- Beginners - Haskell.org

List of a Data Type
by Thomas Jakway 03 Nov '15

03 Nov '15

I've made a new datatype data Visibility a = Hidden a | Shown a I want to create a type that's a list of hidden and visible integers. So something like this: type possiblyVisibleInts = [Visibility Int] The idea is that some of the ints will be hidden and some will be shown. But it doesn't work. Why not? This seemed even odder to me after poking around in ghci: :t [Just 5, Nothing] gives [Just 5, Nothing] :: Num a => [Maybe a] So clearly it's possible to have a list of a type that has 2 constructors--you're not restricted to having a list that only contains Just, for example. ghci is also OK with e.g. :t [Hidden 5, Shown 6] [Hidden 5, Shown 6] :: Num a => [Visibility a]

3 3

Getting Started
by Abhishek Kumar 03 Nov '15

03 Nov '15

Hello Everyone I am new to haskell (learning right now). I want to contribute to data structure and algorithm libraries in haskell.Please guide me with appropriate links and how to start with.I have previous implementation experience in other languages. Thanks Abhishek Kumar

3 4

let , equations, and monads
by PATRICK BROWNE 03 Nov '15

03 Nov '15

{- From Learn Haskell Fast and Hard : 4.3.1. Maybe is a monad http://yannesposito.com/Scratch/en/blog/Haskell-the-Hard-Way/#maybe-monad Concerning the code below I have the following questions: 1) Are eligibleLet and eligibleEquational operationally equivalent (i.e. perform the same operations) and/or semantically equivalent(i.e. Church-Rosser)? 2) Apart from the return type of Maybe instead of Bool how does eligibleMonad differ from eligibleLet? Regards, Pat -} deposit value account = account + value withdraw value account = account - value -- You are eligible for a bonus only if your sequence of transactions stays out of the red. eligibleLet :: (Num a,Ord a) => a -> Bool eligibleLet account = let account1 = deposit 100 account in if (account1 < 0) then False else let account2 = withdraw 200 account1 in if (account2 < 0) then False else let account3 = deposit 100 account2 in if (account3 < 0) then False else let account4 = withdraw 300 account3 in if (account4 < 0) then False else let account5 = deposit 1000 account4 in if (account5 < 0) then False else True -- No let expressions, hence intermediate calculations are performed multiple times. -- Should terminates on first point of failure eligibleEquational :: (Num a,Ord a) => a -> Bool eligibleEquational account = if (deposit 100 account) < 0 then False else if (withdraw 200 (deposit 100 account)) < 0 then False else if (deposit 100 (withdraw 200 (deposit 100 account))) < 0 then False else if (withdraw 300 (deposit 100 (withdraw 200 (deposit 100 account)))) < 0 then False else if (deposit 1000 (withdraw 300 (deposit 100 (withdraw 200 (deposit 100 account))))) < 0 then False else True -- Monadic version depositM :: (Num a) => a -> a -> Maybe a depositM value account = Just (account + value) withdrawM :: (Num a,Ord a) => a -> a -> Maybe a withdrawM value account = if (account < value) then Nothing else Just (account - value) eligibleMonad :: (Num a, Ord a) => a -> Maybe Bool eligibleMonad account = do account1 <- depositM 100 account account2 <- withdrawM 200 account1 account3 <- depositM 100 account2 account4 <- withdrawM 300 account3 account5 <- depositM 1000 account4 Just True main = do print $ eligibleLet 300 -- True print $ eligibleLet 299 -- Falsen print $ eligibleEquational 300 -- True print $ eligibleEquational 299 -- False print $ eligibleMonad 300 -- Just True print $ eligibleMonad 299 -- Nothing -- This email originated from DIT. If you received this email in error, please delete it from your system. Please note that if you are not the named addressee, disclosing, copying, distributing or taking any action based on the contents of this email or attachments is prohibited. www.dit.ie Is ó ITBÁC a tháinig an ríomhphost seo. Má fuair tú an ríomhphost seo trí earráid, scrios de do chóras é le do thoil. Tabhair ar aird, mura tú an seolaí ainmnithe, go bhfuil dianchosc ar aon nochtadh, aon chóipeáil, aon dáileadh nó ar aon ghníomh a dhéanfar bunaithe ar an ábhar atá sa ríomhphost nó sna hiatáin seo. www.dit.ie Tá ITBÁC ag aistriú go Gráinseach Ghormáin – DIT is on the move to Grangegorman <http://www.dit.ie/grangegorman>

2 3

Partition a list recursively
by Alexandre Delanoë 03 Nov '15

03 Nov '15

Hello, I am looking for such function: function :: Ord a => [a] -> [a] -> [[a]] function [1,3,6] [0..6] == [[0,1],[2,3],[4,5,6]] Question: does it exist already ? If no, continue: Let: partIt :: Ord a => a -> [a] -> [[a]] partIt m xs = [takeWhile (<= m) xs, dropWhile (<= m) xs] example: partIt 1 [0..5] == [[0,1],[2,3,4,5]] Question: How to define recursively partIt 1) with a list as parameter ? 2) recursively on the second part of the list ? Thanks for help, -- Alexandre Delanoë

2 2

randmomR produces only even values
by martin 01 Nov '15

01 Nov '15

Hello all When I read: mkStdGen :: Int -> StdGen The function mkStdGen provides an alternative way of producing an initial generator, by mapping an Int into a generator. Again, distinct arguments should be likely to produce distinct generators. I thought, that > fst $ R.randomR (1,10) (R.mkStdGen s) should get me a random value between 1 and 10 and that I get different values depending on the seed s. But this > length $ filter even $ map (\s -> fst $ randomR (1::Int,10) (mkStdGen s))[1..1000] gives my 1000, i.e. all random numbers are even numbers. However, when I use random instead of randomR > length $ filter even $ map (\s -> fst $ random (mkStdGen s) ::Int)[1..1000] I get 499 (okay) Why is that so?

5 5

lists strange behaviour
by galeonet＠tiscali.it 28 Oct '15

28 Oct '15

Hello, if I write down: [(x,y)|x<-[1..5],y<-[1..5]] I obtain as expected: [(1,1),(1,2),(1,3),(1,4),(1,5),(2,1),(2,2),(2,3),(2,4),(2,5),(3,1),(3,2),(3,3),(3,4),(3,5),(4,1),(4,2),(4,3),(4,4),(4,5),(5,1),(5,2),(5,3),(5,4),(5,5)] If I write down: [(x,y)|x<-[1..5],y<-[1..5],x==5] I obtain as expected: [(1,1),(1,2),(1,3),(1,4),(1,5)] but if I write: [(x,y)|x<-[1..5],y<-[1..5],x<-[1]] I obtain: [(1,1),(1,2),(1,3),(1,4),(1,5),(1,1),(1,2),(1,3),(1,4),(1,5),(1,1),(1,2),(1,3),(1,4),(1,5),(1,1),(1,2),(1,3),(1,4),(1,5),(1,1),(1,2),(1,3),(1,4),(1,5)] Why????? Thank you in advance. Regards, Maurizio Connetti gratis il mondo con la nuova indoona: hai la chat, le chiamate, le video chiamate e persino le chiamate di gruppo. E chiami gratis anche i numeri fissi e mobili nel mondo! Scarica subito l’app Vai su https://www.indoona.com/

2 1

New chapter in Learning Haskell: fractal structures
by Manuel M T Chakravarty 27 Oct '15

27 Oct '15

Our tutorial ”Learning Haskell” just gained a new fun chapter on visualising recursion with fractal structures: http://blog.haskellformac.com/blog/fractals-recursion-in-pictures Happy Coding! Manuel PS: This is going to be a good one for anybody who wants to get their kids interested in programming, too.

1 0

How would you implement Instant and Interval
by martin 27 Oct '15

27 Oct '15

Hello all If I define an Instant as a point in Time and an Interval as the difference between two Instants, and I also want to use (+) and (-), how can I do this. My initial thought making them instances of the Num class, but that does not work. (-) is okay on Intervals, but on Instant it returns a different type (Interval). Is it possible at all to define a typeclass with (-) :: Instant -> Instant -> Interval without using language extensions?

3 2

Space consumption and strictness in strict map
by John Lusk 27 Oct '15

27 Oct '15

Hello, beginners(a)haskell.org. I posted this a few days ago, but it looks like attachments get pulled out by Mailman, whether or not they are small text files ( :>( ), so I'm trying again, sans attachment. _________________________________________ Hey, I have a problem I "solved" by sprinkling !s around like pixie dust, but I'd like to know what's going on. My program is below, in all its messy glory, but the whole ball of wax is at https://github.com/JohnL4/PassphraseGenerator (mostFrequent.hs). It takes as input one of the 2012 Google ngram (specifically, 1-gram) raw data files. The expected input format is documented in several places. (For testing, I took the first million lines.) The bang that worked is on the 3rd argument of 'wordCount', the map. I tried commenting out the "(Map.insertWith (+) ngram matchCount aMap)" part (and just returning the input map). When I did that, boy was space consumption small, so that line is part of the problem. So, what's going on without the bang? I guess I'm just building up a bunch of thunks w/that Map.insertWith call, but what kind of thunks? Unevaluated calls to splitOn and (!!) and read and (+) and lines? I thought a strict Map would avoid that. I guess WHNF isn't enough? Looking at https://wiki.haskell.org/Weak_head_normal_form, I don't *think* I have any constructors in there, but profiling with -hd shows me that (:) is the most frequently occurring closure. Where is *that* coming from? Is the occurrence of (:) so high because it's either a built-in function applied to too few arguments or a lambda abstraction? Which one? (Also, is "lambda abstraction" the same as "lambda expresson"?) And then... what does that bang on the map argument do? Does it force evaluation of the passed argument all the way down to primitives, so that we truly get a data structure containing only strings and ints (and no thunks)? What does that do to time complexity? Does it have to traverse the entire map looking for thunks, even though I only added one at some random location in the map? Is there a better way? I guess I need to force strictness somewhere else, but I'm not sure how. I tried using seq (a little half-heartedly), and ($!), but I guess I did it wrong and only wound up with more thunks to seq and id ($!), right? So, I wound up with lazy strictness? Thanks for any help. John. My code: -- | Reads stdin and prints the n most-frequent words occurring after year y to stdout, along with -- | their counts. {-# LANGUAGE BangPatterns #-} import GHC.IO.Handle (hPutStr) import GHC.IO.Handle.FD (stderr) import Data.Char (toLower) import Data.List (intersperse, sortBy) import qualified Data.Map.Strict as Map import Data.List.Split (splitOn) {- Test data: let filetext = "aaa\t1900\t2\naaa\t1950\t3\nbbb\t1950\t5\nbbb_VERB\t1980\t9" -} -- | Number of most-frequent words we want. n :: Int n = 15000 -- | Earliest year we want to count while building frequency table y :: Int y = 1950 main :: IO() main = do {- Read line, split on tabs, take first three fields, which are: 1-gram (word), year, match-count. If year >= y: Trim parts of speech (POS) from 1-gram (leading, trailing known fragments delimited by "_") Known fragments: NOUN, VERB, ADJ, ADV, PRON (pronouns), DET (determiners and articles), ADP (prepositions, postpositions), NUM, CONJ, PRT (particles), X (miscellaneous) Split on "_", discard expected known fragments, complain if there are more than one fragments left. (Note that I have verified that each ngram has 0 or 1 trailing attributes, so the check for an unexpected number of parts is unnecessary.) Find word in dictionary and add match-count to that entry. At end of input, sort dictionary entries by match-counts (descending) and take first n entries for output. -} allInput <- getContents putStrLn (concat (intersperse "\n" (map fst (take n (sortBy countDescending (Map.toList (wordCounts y (lines allInput) Map.empty))))))) hPutStr stderr "Done.\n" -- | Orders inputs by 2nd element (count), descending countDescending :: (String,Int) -> (String,Int) -> Ordering countDescending (_, countA) (_, countB) | countA < countB = GT | countA == countB = EQ | otherwise = LT -- | Returns map of all counts (summed) for words occurring on or after given year. wordCounts :: Int -- ^ Year -> [String] -- ^ Lines in form "word\tyear\tcount\totherStuffWeDontCareAbout" -> Map.Map String Int -- ^ Input map -> Map.Map String Int -- ^ Output map wordCounts _ [] aMap = aMap wordCounts aYear (aLine:restLines) !aMap = let fields = splitOn "\t" aLine ngramParts = splitOn "_" (fields!!0) ngram = map toLower (ngramParts!!0) -- "ngram" is the same as "word", in this case. year = read (fields!!1) :: Int matchCount = read (fields!!2) :: Int in if (year < aYear) then (wordCounts aYear restLines aMap) else (wordCounts aYear restLines (Map.insertWith (+) ngram matchCount aMap) -- aMap )

2 1

Difference of time execution times when measuring with time and profiling
by Javier de Vega Ruiz 26 Oct '15

26 Oct '15

HI all, I am messing around with bang patterns and noticed some huge differences between the total time as reported by the time tool and the .prof file. Below is the code used. Without bang patterns: module Main where import Data.List fastFibs = unfoldr nextFib (1, 1) where nextFib (x, y) = Just $ (x, (y, (x + y))) main = putStrLn $ (show n) ++ "th fib is: " ++ (show $ fastFibs !! (n - 1)) where n = 1000000 With bang patterns: {-# LANGUAGE BangPatterns #-} module Main where import Data.List fastFibs = unfoldr nextFib (1, 1) where nextFib (!x, !y) = Just $ (x, (y, (x + y))) main = putStrLn $ (show n) ++ "th fib is: " ++ (show $ fastFibs !! (n - 1)) where n = 1000000 when looking at the first through time and prof I get the following. Without: real 0m53.501s user 0m0.015s sys 0m0.328s Thu Oct 22 16:46 2015 Time and Allocation Profiling Report (Final) fast-fib.exe +RTS -p -RTS total time = 9.52 secs (9520 ticks @ 1000 us, 1 processor) total alloc = 43,500,223,152 bytes (excludes profiling overheads) Please note the huge difference 53 vs 9 seconds. With: real 0m10.095s user 0m0.031s sys 0m0.344s Thu Oct 22 16:50 2015 Time and Allocation Profiling Report (Final) fast-fib.exe +RTS -p -RTS total time = 8.97 secs (8971 ticks @ 1000 us, 1 processor) total alloc = 43,500,309,960 bytes (excludes profiling overheads) Here differences seem to be much smaller. I am using Windows 8.1 64 bit, GHC 7.8.3 and measuring with the following line: ghc Main.hs -o fast-fib.exe -O2 -prof && time ./fast-fib.exe +RTS -p && cat fast-fib.prof Could someone please explain where the big difference is coming from and how to change the measuring approach to get more consistent results? Best regards, Javier de Vega Ruiz.

2 2