Ocaml function that takes a matrix (how to add without using matching)

Question

How to implement a function sum_matrix that can be used like this?

let m = [[1;2;3]; [4;5;6]];; 
sum_matrix( [[1;2;3]; [4;5;6]]);;

Without using matching, or comparing a list to an empty list. Supposed to use fold.

I understand how to do this wit two separate lists but the matrix aspect adds a level of depth that I do not understand. Also how does OCaml really accept parameters like that?

Answer 1

The function

let sum li = List.fold_left (fun acc elem -> acc + elem) 0 li

adds all elements of a list (they must be integer).

The function

let sum_apply f li = List.fold_left (fun acc elem -> acc + f elem) 0 li

computes the sum of applying f to each element of the input list: sum_apply f [x;y;z] is fx + fy + fz -- this could also be implemented using sum and List.map .

Do you see how to combine these two functions to get the sum of the integers in an int list list ?