May you go out of your life and return to Java parser

A few days ago, a friend of mine gave me a Haskell question

This question is interesting and not difficult.

This problem is called the extensibility problem. In fact, there are two areas that need to be expanded.

0: parser needs to be extended with methods such as open recursion

1: The ADT from parse also needs scalability

There are many requirements in the second half, such as final tagless, dtalc, tree that growth, recurrence scheme style fix... So put down the watch and let's deal with the previous one.

The first half.. Haskell's looked around object system has done it. Of course, they have a little heavy weight. They plan to get a super lightweight one: five lines is enough, and one more line is little Sasha.

Ready？