If you want a hard/interesting parsing challenge, try Clojure’s #_ reader macro. It’s a powerful construct that allows you to comment the next form. If you’re n

If you want a hard/interesting parsing challenge, try Clojure’s #_ reader macro. It’s a powerful construct that allows you to comment the next form. If you’re not used to Clojure, it’s like writing #_ in front of anything --a function, an array, a keyword, etc-- to comment it, even if it’s on multiple lines. For example: #_ (defn foo [x y] (println x y)) This is equivalent to commenting the three lines. Things become even harder when you learn that these thing "swallow" the next form and can be used anywhere: (let [a #_ b 43] #_ #_ hello (H N) (print a)) The code above is equivalent to the following: (let [a 43] (print a)) All the rest is comments. The hard/interesting bit is that to tokenize you must construct a syntax tree in order to correctly parse the next form, but in order to construct a syntax tree you first need to tokenize the code.