- Beginners - Haskell.org

Different versions of ghci REPL in Emacs?
by Lawrence Bottorff 22 Mar '20

22 Mar '20

I'm using Emacs (26.3) and org-mode (9.3.6) and I seem to have two different versions of ghci going, depending on whether I'm using haskell-mode alone with a *.hs file, or whether I am evaluating an org-mode babel code block. If I open, say, test.hs in Emacs, haskell-mode starts up. And if I choose <Start interpreter> from the menu, I get this "lambda prompt" REPL The next big Haskell project is about to start! If I break, you can: 1. Restart: M-x haskell-process-restart 2. Configure logging: C-h v haskell-process-log (useful for debugging) 3. General config: M-x customize-mode 4. Hide these tips: C-h v haskell-process-show-debug-tips λ> But if I call M-run-haskell, I get this "Prelude prompt" REPL Configuring GHCi with the following packages: GHCi, version 8.6.5: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/lawrence/.ghc/ghci.conf Prelude> Prelude> and this babel bit knows and interacts with it #+BEGIN_SRC haskell :results output :set +m doubleMe x = x + x doubleUs x y = doubleMe x + doubleMe y #+END_SRC but then trying to run this code block with lambda prompt REPL, I get the error executing Haskell code block... org-babel-execute:haskell: Buffer *haskell* does not exist or has no process even though the lambda REPL is called *haskell*. Why am I getting two separate REPLs? BTW, which IDE do Haskell big boys and girls use? LB

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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 21 Mar '20

21 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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