Re: divRem by `-' performance

On Wed, Oct 17, 2012 at 06:17:28PM +0300, Roman Cheplyaka wrote:

* Serge D. Mechveliani [2012-10-17 19:02:37+0400]

...

People, consider the following contrived program for division with remainder:

---------------------------------------------------------------- qRem :: Int -> Int -> (Int, Int) qRem m n = if m < 0 || n <= 0 then error "\nwrong arguments in qRem.\n" else qRem' 0 m where qRem' q r = if r < n then (q, r) else qRem' (succ q) (r - n)

You need to force evaluation of 'q' here, otherwise it becomes a growing chain of 'succ' applications.

E.g.

qRem' q r = if r < n then (q, r) else (qRem' $! succ q) (r - n)

This looks as a natural explanation. But it is generally difficult for me to admit that sometimes it is desirable to use strinctess annotation. I programmed DoCon for 6 years, and it does not have any bit of strictness annotation, and I always had an impression that it is all right with performance. And now it occurs that setting $! in some places may make many parts of it somewhat 100 times less expensive -- ? Somehow difficult to admit. Regards, Sergei