Variable Evaluation

Our evaluation function now depends on some environment. We should pass this in as an argument and use it to get a value if we encounter a symbol type. Because our environment returns a copy of the value we need to remember to delete the input symbol lval.

  1. lval* lval_eval(lenv* e, lval* v) {
  2.   if (v->type == LVAL_SYM) {
  3.     lval* x = lenv_get(e, v);
  4.     lval_del(v);
  5.     return x;
  6.   }
  7.   if (v->type == LVAL_SEXPR) { return lval_eval_sexpr(e, v); }
  8.   return v;
  9. }

Because we’ve added a function type, our evaluation of S-Expressions also needs to change. Instead of checking for a symbol type we want to ensure it is a function type. If this condition holds we can call the fun field of the lval using the same notation as standard function calls.

  1. lval* lval_eval_sexpr(lenv* e, lval* v) {
  3.   for (int i = 0; i < v->count; i++) {
  4.     v->cell[i] = lval_eval(e, v->cell[i]);
  5.   }
  7.   for (int i = 0; i < v->count; i++) {
  8.     if (v->cell[i]->type == LVAL_ERR) { return lval_take(v, i); }
  9.   }
  11.   if (v->count == 0) { return v; }
  12.   if (v->count == 1) { return lval_take(v, 0); }
  14.   /* Ensure first element is a function after evaluation */
  15.   lval* f = lval_pop(v, 0);
  16.   if (f->type != LVAL_FUN) {
  17.     lval_del(v); lval_del(f);
  18.     return lval_err("first element is not a function");
  19.   }
  21.   /* If so call function to get result */
  22.   lval* result = f->fun(e, v);
  23.   lval_del(f);
  24.   return result;
  25. }