DrRacket is an IDE for the programming language Racket, a Lisp/Scheme dialect. The language is a good starting point if you want to learn about the functional programming paradigm. This blog entry explains how to install the IDE on your Mac.
To install DrRacket on Mac OS X 10.11.3 El Capitan, you can simply use your existing Homebrew installation like this:
brew cask install racket
This will install racket with some libraries and the IDE DrRacket.
NOTE: If you use the PtlRacket formula for Homebrew,
brew install ptl-racket
you will install a very minimal installation of racket, without the IDE DrRacket and with only a few libraries.