On Thu, 14 Dec 2006 15:31:54 -0800, David Roundy <[EMAIL PROTECTED]> wrote: > import Data.List > > largenum = 1000000 > > main = do putStrLn "strict foldl1" > print $ foldl1' (\a b -> a + 1) $ [1..largenum] > putStrLn "lazy foldl1" > print $ foldl1 (\a b -> a + 1) $ [1..largenum] > > > It gets through the first one, but not the second call, which differs only > in the strictness of the foldl. You can make it use up more memory by > making largenum a hundred times bigger, in which case for some reason it > doesn't seem to have a stack error (although it hasn't completed on my > computer, and uses something like 2G of memory). Perhaps the thunks are > placed on the heap, and only when they are actually evaluated does > anything > go onto the stack?
1) What precisely is a thunk? 2) Let me see if I get this right. The strict version runs in constant space because in the expression (((1 + 1) + 1) ... + 1) the innermost (1 + 1) is reduced to 2 right away. The lazy version first uses a huge amount of heap space by building the entire expression (((1 + 1) + 1) ... + 1) on the heap. The evaluation of this expression starts by placing the outermost + 1 on the stack and continues inward, not actually reducing anything, before everything has been placed on the stack, which causes the overflow. Correct? Thanks for your help! Felix _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
