Condition CircleCI build on what part of monorepo changed?

I'm working on a monorepo project that contains a few different subprojects in different languages. Right now I run three different CircleCI "jobs" on each commit. However, each of these packages compile and test themselves independently. Hence I don't actually have to spend time on running all three jobs every time.

Is there some way to condition a CircleCI job on which directory you committed code to?

This discussion gives a pretty good suggestion on how to accomplish what I want.

https://discuss.circleci.com/t/does-circleci-2-0-work-with-monorepos/10378/11

StackExchange DevOps Q#2558, answer score: 3