- Beginners - Haskell.org

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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Re: [Haskell-beginners] Parallel Processing in Libraries?
by Sébastien Leblanc 19 Jan '20

19 Jan '20

> I'd like to have a library which utilized parallel programming (mostly for map-reduce tasks). Since parallel computation is a complex topic, there are many solutions that might not apply to a problem. I find that often, the simplest, often overlooked method of implementing parallel computation is by breaking down a problem into chunks that can easily be computed by a single core. Multi-threading is another topic entirely, and while it is often related to parallelization, it is also often used only to allocate more cores (improve performance) rather than implement an algorithm that is otherwise serial, but then you get to have to deal with all the difficulty of multi-threading like synchronization, shared memory access, thread-safe code, having to deal with errors potentially affecting the whole process vs. a single worker. Thus, once you break down the problem into individual components, one way to implement parallelization is by using a message queue task list system. The principle is pretty simple, using a message broker such as RabbitMQ or ZeroMQ, workers connect to this message queue and listen to messages from a controller telling them to process some data using some function. First come, first served. Once the worker is done, the reply is sent back to the message queue or it is made available by any other means (for example in a folder with shared access). The worker could then notify other workers to further process this data. The elegance of this system lies in how flexible the actual hardware architecture that processes the load can be. For example, using some cloud provider, you can automatically spawn more VMs to process a higher load of data, and discard the VMs once they get idle long enough. On a single machine, you can fire up one or as many processes as you have computing cores (if your workload is almost 100% CPU-bound) and let the OS take care of scheduling the tasks. I am not aware of a complete task processing library for Haskell; for an example of a mature project that provides such features, check out Celery on Python. If you cannot find a substitute, I suppose you could always use Celery and run your Haskell code from inside the Python interpreter.

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Parallel Processing in Libraries?
by Leonhard Applis 16 Jan '20

16 Jan '20

Hi, I'm starting to dip into Parallel Processing with Haskell while reading "Parallel and Concurrent Programming in Haskell". All the Examples in the Book construct a program which is doing all the work, so with all files in the same program which has a main method. I'd like to have a library which utilized parallel programming (mostly for map-reduce tasks). Is this possible? My first approach of putting the par-code in my library and running the program threaded does not seem to use multiple cores. Also I have not seen any parallel programming in "popular" libraries (such as QuickCheck) which makes me think that I'm on the wrong path. To summarize my goal: I want to build a computation-intensive library, which utilizes multiple cores when used from an executable. Thank you Leonhard

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Code review request?
by Jake Vossen 15 Jan '20

15 Jan '20

Hey everyone, Let me know if this is not the right place for this, but I am curious if someone could take a look at my code and maybe share some feedback / how a more experienced haskeller would approach this problem. New to Haskell, pretty experienced with imperative languages. I have solved the following problem in Haskell: > Given a non-empty array, return true if there is a place to split the > array so that the sum of the numbers on one side is equal to the sum > of the numbers on the other side. > canBalance([1, 1, 1, 2, 1]) -> true > canBalance([2, 1, 1, 2, 1]) -> false > canBalance([10, 10]) -> true Here is my code (my solution uses `can_split_even` not `canBalance`) ``` can_split_even :: (Num a) => (Eq a) => [a] -> Bool can_split_even xs = True `elem` is_even_at_each_spot where is_even_at_each_spot :: [Bool] is_even_at_each_spot = map (is_split xs) [1 .. (length xs - 1)] where is_split :: (Num a) => (Eq a) => [a] -> Int -> Bool is_split xs index = sum (take index xs) == sum (drop index xs) ``` Thanks so much! -- Jake Vossen Colorado School of Mines, Class of 2022 B.S. Computer Science PGP: 08CD 67DA FE3A 0AE7 B946 6AC3 B812 7052 D9E3 817B https://jake.vossen.dev

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Prime numbers -- specific list comprehension explained
by Alexander Chen 15 Jan '20

15 Jan '20

Hi, This could gives you for any given integer the list of prime numbers. source is from a stack overflow snippet. helper function: isqrt :: Integral a => a -> a isqrt = floor . sqrt . fromIntegral main function: primes 1 = [] primes n = 2:[i | i <- [3,5..n], and [mod i k /= 0 | k <- primes (isqrt i)]] my main unclarity is how I should interpret i and k in de mod part. At every step of the recursion. example: primes 10. it should generate [2,3,5,7,9] if you ignore the second part. however, when I look at the second part then first I need to do this primes (isqrt 10), which give primes 3. which gives met [2,3]. first question) I haven't reached base case in this situations so can i or can i not meaningfully evaluate mod i k /= 0? Independent of that I now go deeper to reach base case. So I go to primes (isqrt 3) which gives me primes 1 which is []. Now I hit the base case and need to definitely evaluate mod i k /= 0. second question) but in this case what is i and what is k? If I have it completely wrong, then please be patience. I appreciate the input! best,

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Re: [Haskell-beginners] Beginners Digest, Vol 139, Issue 2
by Alex Zarebski 09 Jan '20

09 Jan '20

If the goal is to filter a list for numbers which are multiples of some other number, perhaps the =mod= function would be a neater way to do this. It is part of the Prelude with documentation here: https://hackage.haskell.org/package/base-4.12.0.0/docs/Prelude.html#v:mod On Thu, Jan 9, 2020 at 12:01 PM <beginners-request(a)haskell.org> wrote: > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. why doesn't this work? (Alexander Chen) > 2. Re: why doesn't this work? (Oliver Charles) > 3. Re: why doesn't this work? (Ut Primum) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Wed, 8 Jan 2020 16:01:20 +0100 (CET) > From: Alexander Chen <alexander(a)chenjia.nl> > To: beginners(a)haskell.org > Subject: [Haskell-beginners] why doesn't this work? > Message-ID: <675050120.318296.1578495680883(a)ichabod.co-bxl> > Content-Type: text/plain; charset="utf-8" > > hi, > > for a List comprehension I want to only include the integers of a > division. In my beginners mind this should work: > > (round (x / y)) == (x / y) > > however, i get a nontrivial error blurp. > > what am i doing wrong? > > best, > > Alexander >

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