Rather than evaluate expressions numerically as a non-CAS calculator would, a CAS interprets and evaluates expressions symbolically. This allows the software to solve algebraic equations, give exact forms, factor and expand expressions, and more.
To take input, this implementation uses a sort of makeshift "shell" that the user
types their expression into and presses ENTER to evaluate it.
To parse this user input, this CAS uses ANTLR.
I got this idea because I thought it would be cool to figure out how a CAS works by making one. I own a calculator that has a CAS and was really interested in how it works.
At this point, my implementation is just a glorified calculator. It can evaluate expressions and equations, and store variables. I'd like to eventually learn more about how computational algebra works so I can implement a better solution.
I'm currently reading this paper from Philipps–Universität Marburg in Germany to help me understand more.