On Tue, May 06, 2003 at 05:02:56AM -0500, Galen Menzel wrote:
makeList :: IO [Int] makeList = do s <- getLine case read s of 0 -> return [] x -> do xs <- makeList return (x:xs)
I changed the if to a case so that (read s) doesn't get called twice.
Does the compiler not optimize away identical calls? Or is it not allowed to, for some reason? I agree that the case looks nicer, and would do it in general, but have often wondered how hard the compilers try (or are allowed to try... as it might mess up space usage) to optimize away multiple identical calls. Basically, if I write: thrice_length s = length s + length s + length s (A very stupid example...) Is my program really going to run length s three times, or will it just call it once (assuming I have full optimizations on)? I would have thought that one of the major advantages of a purely functional language would be that the compiler could safely eliminate common subexpressions, since it knows that they will always be identical. On the other hand, if I write a function like: long_list = [1..100000] ++ [1..100000] ++ [1..100000] ++ [1..100000] And then run a putStr $ unlines $ map show long_list I would expect this to take very little memory, which would mean that the [1..100000] would need to be calculated four separate times rather than being calculated once and stored... -- David Roundy http://www.abridgegame.org