Hello Malcolm, Thursday, August 31, 2006, 2:29:46 PM, you wrote:
in general, for any function we can either implement 1) good consumer based on using foldr 2) tail-recursion
are you agree?
I'd like to point out that, if you are prepared to abandon foldr/build fusion in favour of hylo fusion, you can code good consumers using either foldr or foldl.
looks promising. but even if GHC HQ will accept your plan, something will still remain to fix: there are some functions, such as 'filter', that can be implemented via foldr but not via foldl. such functions has two possible implementations: 1) using foldr, which make them goo producers 2) using reverse+foldl which makes them tail-recursive which makes situation more interesting is that even without fusion foldr=based solution will be faster for most lists - as long as whole processed data fits in L2 processor cache (at least, this says results of my tests of replicateM implementations) .... browsing GHC.List, i've found the following: #ifdef USE_REPORT_PRELUDE and = foldr (&&) True or = foldr (||) False #else and [] = True and (x:xs) = x && and xs or [] = False or (x:xs) = x || or xs {-# RULES "and/build" forall (g::forall b.(Bool->b->b)->b->b) . and (build g) = g (&&) True "or/build" forall (g::forall b.(Bool->b->b)->b->b) . or (build g) = g (||) False #-} #endif why we can't use foldl in definition and at the same time have rules for using with build ??? i understand that it's impossible for and/or, but for 'length' this should work, i think? -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com