Implications of foldr vs. foldl (or foldl')?

The recursion for foldr f x ys where ys = y1,y2,...,yk looks like.

Their semantics differ so you can't just interchange foldl and foldr. The one folds the elements up from the left, the other from the right. That way, the operator gets applied in a different order. This matters for all non-commuative operations, such as subtraction.

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