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

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