Reduce a list by applying a binary operator, from left to right, e.g. foldl' op nul [x0 x1 x2 ...] = op (op (op nul x0) x1) x2) .... For example, foldl' (x: y: x + y) 0 [1 2 3] evaluates to 6. The return value of each application of op is evaluated immediately, even for intermediate values.