We can also add a constructor for this variation.

    2. lval* lval_qexpr(void) {
    3.   lval* v = malloc(sizeof(lval));
    4.   v->type = LVAL_QEXPR;
    5.   v->count = 0;
    6.   v->cell = NULL;
    7.   return v;
    8. }
    1. void lval_del(lval* v) {
    3.     case LVAL_NUM: break;
    5.     case LVAL_SYM: free(v->sym); break;
    7.     /* If Qexpr or Sexpr then delete all elements inside */
    8.     case LVAL_QEXPR:
    9.     case LVAL_SEXPR:
    10.       for (int i = 0; i < v->count; i++) {
    11.         lval_del(v->cell[i]);
    12.       }
    13.       free(v->cell);
    14.     break;
    17.   free(v);
    18. }

    Using these simple changes we can update our reading function lval_read to be able to read in Q-Expressions. Because we reused all the S-Expression data fields for our Q-Expression type, we can also reuse all of the functions for S-Expressions such as lval_add. Therefore to read in Q-Expressions we just need to add a special case for constructing an empty Q-Expression to lval_read just below where we detect and create empty S-Expressions from the abstract syntax tree.

    1. if (strcmp(t->children[i]->contents, "(") == 0) { continue; }
    2. if (strcmp(t->children[i]->contents, ")") == 0) { continue; }
    3. if (strcmp(t->children[i]->contents, "}") == 0) { continue; }
    4. if (strcmp(t->children[i]->contents, "{") == 0) { continue; }

    Because there is no special method of evaluating Q-Expressions, we don’t need to edit any of the evaluation functions. Our Q-Expressions should be ready to try. Compile and run the program. Try using them as a new data type and ensure they are not evaluated.