[Haskell-beginners] Hello (First message on the mailing list)
Francesco Ariis
fa-ml at ariis.it
Mon Feb 10 11:53:14 UTC 2020
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
More information about the Beginners
mailing list