REPL：把前面所有拼成可交互解释器

// repl.js —— 完整可运行：复用前几节的 parse/ev/Environment/标准库
function createGlobalEnv() {
  const env = new Environment()
  env.define('print', (...xs) => console.log('»', ...xs))
  env.define('len', x => x.length)
  env.define('map', (a, fn) => a.map(e => fn(e)))
  return env
}

// 执行一段源码（可包含多条语句），返回最后一条表达式的值
function run(src, env) {
  const ast = parse(src)
  let last
  for (const s of ast) last = ev(s, env)
  return last
}

// REPL 主循环：读一行 -> 求值 -> 打印
function repl(lines) {
  const env = createGlobalEnv()
  for (const line of lines) {
    if (line.trim() === '') continue
    if (line.trim() === ':q') { console.log('bye'); return }
    try {
      const v = run(line, env)
      if (v !== undefined) console.log('=>', v)
    } catch (e) {
      console.log('错误:', e.message)
    }
  }
}

// ===== 调用示例（模拟用户在终端逐行输入）=====
repl([
  'var x = 10;',
  'x = x * x;',
  'print(x);',
  'print(len("hi"));',
  ':q',
])