- Beginners - Haskell.org

curious behavior of Haskell wiki
by Doug McIlroy 21 Mar '20

21 Mar '20

Why does a search for "bitwise" in the Haskell wiki find an instance of "bitwise AND", whereas a search for "bitwise AND" or "bitwise and" (in quotes) finds nothing? Is this a bug or a (mis)feature? Doug McIlroy

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about "putStr"
by nowsnow 20 Mar '20

20 Mar '20

GHC 8.6.5 ===== import Data.Char main = do putStr "Give me some input: " l <- getLine putStrLn $ map toUpper l ====run==== my str Give me some input:MY STR ===not:=== Give me some input:my str MY STR -- nowsnow <nowsnow(a)163.com>

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Re: [Haskell-beginners] Prelude source code
by Doug McIlroy 27 Feb '20

27 Feb '20

> Thank you. Ive also found other references online, but I want to see the > one installed locally in my system. > > I come from Smalltalk where everything is accessible and modifiable. > > Is the Prelude and other base modules distributed in compiled form only? One of the joys of Hugs is that it includes the H98 Prelude, a real vade mecum for both beginners and experts. It is one of the reasons that for everyday use I resort more often to Hugs than to ghci. The modern Prelude, spread over dozens of files, and programmed at a higher level of abstraction, will not be as useful to a beginner as the H98 Prelude is. Nevertheless it's a valuable and fundamental resource. And it's real code, snatches of which one may borrow as a basis for special modification. That is not true of stuff served up by Hoogle. Quite properly, the ghc distribution contains a compiled version of the Prelude. But that doesn't preclude including the source. It would be a public service to do so. ["You can always get it with Cabal" is unreasonably dismissive. Why should beginners have to master another arcane subject just to get a handbook for their newest toy?] Doug McIlroy

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Prelude source code
by Francisco Garau 26 Feb '20

26 Feb '20

I want to see the source code of the Prelude. Is that available in the standard distribution? Any hints in which directory should I be looking? I've installed stack on OSX 10.11 with the below command curl -sSL https://get.haskellstack.org/ | sh Thanks, Francisco

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2D drawing library for Beginner
by Olivier Revollat 14 Feb '20

14 Feb '20

HI, I would like to experiment with 2D graphics with Haskell (to do for example generative art, ...) I found Diagrams library which seem very powerful but I find it a bit overwhelming for beginner, so do you have anything I can start with ? maybe even a game engine .... Thanks ;)

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Hello (First message on the mailing list)
by Olivier Revollat 14 Feb '20

14 Feb '20

Hi everybody, it's my first message on this ML :) I don't know if it's appropriate to post this here but I would like to have some feedback with one of my first Haskell code. I've been inspired by a recent Numberphile video ( https://www.youtube.com/watch?v=HJ_PP5rqLg0) how explain the "Russian Peasant" algorithm to do multiplication (here in a nutshell : https://www.wikihow.com/Multiply-Using-the-Russian-Peasant-Method) So I decided I give it a go in Haskell, here is my solution, I appreciate if you give me some feedback on how to improve this code (make it more "idiomatic Haskell") NB : I apologize if it's not the right place to ask this kind of review ... in that case, where can I post this ? Thanks ! module DivRusse where main :: IO () main = do putStrLn "13 x 12 is" print $ russmul 13 12 russmul :: Int -> Int -> Int russmul a b = let filteredPair = filter (\pair -> (fst pair) `mod` 2 /= 0 ) $ (a,b) : russmulList a b in foldr (\pair acc -> snd pair + acc) 0 filteredPair russmulList :: Int -> Int -> [(Int, Int)] russmulList 1 _ = [] russmulList a b = let a' = a `div` 2 b' = b * 2 in (a', b') : russmulList a' b'

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Memory usage of if-then-else vs guards
by Miguel Angel Ordoñez Silis 14 Feb '20

14 Feb '20

Hí everybody, Earlier today I was testing some code in ghci with ":set +s" enabled. For some reason I changed an if-then-else expressions to guards and I was surprised to find out that the memory usage declined significantly (around 20%). Here is a function call output using if-then-else: λ> inarow1 ls 1 (1.28 secs, 401,690,400 bytes) And here is a function call output using guards: λ> inarow1 ls 1 (1.18 secs, 313,690,576 bytes) I ran both versions many times and this difference was consistent. Could you help me understand why? Thanks Miguel P.S. I don't think it should matter but maybe it does, here is the function definition: inarow1 :: forall a. Eq a => [a] -> Int inarow1 [] = 0 inarow1 (x:[]) = 1 inarow1 ls = aux 0 1 ls where aux :: Int -> Int -> [a] -> Int aux top curr (x:y:[]) = max top $ if x == y then curr + 1 else curr aux top curr (x:xs@(y:ys)) | x == y = aux top (curr + 1) xs | otherwise = aux (max top curr) 1 xs And the list: ls = [1..1000000]

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Need some understanding on this behavior.
by Alexander Chen 05 Feb '20

05 Feb '20

hi, I need some understanding on the following behavior: a = [1..] [x | x <-a, x <= 100] [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100 (note the missing ']' which results in the command not finishing) b = take 1000 a [x | x <-b, x <= 100] [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100] Note, I took more for b just for ease sake. But now the command finishes correctly. Question: why can i not use a lazy list directly in a list comprehension? best,

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using both my personal .hs and a library simultaneously + seeing a function
by Alexander Chen 30 Jan '20

30 Jan '20

Hi, 1) I have my own functions in a .hs file which i can load in winGCHI and use the functions without a problem. But I also need 1 function from a hackage library, which I have downloaded in my haskell folder. I can 'toggle' between both my personal .hs and the library but I cannot use them both at the same time. The library is Primes(just for clarity). How can I merge these two so I can continue my training? 2) Is there a way to see a function if you call it in winGCHI. I don't mean the type structure which you do with :t but really the function itself for instance(see bold): circular_times :: Int -> [Int] circular_times n = take (length (show n)) (iterate circular n) thanks in advance. best, Alexander

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Re: [Haskell-beginners] Parallel Processing in Libraries?
by Leonhard Applis 20 Jan '20

20 Jan '20

Thank you Akhra, Thank you Sebastian, I appreciate your help and feedback. However my Question was maybe not clear enough. So in general my task is perfectly fit for Control.Parallel and if I'd make a program including the Code I want I could just put control.parallel in the right places and run the program threaded. Lets call this Application A. However for separation I'd like to have my primary code, the one which should be run parallel in a separate library, call it Library A. Library A should be utilizing Control.Parallel. I then want to have Application B using Library A with multiple Cores. The code of Library A has still all the control.parallel in place which worked perfectly fine when run running Application A threaded. When I run Application B only one core seems to be utilized. I also don't see any kind of parallel Code in popular frameworks such as QuickCheck2 (which seems very fit for parallel approaches) which makes me wonder if what I want is "not possible" the way I want it. I'd like to get Application B (and potential more Application C,D) to utilize library A properly. I am at the moment not looking for mitigations or parallel-enabling frameworks/libraries. thank you in advance and best regards Leonhard

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