How these source strings are evaluated in XGR is grammar dependent. In theory, Whigham's technique makes it possible to evolve programs in any language which can be defined in a BNF. However, in practice, it requires a way of evaluating the program snippets in order to assign a fitness score. So in order to evolve Java, Lisp or Ruby code, some form of interpreter for that language is required. EpochX provides a way of plugging in interpreters for different programming languages, and provides interpreters out of the box for the following:
Of course it is also possible to evaluate a candidate solution manually without an interpreter.