Reveal now highlights illegal booleans in Clojure

I made Reveal — Read Eval Visualize Loop for Clojure — a set of visual tools aiming to improve the development experience by providing better access to the data in your JVM. One of the core ideas in Reveal is using value-aware syntax-highlighting to display the output to the user, so you can immediately see a difference between e.g. a symbol clojure.lang.Ratio and a class clojure.lang.Ratio. The newest version of Reveal (Free 1.3.280, Pro 1.3.359) introduces a new highlight.

What are illegal booleans

An illegal boolean is an instance of Boolean that is neither Boolean/TRUE nor Boolean/FALSE, e.g. a manually created instance of a Boolean like (Boolean. true). They are problematic in Clojure because of its implementation of truthiness — while the semantics define truthiness as “everything is truthy except nil and false”, it is implemented as “everything is truthy except nil and Boolean/FALSE”. This means (Boolean. false) is truthy in Clojure. It’s bad. It’s a great time sink that can drive you wild if you don’t know about it and you see a false acting as true in your code.

And what’s worse, you can get those pesky little booleanses accidentally.

When can you get illegal booleans

Of course, no one ever writes (Boolean. false), but new boolean instances will be created if you use reflection:

(let [eq-meth (.getDeclaredMethod Object "equals" (into-array Class [Object]))]
  (def eq? #(.invoke eq-meth %1 (into-array Object [%2]))))

(eq? 1 2)
=> false ;; So far so good...

(if (eq? 1 2) :equal :not-equal)
=> :equal ;; What the fuuuu....

…Yep. The Boolean constructor is deprecated since Java 9, but it’s left for compatibility and JVM will create new booleans in cases such as this.

How Reveal helps you spot the problem

In the newest release of Reveal, you will now be able to spot the problem earlier:


Neat, isn’t it?

How to fix the problem

If you find illegal booleans in your code, you should find the place where they originate from and wrap it with boolean call like so:

(let [eq-meth (.getDeclaredMethod Object "equals" (into-array Class [Object]))]
  (def eq? #(boolean (.invoke eq-meth %1 (into-array Object [%2])))))

(eq? 1 2)
=> false ;; So far so good...

(if (eq? 1 2) :equal :not-equal)
=> :not-equal ;; Whew!

Hope this helps!