Hi, I've just started messing around with strictness. My goal now is understanding when and how to use it. I began with simple examples like making a strict foldl. When trying to sum a list of 60000 elements with lazy foldl I'd get a stack space overflow, with a strict foldl I was able to sum a list of 1E8 elements :) Then I thought I'd mess with the fibonnaci function. I began with:
fibac :: IntPos -> (IntPos,IntPos) -> (IntPos,IntPos) fibac n (a,b) | n == 0 = (a,b) | otherwise = fibac (n-1) (b,a+b)
Then I tried:
sfibac :: IntPos -> (IntPos,IntPos) -> (IntPos,IntPos) sfibac n (a,b) | n == 0 = (a,b) | otherwise = sfibac (n-1) (b, (+b) $! a)
I tried both functions with ghc 5.00.1 The strict version was not only slower, but I'd get stack overflows for numbers that worked with the 1st version!! (like 100000) I expect sfibac and fibac work this way: assuming that a+b = c b+c = d c+d = e sfibac: sfibac 4 (a, b) -> sfibac 3 (b, a+b) -> sfibac 2 (a+b, b+(a+b)) -> sfibac 1 (b+(a+b), (a+b)+(b+(a+b))) -> sfibac 0 ((a+b)+(b+(a+b)), (b+(a+b)) + (a+b)+(b+(a+b))) now if I had to evaluate the first element I'd get (a+b)+(b+(a+b)) -> c+(b+c) -> c+d -> e fibac: sfibac 4 (a, b) -> sfibac 3 (b, a+b) -> sfibac 2 (c, b+c) -> sfibac 1 (d, c+d) -> sfibac 0 (e, d+e) I'd expect sfibac to be faster, but what I find even more strange is the stack space overflow in sfibac! why? Greetings, J.A.