tiny lisp interpreter

lisper.js

let env = {
  '+': (...args)=> args.reduce((mem, cur)=> mem + cur),
  '*': (...args)=> args.reduce((mem, cur)=> mem * cur),
  '-': (...args)=> args.reduce((mem, cur)=> mem - cur),
  '/': (...args)=> args.reduce((mem, cur)=> mem / cur),
};

function tokenize(expr) {
  return expr.replace(/\(|\)/g, ' $& ').trim().split(/\s+/);
}

// TODO
function evaluate(ast) {
  return ast || 'Invalid expression';
}

function parse(expr) {
  const stack = [];
  let curAst = [];

  for (const token of tokenize(expr)) {
    if (token === '(') {
      curAst = [];
      stack.push(curAst);
    } else if (token === ')') {
      const done = stack.pop();
      curAst = stack[stack.length - 1] || [];
      curAst.push(done);
    } else {
      curAst.push(token);
    }
  }
  if (stack.length !== 0) return null;
  return curAst[0];
}

function main() {
  process.stdin.resume();
  process.stdin.setEncoding('utf8');
  process.stdout.write('> ');

  process.stdin.on('data', (text)=> {
    if (text === 'quit\n') {
      process.exit();
    } else {
      console.log(evaluate(parse(text.trim())));
    }
    process.stdout.write('> ');
  });

  process.on('exit', ()=> {
    console.log('Bye');
  });
}

if (require.main === module) {
  main();
}