Add nth builtin

src/builtins.c

@@ -69,6 +69,13 @@ LISP_FUNC_1(ucl_builtin_cdr, state, arg) {
     return ucl_cdr(arg);
 }
 
+LISP_FUNC_2(ucl_builtin_nth, state, n, list) {
+    UCL_COND_OR_RET_ERROR(n->type == UCL_TYPE_INT, "First argument to nth must be an integer");
+    UCL_COND_OR_RET_ERROR(list->type == UCL_TYPE_CELL, "Second argument to nth must be a list");
+
+    return ucl_list_nth(list, n->integer);
+}
+
 LISP_FUNC_2(ucl_builtin_add, state, arg0, arg1) {
     if (arg0->type != UCL_TYPE_INT) {
         return ucl_error_create("Invalid type of argument 0 to 'add'");

src/builtins.h

@@ -21,6 +21,7 @@ struct ucl_object *ucl_builtin_now_millis_mono(struct ucl_state *state, struct u
 
 struct ucl_object *ucl_builtin_car(struct ucl_state *state, struct ucl_object *args);
 struct ucl_object *ucl_builtin_cdr(struct ucl_state *state, struct ucl_object *args);
+struct ucl_object *ucl_builtin_nth(struct ucl_state *state, struct ucl_object *args);
 struct ucl_object *ucl_builtin_list(struct ucl_state *state, struct ucl_object *args);
 struct ucl_object *ucl_builtin_print(struct ucl_state *state, struct ucl_object *args);
 struct ucl_object *ucl_builtin_printl(struct ucl_state *state, struct ucl_object *args);

src/main.c

@@ -49,11 +49,11 @@ int main(int argc, const char **argv) {
     ucl_state_put(state, "list-p", ucl_builtin_create(ucl_builtin_list_p));
     ucl_state_put(state, "car", ucl_builtin_create(ucl_builtin_car));
     ucl_state_put(state, "cdr", ucl_builtin_create(ucl_builtin_cdr));
+    ucl_state_put(state, "nth", ucl_builtin_create(ucl_builtin_nth));
     // TODO:
     //   - equality
     //   - map
     //   - reduce
-    //   - nth
     //   - booleans (e.g. not)
 
     if (argc < 2) {