Today, programmers type code into text editors and are forced to re-construct program behavior in their heads, an arduous task that limits accessibility for beginners and feels unpleasant for experts. Is it possible to create user interfaces that provide useful feedback about program behavior in support of programmer experience?

REPLugger (REPL + debugger) is a research prototype code editor attempting to provide this kind of immediate feedback. REPLugger integrates the liveliness of a REPL and the comprehensiveness of a debugger directly into the code-editing experiencing. By constructing a sample program environment, REPLugger allows programmers to examine the effects of their code as-they-type. Additionally, REPLugger supports the easy creation and saving of what-if scenarios, allowing programmers to quickly test and validate hypotheses about program behavior in complex systems. The result is a surprisingly fluid and pleasant editing experience that can work with programs of many thousands of lines.