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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. Hello (First message on the mailing list) (Olivier Revollat) 2. Re: Hello (First message on the mailing list) (Francesco Ariis) ---------------------------------------------------------------------- Message: 1 Date: Mon, 10 Feb 2020 10:56:40 +0100 From: Olivier Revollat <revol...@gmail.com> To: beginners@haskell.org Subject: [Haskell-beginners] Hello (First message on the mailing list) Message-ID: <ca+nxgrxjjpqeycsuw-ayjiuza9ofmgag3fx8nvs9xvguk1j...@mail.gmail.com> Content-Type: text/plain; charset="utf-8" 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' -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.haskell.org/pipermail/beginners/attachments/20200210/5cfd9404/attachment-0001.html> ------------------------------ Message: 2 Date: Mon, 10 Feb 2020 12:53:14 +0100 From: Francesco Ariis <fa...@ariis.it> To: beginners@haskell.org Subject: Re: [Haskell-beginners] Hello (First message on the mailing list) Message-ID: <20200210115314.GB1097@aspire.station> Content-Type: text/plain; charset=us-ascii Hello Olivier, On Mon, Feb 10, 2020 at 10:56:40AM +0100, Olivier Revollat wrote: > 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. It is an appropriate post in the appropriate list! > 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") Ok, the problems I see with russmulList are: > russmulList :: Int -> Int -> [(Int, Int)] > russmulList 1 _ = [] > russmulList a b = > let a' = a `div` 2 > b' = b * 2 > in (a', b') : russmulList a' b' - russmulList does not handle 0 gracefully (try `russmulList 0 10`) - russmulList should _not_ discard the factors from the top of the list (or you have to awkwardly re-add them as you did in filteredPair) This or similar will do: russmulList :: Int -> Int -> [(Int, Int)] russmulList 0 b = [] russmulList a b = let a' = a `div` 2 b' = b * 2 in (a, b) : russmulList a' b' Now let's go through `russmul`: > 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 - `(a,b) :` is needed no more - in filteredPair you can drop the parentheses around `fst pair` - use `odd` instead of "`mod` 2 /= 0`" - in any case you should express the predicate in point-free style as `even . fst` - `foldr` part can be made much clearer with sum (map snd ...) So: russmul :: Int -> Int -> Int russmul a b = let filteredPair = filter (odd . fst) (russmulList a b) in sum (map snd filteredPair) Was this clear/useful? If not, fire again and welcome to the functional world! -F ------------------------------ 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 4 *****************************************