computes the length of list which contains list (SML)

Question

SML length function:

fun length(L) =
    if (L=nil) then 0
    else 1+length(tl(L));

For example:

length [1,2,3] = 3;
length [ [5], [4], [3], [2,1] ] = 4;

Based on the code, how do I change it if I also want to count the elements in the list of the list?

For example:

length [ [5], [4], [3], [2,1] ] = 5;

Answer 1

You could create another function that will use your function as follows:

fun d_length ( [] ) = 0
| d_length ( l :: l' ) = length(l) + d_length(l');

d_length[ [5], [4], [3], [2,1] ];

Or alternatively, use the build in reducer:

List.foldl (fn(e,a) => length(e) + a) 0 [ [5], [4], [3], [2,1] ];