Hi all, I'm about to optimize a fairly large Haskell program, which uses partition quite often (and a fold immediately afterwards). The GHC User guide section about RULE pragmas says that filter is a good producer, whereas partition is not mentioned (therefore, I assume it is not). I put a little test program together:

fuse1 :: [Bool] -> (Bool,Bool) fuse1 xs = (x,y) where (fs,gs) = partition id xs x = foldr (&&) True fs y = foldr (||) False gs

fuse2 :: [Bool] -> (Bool,Bool) fuse2 xs = (x,y) where fs = filter id xs gs = filter not xs x = foldr (&&) True fs y = foldr (||) False gs

Compiling it via ghc -O and loading the compiled object to ghci results in: Prelude Test> fuse2 (replicate 10000000 False) (True,False) (8.19 secs, 241174592 bytes) Prelude Test> fuse1 (replicate 10000000 False) (*** Exception: stack overflow (6.18 secs, 48472660 bytes) I understand the results: Using partition an intermediate list seems to be build. But WHY? Is it simply because of a missing RULE pragma for partition or is there a "deeper" reason? Cheers, Jan