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Prelude> foldl (+) 0 [1..100000000]
<interactive>: out of memory

Prelude> foldr (+) 0 [1..100000000]
<interactive>: out of memory

Prelude> id id id id id id id id id id id id id id id id id id id id id id id id id id id id id id id id id id id id id id id id id id 1
<interactive>: out of memory

>>9
The key to "not actually dying" in foldr is to take advantage the supplied function's associativity. Then substitute a working foldl or do a reasonable parallelization.

In foldl, the runtime should be smarter about pathological laziness (like it seems to be about memoization.) foldl' shouldn't be needed to prevent oom conditions (though I wouldn't get rid of it.)