Harper gives an improved version of reduce()
fun staged reduce (unit, opn) = let fun red nil = unit | red (h::t) = opn (h, red t) in red end
and states:
We could just as well have replaced the body of the let expression with the function fn l => red l but a momentÂ’s thought reveals that the meaning is the same.
now, how do we call this subroutine?
hang onto your hats:
staged_reduce (0, (op +)) [4,5,6] ;