Don't understand the choices function in the example

[ctataryn]

In your example you have the following for the thirdQuestion:

  choices: answers => index => {
    switch (index) {
      case 0:
        return ['second line when selected first', 'third line when selected first'];
      case 1:
        return ['first line when selected second', 'third line when selected second'];
      case 2:
        return ['first line when selected third', 'second line when selected third'];
      default:
        return ['first line', 'second line'];
    }
  },

The inquirer documentation for the choices property of a question states:

choices: (Array|Function) Choices array or a function returning a choices array. If defined as a function, the first parameter will be the current inquirer session answers.

So in your example I understand using the short-form arrow (=>) is used to declare a function that takes an answers parameter, however I don't understand the second arrow (=>index). To me this almost looks as if it's a short-form for apply on an array?