Comments on: Prelude golf: what can we learn by rewriting the partition function?

By: loopzy

loopzy — Thu, 03 Dec 2009 15:21:03 +0000

Thanks this was a good read

By: thomashartman1

thomashartman1 — Fri, 30 Oct 2009 13:35:16 +0000

I tested this, and it’s good — same time profile as partitionSL.

Interesting point, for your function it was fast in ghci without precompiling, whereas for partitionSL I had to ghc –make first to get reasonable performance.

The filter version is present in the current ghc prelude as a commment prior to the real definition.

By: Kamil Dworakowski

Kamil Dworakowski — Thu, 29 Oct 2009 20:16:58 +0000

Here is a definition of partition function in terms of foldr which works for infinite lists.

import Control.Arrow
part f xs = foldr g ([],[]) xs where
g x rec = let mod = if f x then first else second in mod (x:) rec

The trick is not to force rec to soon. In you definition you have a pattern matching, which is convenient but forces the recursive call which then loops until stack overflow.

By: Matt Brubeck

Matt Brubeck — Wed, 28 Oct 2009 21:56:18 +0000

Very interesting! By the way, my initial approach was this very short version:

partition p xs = (filter p xs, filter (not . p) xs)

which I believe is inefficient because it will call p twice on each element of the list, but on the other hand it’s both short and lazy.