Open Source as a business
expression evaluation: pld.cs.luc.edu/courses/271/spr18/demos/expressionsij.zip.
Demo of command-line expressions
With a calculator interface, things are a little different. Generally, the '(' key clears the display and starts a new subexpression. Operator keys might display the current operator on the screen. The ')' key acts like the '=' key for the current expression, displaying the results so far. galculator takes this approach.
The other alternative is to display the entire expression on the display line. qalculate does this.