 Continuation representation of a left fold

A right fold can build a function that accepts the initial accumulator later. Its empty result is id; its step prepends one update before the continuation supplied by the tail. The invariant foldr c id xs a = foldl s a xs, quantified over all a, follows from the two fold equations. Both applied sides give the bottom value on an undefined spine; finite approximation then covers infinite lazy lists. The larger result type retains information that a direct same-result-type fold can lose.