[Haskell-cafe] is there a way to prove the equivalence of these two implementations of (Prelude) break function?

[David Virebayre]

On Sun, Jun 6, 2010 at 5:10 AM, Thomas Hartman <tphyahoo at 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.
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