添加列表列表

Question

I am new to SML and am working on some list arithmitic. I am trying to add a list of lists together using the fold function, without any iteration. The idea is

[[1,2,3],[4],[1]] = 11

This is the fold function I am using

fun fold f base [] = base
| fold f base (x::xs) = f x (fold f base xs);

Because I am using fold, I am not sure if there is a way to use the fold function to make the list of lists one single list, and then it is pretty easy to add them together, or if I am just missing the ball completely.

Here are the added functions, they add up the contents of a single list.

fun add x y = x+y;
fun sumList L = fold add 0 L;

Answer 1

I suggest that you first try to write a function that adds all numbers in a list of intergers and adds this number to a start value (hint: this can be done using fold as well).

You can then use this function as a function argument to fold to achieve your goal.