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Today's Topics:
1. Hello (First message on the mailing list) (Olivier Revollat)
2. Re: Hello (First message on the mailing list) (Francesco Ariis)
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Message: 1
Date: Mon, 10 Feb 2020 10:56:40 +0100
From: Olivier Revollat <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] Hello (First message on the mailing list)
Message-ID:
<ca+nxgrxjjpqeycsuw-ayjiuza9ofmgag3fx8nvs9xvguk1j...@mail.gmail.com>
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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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Message: 2
Date: Mon, 10 Feb 2020 12:53:14 +0100
From: Francesco Ariis <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Hello (First message on the mailing
list)
Message-ID: <[email protected]>
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
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