[Haskell-beginners] Hello (First message on the mailing list)

[Olivier Revollat]

Thank you Lyndon,
Your comments will help me a lot !

I'm glad the haskell community is kind with beginner !


Le jeu. 13 févr. 2020 à 05:25, Lyndon Maydwell <maydwell at gmail.com> a
écrit :

> 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 <revollat at 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'
>>
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