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
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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