Languages can be split into two components, their syntax and their semantics. It's your understanding of English syntax that tells you the stream of words "Sleep furiously green ideas colorless" is not a valid sentence. Semantics is deeper. Even if we rearrange the above sentence to be "Colorless green ideas sleep furiously", which is syntactically correct, it remains nonsensical on a semantic level. With Treetop, you'll be dealing with languages that are much simpler than English, but these basic concepts apply. Your programs will need to address both the syntax and the semantics of the languages they interpret. Treetop equips you with powerful tools for each of these two aspects of interpreter writing. You'll describe the syntax of your language with a parsing expression grammar. From this description, Treetop will generate a Ruby parser that transforms streams of characters written into your language into abstract syntax trees representing their structure. You'll then describe the semantics of your language in Ruby by defining methods on the syntax trees the parser generates.