“It's a motivating force to implement something; I really recommend it. If you want to understand C, write a C compiler. If you want to understand Lisp, write a Lisp compiler or a Lisp interpreter. I've had people say, “Oh, wow, it's really difficult writing a compiler.” It's not. It's quite easy. There are a lot of little things you have to learn about, none of which is difficult. You have to know about data structures. You need to know about hash tables, you need to know about parsing. You need to know about code generation. You need to know about interpretation techniques. Each one of these is not particularly difficult. I think if you're a beginner you think it's big and complicated so you don't do it. Things you don't do are difficult and things you've done are easy. So you don't even try. And I think that's a mistake.”