Hi, I see a stack overflow with this code, but I don't understand why. Test.hs -------------- main :: IO () main = do let list = ([1..3*1000*1000] :: [Int]) let !x = foldl'2 (flip seq) () list return x -- foldl'2 is just the __GLASGOW_HASKELL__ version of Data.List.foldl'. -- http://www.haskell.org/ghc/docs/6.12.2/html/libraries/base-4.2.0.1/ -- src/Data-List.html -- The test case behaves the same if I call foldl' instead of foldl'2. foldl'2 :: (a -> b -> a) -> a -> [b] -> a foldl'2 f z0 xs0 = lgo z0 xs0 where lgo z [] = z lgo z (x:xs) = let z' = f z x in z' `seq` (lgo z' xs) $ ghc Test.hs -o Test $ time ./Test real 0m0.087s user 0m0.084s sys 0m0.000s $ ghc Test.hs -o Test -O $ time ./Test Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize -RTS' to increase it. real 0m1.780s user 0m1.764s sys 0m0.004s $ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.12.3 I looked at the Core output with ghc-core, with or without optimizations, and I see a $wlgo recursive function that doesn't appear to end in a tail call. I don't see any let expressions in the folding code, so I assume no thunks are being created. I can make a tail call appear by doing either of two things: 1. Replace "lgo z []" with "lgo !z []". This suggestion came from an email on haskell-beginners that I can't find right now. It was a few months ago. 2. Instead of using the __GLASGOW_HASKELL__ version of foldl', use the other version: foldl' f a [] = a foldl' f a (x:xs) = let a' = f a x in a' `seq` foldl' f a' xs My test case is contrived. Originally, I had a program that read lines from a file as Data.ByteString values, and I was trying to debug a stack overflow. I added the foldl' call to force the evaluation of the ByteString lines, but the foldl' call itself overflowed the stack. I might have fixed my original stack overflow problem. I was applying sum to a large list of integers, and sum is lazy. I don't think I have any real code anymore that overflows the stack, but I'm uncomfortable because I don't know why my test case doesn't work. Is the foldl' call in my test case allowed to have linear stack usage? Thanks, -Ryan