No, it would work with strict foldl too. In fact in the absence of optimization it would work better (uses less time and space). The optimization required is inlining and strictness analysis.
Is this also true if your just going to use the first few elements after reversing it?
A function which requires lazy foldl for correctness would have to be sometimes lazy in its first argument, and at the same time some partial results would have to be undefined. By "function" I mean the first argument of foldl, treated as a binary function.
Here this doesn't apply because flip (:) x y is always defined. And another common case for foldl, sum, doesn't apply because (+) is usually strict on both arguments (although in principle it does not have to be true because of overloading, which implies that a compiler can only optimize particular specializations of sum, not generic sum).
I don't know of any real-life example.
Yes you are right, my bad. I was thinking as if lists were not lazy on their elements, therefore my second example... But yes, now it is not a common example anymore.
Perhaps there are cases where evaluating partial results is correct but inefficient. I don't know such case either.
Just replace the errors on my second example by some big computations... J.A.