Function Interpretation

Functions in Egg get their own local scope. The function produced by the fun form creates this local scope and adds the argument bindings to it. It then evaluates the function body in this scope and returns the result.

specialForms['->'] =specialForms['fun'] = function(args, env) {
  if (!args.length) {
    throw new SyntaxError('Functions need a body.')
  }

  function name(expr) {
    if (expr.type != 'word') {
      throw new SyntaxError('Arg names must be words');
    }
    return expr.name;
  }

  let argNames = args.slice(0, args.length - 1).map(name);
  let body = args[args.length - 1];

  return function(...args) {
    if (args.length > argNames.length) {
      throw new TypeError(`Wrong number of arguments. Called with ${args.length} arguments and declared ${argNames.length} parameters`);
    }

    let localEnv = Object.create(env);
    for (let i = 0; i < args.length; i++) {
      localEnv[argNames[i]] = args[i];
    }

    return body.evaluate(localEnv);
  };
};