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You can reach the person managing the list at beginners-ow...@haskell.org When replying, please edit your Subject line so it is more specific than "Re: Contents of Beginners digest..." Today's Topics: 1. Re: Hello (First message on the mailing list) (Lyndon Maydwell) ---------------------------------------------------------------------- Message: 1 Date: Thu, 13 Feb 2020 15:25:21 +1100 From: Lyndon Maydwell <maydw...@gmail.com> To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell <beginners@haskell.org> Subject: Re: [Haskell-beginners] Hello (First message on the mailing list) Message-ID: <cam5qztxsamlmzoonutlekjyvfaxqox4nxwusmrdr7dv3y0_...@mail.gmail.com> Content-Type: text/plain; charset="utf-8" Hi Olivier, I had a quick look, I think it's a great first try. Here are my thoughts! module DivRusse where {- Comments: * This seems unsafe in that it doesn't handle negative numbers well. * This can be evidenced by needing a guard on our property. * Could be addressed by using a safe newtype. * Define properties and use quickcheck to test them. * Favor pattern-matching over use of `fst`, `snd`. * Use `where` over `let` to highlight what the final result is. * Rewrite folds to more wholemeal approach. e.g. `sum $ map snd filteredPair` * Use standard functions and composition to eliminate lambdas like this: `(\(x, _) -> x `mod` 2 /= 0 )` = `(odd . fst)`. * `russmulList` could go into an infinite loop for negative numbers. Either prevent this with types (preferred), or return an error somehow. -} main :: IO () main = do putStrLn "13 x 12 is" print $ russmul 13 12 -- Property: Does russmul = *? prop_russmul :: Int -> Int -> Bool prop_russmul a b | a > 0 && b > 0 = russmul a b == a * b | otherwise = True russmul :: Int -> Int -> Int russmul a b = sum $ map snd filteredPair where filteredPair = filter (odd . fst) $ (a,b) : russmulList a b russmulList :: Int -> Int -> [(Int, Int)] russmulList 1 _ = [] russmulList a b = (a', b') : russmulList a' b' where a' = a `div` 2 b' = b * 2 Warm Regards, - Lyndon On Mon, Feb 10, 2020 at 8:55 PM Olivier Revollat <revol...@gmail.com> wrote: > 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' > > _______________________________________________ > Beginners mailing list > Beginners@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.haskell.org/pipermail/beginners/attachments/20200213/d300049d/attachment-0001.html> ------------------------------ Subject: Digest Footer _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners ------------------------------ End of Beginners Digest, Vol 140, Issue 6 *****************************************