I have a function

myLength = foldl (\ x _ -> x + 1) 0

which fails with stack overflow with input around 10^6 elements (myLength [1..1000000] fails). I believe that is due to the thunk build up since when I replace foldl with foldl', it works. So far so good.

But now I have another function to reverse a list :

myReverse = foldl (\ acc x -> x : acc) []

which uses the lazy version foldl (instead of foldl')

When I do myLength . myReverse $ [1.1000000]. This time it works fine. I fail to understand why foldl works for the later case and not for former?