Implementing Value Storage and Error Handling in a C Lisp Interpreter

Value Storage in Lispy Interpreter

After implementing a Polish notation parser, we need a value storage called Lispy Values to hold AST values and error states.

Value Types

We define two value types: numeric operands and error messages.

typedef struct {
  int type;
  long num;
  int err;
} lval;

Enumeration for value types:

enum { LVAL_NUM, LVAL_ERR };

Enumeration for error types:

enum { LERR_DIV_ZERO, LERR_BAD_OP, LERR_BAD_NUM };

Constructors

lval lval_num(long x) {
  lval v;
  v.type = LVAL_NUM;
  v.num = x;
  return v;
}

lval lval_err(int x) {
  lval v;
  v.type = LVAL_ERR;
  v.err = x;
  return v;
}

Printing Functions

void lval_print(lval v) {
  switch (v.type) {
    case LVAL_NUM:
      printf("%li", v.num);
      break;
    case LVAL_ERR:
      if (v.err == LERR_DIV_ZERO) printf("Error: Division By Zero!");
      else if (v.err == LERR_BAD_OP) printf("Error: Invalid Operator!");
      else if (v.err == LERR_BAD_NUM) printf("Error: Invalid Number!");
      break;
  }
}

void lval_println(lval v) {
  lval_print(v);
  putchar('
');
}

Semantic Error Detection

During evaluation we propagate errors and check for division by zero, unknown operators, and invalid numbers.

lval eval_op(lval x, char *op, lval y) {
  if (x.type == LVAL_ERR) return x;
  if (y.type == LVAL_ERR) return y;
  if (strcmp(op, "+") == 0) return lval_num(x.num + y.num);
  if (strcmp(op, "-") == 0) return lval_num(x.num - y.num);
  if (strcmp(op, "*") == 0) return lval_num(x.num * y.num);
  if (strcmp(op, "/") == 0) {
    if (y.num == 0) return lval_err(LERR_DIV_ZERO);
    return lval_num(x.num / y.num);
  }
  return lval_err(LERR_BAD_OP);
}

The main evaluation function walks the AST, evaluates sub‑expressions, and combines them using eval_op. The main function sets up the parsers, reads input, parses, evaluates, prints the result, and cleans up.