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

[Olivier Revollat]

Hello Francesco,
This is very clear. Thanks for your help !
Your version is much more readable and elegant !

Le lun. 10 févr. 2020 à 12:53, Francesco Ariis <fa-ml at ariis.it> a écrit :

> 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
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