[Haskell-cafe] Stack vs Heap allocation

[Edsko de Vries]

Hi,

How can I know whether something will be stack or heap allocated? For
example, in the standard example of why 

foldl (+) 0

will fail to evaluate a long list of integers due to a stack overflow,
but foldl' won't, it is pointed out that foldl starts building up
unevaluated thunks. So, apparently, these thunks are allocated on the
stack rather than on the heap with a pointer to the thunk on the stack.
(I understand that foldl' is asymptotically better than foldl
space-wise; that is irrelevant to my question.)

On the other hand, in this version that sums all the values in a tree

data Tree = Leaf Int | Node Tree Tree

sum :: Tree -> Int
sum t = sum' [t] 0
  where
    sum' [] acc = acc
    sum' (Leaf i : ts) acc = sum' ts $! (i + acc)
    sum' (Node l r : ts) acc = sum' (l : r : ts) acc

we are building up a (potentially) large lists of trees yet to be
processed, but never run out of stack space. Apparently, the list is
built up on the heap rather than on the stack. 

What is the fundamental difference between these two examples? Why is
the list of trees yet to be processed allocated on the heap (with a
pointer on the stack, presumably) but the unevaluated thunks in the
foldl example allocated on the stack?

Thanks,

Edsko