How do you return from a function early in Clojure?

Question

Common Lisp has return-from; is there any sort of return in Clojure for when you want to return early from a function?

Answer 1

There isn't any explicit return statement in clojure. You could hack something together using a catch/throw combination if you want to, but since clojure is much more functional than common lisp, the chances you actually need an early return right in the middle of some nested block is much smaller than in CL. The only 'good' reason I can see for return statements is when you're dealing with mutable objects in a way that's not idiomatic in clojure.

I wouldn't go as far as saying that it's never useful, but I think in Clojure, if your algorithm needs a return statement, it's a major code smell.

Answer 2

When you need to bail out of a computation early, you need a way to do that, not an argument from purists. Usually you need it when you're reducing a big collection and a certain value indicates that there's no point in further processing the collection. To that end, the ever-practical Clojure provides the reduced function.

A simple example to illustrate is that when multiplying a sequence of numbers, if you encounter a zero, you already know that the final result will be zero, so you don't need to look at the rest of the sequence. Here's how you code that with reduced:

(defn product [nums]   (reduce #(if (zero? %2)                (reduced 0.0)                (* %1 %2))           1.0           nums)) 

reduced wraps the value you give it in a sentinel data structure so that reduce knows to stop reading from the collection and simply return the reduced value right now. Hey, it's pure-functional, even!

You can see what's going on if you wrap the above if in a do with a (println %1 %2):

user=> (product [21.0 22.0 0.0 23.0 24.0 25.0 26.0]) 1.0 21.0 21.0 22.0 462.0 0.0 0.0  user=> (product [21.0 22.0 23.0 24.0 25.0 26.0]) 1.0 21.0 21.0 22.0 462.0 23.0 10626.0 24.0 255024.0 25.0 6375600.0 26.0 1.657656E8