The case for and against Clojure predicates returning nil

Last week, there was a mailing list post on the clojure-dev mailing list noting that several of the new predicates in Clojure 1.9 alphas return true or nil. The three new predicates are: qualified-keyword?, qualified-ident?, and qualified-symbol?. Previously, all predicate functions in Clojure core, i.e. functions that end in a ? have returned true/false. In this post I want to examine the pros and cons of this new behaviour.

If I’ve missed anything here, please get in contact and I’ll update the post.

Pros to some predicates returning nil

The first two points aren’t strong arguments for returning nil, rather they argue that it doesn’t matter whether the functions return nil or false.

Cons to some predicates returning nil:


Personally, I don’t see a strong reason to keep the current behaviour of the new qualified predicate functions, but you might see things differently. Whatever the outcome, I’m glad that in Clojure we don’t have to name all of our boolean functions with an “Eh” suffix.


Alex Miller suggested to file a ticket about this behaviour, so I opened CLJ-2141. After some discussion, a patch was made to coerce the output of these new functions to a boolean.