parser.c

#include <stdio.h>

/**
 * Grammar for this parser
 *
 * S -> E
 * E -> E '+' F | F
 * F -> number | '(' E ')'
 *
 * Without immediate left recursion
 * S  -> E
 * E' -> '+' F E | λ
 * E  -> F E'
 * F -> number | '(' E ')'
 */

// Given
int lex();
void unlex();
void peek();
void match();

// to implement
int S();
int E();
int E_prime();
int F();

void error();

int main(int argc, char const *argv[]) {
  S();
  return 0;
}

int S() { return E(); }

int E() {
  F();
  E_prime();
}

int E_prime() {
  char token;
  switch (token = lex()) {
  case '+':
    F();
    E_prime();
    break;
  default:
    // 'lambda'
    break;
  }
}

int F() {
  char token;
  switch (token = lex()) {
  case number:
    break;
  case '(':
    E();
    if (lex() != ')') {
      error();
    }
    break;
  default:
    error();
    break;
  }
}