Custom internal Runner example?

I have a custom language plugin and I would like to add an ability to run and debug the code written in this language - however, there's no command line or external tool for this, at the end of the day it's just a matter of calling a Java method, e.g. String response = MyLanguage.runScript(script); So my question is, how do I go about implementing an internal runner? All examples I could find start and control the external OS processes. Can anyone recommend any good example of an internal runner?