0) : (1,3,4.0) : (1,4,3.0) : (1,5,3.0) : .... [SVMCF.hs:125:13-86] *Main> :lis 124 points :: A.Array Int [Int] 125 points = assert (len > 0) $ A.listArray (1::Int, len) $ map (\(u,i,r) -> [u,i]) rts 126 values :: UA.UArray Int Double [SVMCF.hs:125:13-86] *Main> :ste Stopped at SVMCF.hs:125:13-28 _result :: UA.Array Int [User] -> UA.Array Int [User] = _ len :: Int = 90570 [SVMCF.hs:125:13-28] *Main> :ste Stopped at SVMCF.hs:125:21-27 _result :: Bool = _ len :: Int = 90570 [SVMCF.hs:125:21-27] *Main> :ste Stopped at SVMCF.hs:125:32-86 _result :: UA.Array Int [User] = _ len :: Int = 90570 rts :: [RTuple] = (1,1,5.0) : (1,2,3.0) : (1,3,4.0) : (1,4,3.0) : (1,5,3.0) : .... [SVMCF.hs:125:32-86] *Main> :ste Stopped at SVMCF.hs:125:32-56 _result :: [[User]] -> UA.Array Int [User] = _ len :: Int = 90570 [SVMCF.hs:125:32-56] *Main> :lis 124 points :: A.Array Int [Int] 125 points = assert (len > 0) $ A.listArray (1::Int, len) $ map (\(u,i,r) -> [u,i]) rts 126 values :: UA.UArray Int Double [SVMCF.hs:125:32-56] *Main> len 90570 [SVMCF.hs:125:32-56] *Main> :ste Stopped at SVMCF.hs:125:60-86 _result :: [[User]] = _ rts :: [RTuple] = (1,1,5.0) : (1,2,3.0) : (1,3,4.0) : (1,4,3.0) : (1,5,3.0) : .... [SVMCF.hs:125:60-86] *Main> :ste *** Exception: Ix{Int}.index: Index (1) out of range ((1,-193459561)) [1]: http://hpaste.org/47460/haskell_array_index_out_of_ran

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What's wrong with my code?
by Zhi-Qiang Lei 06 Jun '11

06 Jun '11

Dear All, I almost finish the chapter 3 of Real World Haskell. The last exercise blocks me. My code will crash while running. Does anyone can tell me which part is wrong in my code? Thanks. Question: Using the code from the preceding three exercises, implement Graham's scan algorithm for the convex hull of a set of 2D points. You can find good description of what a convex hull. is, and how the Graham scan algorithm should work, on Wikipedia. Answer: -- Graham Scan, get the convex hull. -- Implement the algorithm on http://en.wikipedia.org/wiki/Graham_scan import Data.List import Data.Ord data Direction = TurnLeft | TurnRight | GoStraight deriving (Eq, Show) -- Determine if three points constitute a "left turn" or "right turn" or "go straight". -- For three points (x1,y1), (x2,y2) and (x3,y3), simply compute the direction of the cross product of the two vectors defined by points (x1,y1), (x2,y2) and (x1,y1), (x3,y3), characterized by the sign of the expression (x2 − x1)(y3 − y1) − (y2 − y1)(x3 − x1). If the result is 0, the points are collinear; if it is positive, the three points constitute a "left turn", otherwise a "right turn". direction a b c = case compare ((x2 - x1) * (y3 - y1)) ((y2 - y1) * (x3 - x1)) of EQ -> GoStraight GT -> TurnLeft LT -> TurnRight where x1 = fst a y1 = snd a x2 = fst b y2 = snd b x3 = fst c y3 = snd c grahamScan points = scan (sort points) -- For each point, it is determined whether moving from the two previously considered points to this point is a "left turn" or a "right turn". If it is a "right turn", this means that the second-to-last point is not part of the convex hull and should be removed from consideration. This process is continued for as long as the set of the last three points is a "right turn". As soon as a "left turn" is encountered, the algorithm moves on to the next point in the sorted array. (If at any stage the three points are collinear, one may opt either to discard or to report it, since in some applications it is required to find all points on the boundary of the convex hull.) where scan (a : (b : (c : points))) | (direction a b c) == TurnRight = scan (a : (c : points)) | otherwise = a : (scan (b : (c : points))) scan (a : (b : [])) = [] -- Put prime to the head. sort points = prime : (reverse (sortBy (compareByCosine prime) rest)) -- Sort a list to compute. The first step is to find a point whose y-coordinate is lowest. If there are more than one points with lowest y-coordinate, take the one whose x-coordinate is lowest. I name it prime. where compareByYAndX a b | compareByY == EQ = comparing fst a b | otherwise = compareByY where compareByY = comparing snd a b prime = minimumBy compareByYAndX points -- Delete prime from the candidate points. Sort the rest part by the cosine value of the angle between each vector from prime to each point. Reverse it and put prime to the head. rest = delete prime points compareByCosine p a b = comparing cosine pa pb where pa = (fst a - fst p, snd a - snd p) pb = (fst b - fst p, snd b - snd p) cosine v = x / sqrt (x ^ 2 + y ^ 2) where x = fst v y = snd v Best regards, Zhi-Qiang Lei zhiqiang.lei(a)gmail.com

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Seeking some clarification...
by Sean Charles 06 Jun '11

06 Jun '11

I have a CSV file containing some data about trains: stations from, to, times etc. and I wanted to 'learn some more Haskell' and, to my astonishment, I have gotten thus far but I am not sure *why* it works or *how* I got there! LMAO Here is the relevant code ... ====> trains :: String -> IO () trains csvfile = do legodata <- parseCSVFromFile csvfile case legodata of Left error -> print error Right legodata -> mapM_ putStrLn (trainCodes legodata) -- Assumes row 1 contains cell header information -- Note: the train-code is always the third cell trainCodes :: [Record] -> [String] trainCodes = nub . map (!! 2) . tail ====> I was chuffed with writing the trainCodes as a point-free function, that sort of thing is getting a little easier to work with but I still have real head-banging frustrations sometimes with seemingly simple things, like looping and just printing stuff out, despite having taught myself LISP six years ago and Erlang in recent years! I quit!! I really do!!! My confusion arises over: mapM_ putStrLn (traincodes legodata) Given that: mapM_ :: Monad <http://haskell.org/ghc/docs/6.12.2/html/libraries/base-4.2.0.1/Control-Mona…> m => (a -> m b) -> [a] -> m () <http://haskell.org/ghc/docs/6.12.2/html/libraries/ghc-prim-0.2.0.0/GHC-Unit…> Here's how I read it: For any m that is a Monad, mapM_ takes a function that "takes an 'a' and returns it wrapped in a monad", a "list of a's" and returns a "monad containing 'unit'", the empty-list LISP-() undefined voidy thing. Given that: putStrLn :: String <http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.ht…> -> IO <http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.ht…> () <http://hackage.haskell.org/packages/archive/base/latest/doc/ghc-prim-0.2.0.…>, this means that 'a' is String and 'm b' is IO () and my list of [a] is the result of calling 'traincodes legodata'. trainCodes = nub . map (!! 2) . tail legodata is [Record] (from Text.CSV) and so, 'tail' removes the header row from the data, 'map (!! 2)' extracts the third field from each row and finally 'nub' removes the duplicates. Thus the return type from trainCodes is [String]. Gluing it all together: mapM_ :: Monad <http://haskell.org/ghc/docs/6.12.2/html/libraries/base-4.2.0.1/Control-Mona…> m => (a -> m b) -> [a] -> m () <http://haskell.org/ghc/docs/6.12.2/html/libraries/ghc-prim-0.2.0.0/GHC-Unit…> putStrLn :: String <http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.ht…> -> IO <http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.ht…> () <http://hackage.haskell.org/packages/archive/base/latest/doc/ghc-prim-0.2.0.…> trainCodes :: [Record] -> [String] the type of my call then would seem to be: String -> IO () -> [String] -> IO () "putStrLn" -> (trainCodes legodata) -> IO () which means that not only have I got the types correct for the call but the result type of 'IO ()' also satisfies the type return for my function and hence it executes from 'main' (where it is called from) with no issues. So, am I finally beginning to get a grip on it all ? This list is a constant source of education and I don't post very often as you guys give me far too much stuff to be reading all the time! :) I am using Text.CSV to read my file and all I wanted to do was to output a list of unique codes from column three of the spreadsheet data, one per line, so that I can use this Haskell as part of a bigger 'bash' script. Any detailed explanations that might help me better understand my solution would be welcome; right now I feel I 'just got lucky' although there must be a glimmer of understanding somewhere! LOL Thanks, Sean. PS: Phew!

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My notes on a YouTube video presentation on Haskell
by Costello, Roger L. 05 Jun '11

05 Jun '11

Hi Folks, Here are my notes on a YouTube video on Haskell (http://www.youtube.com/watch?v=cXY4fSA7DnM) If you wish to view the video, I recommend skipping over the first part. The Haskell presentation starts at 3:10 Here's what the speaker (Sasha Rush) says: Haskell is an avant-garde programming language. It's not something that you will see a lot of companies using these days. If you want to get a job, you should be a really good C programmer. If you want to program the future, you should learn Haskell. Haskell is fast like C. It's not as fast, but it's getting there. It on the same order of magnitude. Here's a comparison of the speeds of several languages: (Optimized) Haskell is 1.5x slower than C Schema 9.4x Python 16x PHP 23x Ruby 57x Let's take an example -- the Fibonacci sequence. Recall that the Fibonacci sequence begins with the number 1, then another 1, then the following numbers are the sum of the previous two numbers: 1, 1, 2, 3, 5, 8, 13, 21, ... Here's the Fibonacci sequence implemented in Java: List <int> fib(int n) { List <int> seq = new ArrayList(n) seq[0] = 1; seq[1] = 1; for (int i = 2; i < n, i++) { seq[i] = seq[i-2] + seq[i-1]; } return seq; } Read as: The function, fib, takes an integer, n, and returns a list of integers. First thing to do is to declare the list. Then set the first value, then the second value. Then do a for-loop. Here's the Fibonacci sequence implemented in Haskell: fib = 1 : 1 : zipWith (+) fib (tail fib) Read as: The Fibonacci sequence is a list: the first item is the number 1, then the number 1, and then the following numbers are the zipping together (adding values at the same list position) of the Fibonacci sequence and the tail of the Fibonacci sequence. I created a graphic to illustrate: http://www.xfront.com/Haskell/Fibonacci-Sequence-in-Haskell.png /Roger

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Re: [Haskell-beginners] nub... almost
by Benjamin Edwards 05 Jun '11

05 Jun '11

nub = toList . foldl (flip insert) empty But it's crap because it doesn't respect the ordering on the list. On 4 Jun 2011 17:53, "Christopher Howard" <christopher.howard(a)frigidcode.com> wrote: On 06/04/2011 12:20 AM, Iustin Pop wrote: > On Fri, Jun 03, 2011 at 11:49:01PM -0800, Christopher Ho... @iustin: Thanks, that should work well enough. @Edwards: I'm not familiar with Data.Set, so I do not understand your solution. -- frigidcode.com theologia.indicium.us _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners

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nub... almost
by Christopher Howard 05 Jun '11

05 Jun '11

Weird request: For a certain application, the "nub" function from Data.List is exactly what I need... almost. Nub removes duplicates, keeping the /first/ occurrence of each element. However, I need a function that removes duplicates, keeping the /second/ occurrence of each element. There wouldn't happen to be a function already that does this, would there? I'm working with a custom type which is a member of Eq, but some of the data in the type is not used in the comparison. So it is important which of the two "duplicates" are actually saved. -- frigidcode.com theologia.indicium.us

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