Regarding
fun add up nil = 0 | add up (h::t) = h + add up t fun mul up nil = 1 | mul up (h::t) = h * mul up t
Harper states:
What precisely is the similarity? We will look at it from two points of view. One view is that in each case we have a binary operation and a unit element for it.
And my question is: by unit element does he mean identity value?
Yes. -- AdamChlipala