Source code in the freebasic programming language

#define MAXLEN 64

type listitem                  ' An item of a list may be a number
    is_num as boolean       ' or another list, so I have to account
    union                      ' for both, implemented as a union.
        list as any ptr        ' FreeBASIC is twitchy about circularly
        num as uinteger        ' defined types, so one workaround is to
    end union                  ' use a generic pointer that I will cast
end type                       ' later.

type list
    length as uinteger              'simple, fixed storage length lists
    item(1 to MAXLEN) as listitem   'are good enough for this example
end type

sub print_list( list as list )
    print "{";
    if list.length = 0 then print "}"; : return
    for i as uinteger = 1 to list.length
        if list.item(i).is_num then
            print str(list.item(i).num);
        else     'recursively print sublist
            print_list( *cast(list ptr, list.item(i).list) )
        end if
    if i
    next i                                              'gracefully
    return
end sub

function cartprod( A as list, B as list ) as list
    dim as uinteger i, j
    dim as list C
    dim as list ptr inner  'for brevity
    C.length = 0
    for i = 1 to A.length
        for j = 1 to B.length
            C.length += 1
            C.item(C.length).is_num = false   'each item of the new list is a list itself
            inner = allocate( sizeof(list) )     'make space for it
            C.item(C.length).list = inner
            inner->length = 2                    'each inner list contains two items
            inner->item(1) = A.item(i)           'one from the first list
            inner->item(2) = B.item(j)           'and one from the second
        next j
    next i
    return C
end function

dim as list EMPTY, A, B, R
EMPTY.length = 0
A.length = 2 
A.item(1).is_num = true : A.item(1).num = 1
A.item(2).is_num = true : A.item(2).num = 2
B.length = 2 
B.item(1).is_num = true : B.item(1).num = 3
B.item(2).is_num = true : B.item(2).num = 4

R = cartprod(A, B)
print_list(R) : print   'print_list does not supply a final newline
R = cartprod(B, A) : print_list(R) : print
R = cartprod(A, EMPTY) : print_list(R) : print
R = cartprod(EMPTY, A) : print_list(R) : print