On Sun, Jun 6, 2010 at 5:10 AM, Thomas Hartman <tphyahoo@gmail.com> wrote:
> Here's two implementations of break, a snappy one from the prelude,
...
> prelbreak p xs = (takeWhile (not . p) xs,dropWhile (not . p) xs) --
> fast, more or less as implemented in prelude iiuc

I had a look at the prelude, and I was surprised to see there's 2 versions, depending on a flag :

#ifdef USE_REPORT_PRELUDE
break p = span (not . p)
#else
-- HBC version (stolen)
break _ xs@[] = (xs, xs)
break p xs@(x:xs')
| p x = ([],xs)
| otherwise = let (ys,zs) = break p xs' in (x:ys,zs)
#endif

I'm curious why is it so, and which version is compiled in the platform or the ghc binaries.

( my guess is USE_REPORT_PRELUDE compiles functions as defined in the haskell report, but the other version is faster and used by default. )

David.