Comments on: Playing with Haskell’s lazy lists

By: Bookmarks about Dev

Bookmarks about Dev — Sun, 17 Aug 2008 17:45:12 +0000

[...] - bookmarked by 4 members originally found by christopherllc on 2008-07-24 Playing with Haskell’s lazy lists http://www.hvergi.net/2008/07/playing-with-haskells-lazy-lists/ - bookmarked by 3 members [...]

By: A Better ‘nub’ • OJ’s rants

A Better ‘nub’ • OJ’s rants — Mon, 28 Jul 2008 10:17:01 +0000

[...] my TODO list, but I haven’t yet got round to it. So you can imagine my delight when I found this comment by Jedai over at hvergi.net. He’d pointed out that nub was indeed inefficient, and showed an [...]

By: OJ

OJ — Mon, 28 Jul 2008 01:05:22 +0000

Great tidbit Jedai! I’ll be adding that to the memory banks. Might even post about it to share the knowledge. :)

By: Arnar

Arnar — Thu, 24 Jul 2008 18:51:20 +0000

@Jedai: thanks. I wonder why nub isn’t implemented like this in the first place, relying on Ord rather than Eq.

By: Jedai

Jedai — Thu, 24 Jul 2008 14:57:42 +0000

Note that nub only demands that the list element be part of the Eq typeclass. As a result it is very inefficient and a better solution must always be prefered whenever the nature of the elements allows it.

import qualified Data.Set as S
nub' :: (Ord a) => [a] -> [a]
nub' = go S.empty
  where go _ [] = []
        go s (x:xs) | S.member x s = go s xs
                    | otherwise    = x : go (S.insert x s) xs

La différence de performance est énorme.

By: OJ

OJ — Thu, 24 Jul 2008 11:52:50 +0000

I suffer from the same kind of thing myself mate. I'm still relatively new to Haskell, and have a lot to learn. It's a fantastic language though.

Keep up the great work with the blog, and thanks for offering your advice on my Haskell solutions to Project Euler :) Cheers!

By: Arnar

Arnar — Thu, 24 Jul 2008 10:40:58 +0000

@OJ: Ah, yes.. I agree, the list comprehension is much more readable. I guess I should have that in mind whenever I find myself writing something with the form map (\x -> term) xs.

Thanks!

By: OJ

OJ — Thu, 24 Jul 2008 04:26:37 +0000

While this is functionality equiv (I think ... I haven't compiled it :)), I think it's a better option than the version listed above, as I find it easier on the eyes :)

let a' = (1 ,7) : [ (n + 1, x + (gcd (n + 1) x)) | (n, x) <- a' ]

Perhaps it's just me, but list comprehensions tend to win the readability award when compared to lambda expressions.

Nice blog! Cheers.