Re: [commit: ghc] master: Replace some `length . filter` with `count` (02614fd)

Hi, Am Freitag, den 05.08.2016, 22:41 +0000 schrieb git@git.haskell.org:

commit 02614fd61f57b599c5e4fd5e85f00a4e1ce37bc7 Author: Ömer Sinan Ağacan Date:   Fri Aug 5 20:58:02 2016 +0000

    Replace some `length . filter` with `count`

Well intended! I wouldn’t be surprised if this is actually making the code worse, considering that "count" is implemented in the “wrong”, stackeating way (whereas lenght . filter could, thanks to list fusion, produce the desired code) count :: (a -> Bool) -> [a] -> Int count _ [] = 0 count p (x:xs) | p x       = 1 + count p xs                | otherwise = count p xs I would say that the regression reported by  https://perf.haskell.org/ghc/#revision/02614fd61f57b599c5e4fd5e85f00a4e1ce37... confirms this, but T6048 is simply unreliable. So practically, it does not seem to matter either way. I’ll change the implementation of count. (Although even then it is not clear that the change is a win, as "length . filter" might fuse with the producer, whereas "count" without a lot of extra hassle will not.) Joachim -- Joachim “nomeata” Breitner   mail@joachim-breitner.de • https://www.joachim-breitner.de/   XMPP: nomeata@joachim-breitner.de • OpenPGP-Key: 0xF0FBF51F   Debian Developer: nomeata@debian.org