Honza Pokorný

A personal blog

Lisp parentheses

Perhaps the number one reason why people are afraid to try Lisp or don’t like it is the huge amounts of parentheses cluttering up the code. It’s said to be hard to read the code when it’s full of parentheses.

Any experienced Lisp programmer will tell you that the parentheses disappear fairly early on. After a while, you hardly notice them as something annoying. In fact, going back to C-family languages will make you feel like you need to type all kinds of crazy punctuation.

While Clojure technically doesn’t use significant whitespace like Python, in reality, careful identation is crucial to writing clear code.

(defn crop-photo [user photo]
  (when (authenticated? user)
    (when (admin? user)
      (crop photo))))

In this snippet, there are four levels of indentation, four nested expressions. It’s easy to quickly scan this function guess what it does. When a user is authenticated and when they are an admin, crop the photo. If any of the when expressions return a falsy value, the whole function will return nil. All of this is possible because Clojure uses prefix notation. This means that the first element in the (...) form is the name of the function. Therefore, you only need to scan the beginnings of lines to see what functions are being called. Also, you never have to pay attention to closing parentheses because they are all sitting together at the end of the function.

In Clojure, it’s also idiomatic to put function arguments on new lines and align them.

(or (admin? user)
    (staff? user))

In this example, the or macro usually takes two arguments. We put each argument on its own line and align them. This way it’s visually clear what the code does.

Finally, when writing Clojure code, you rarely have to worry about matching up your parentheses. This is a job for your text editor. Inserting a new expression usually involves typing the ( key and having its friend ) inserted for you.

This article was first published on December 12, 2013. As you can see, there are no comments. I invite you to email me with your comments, criticisms, and other suggestions. Even better, write your own article as a response. Blogging is awesome.