Opal is a source to source compiler. It accepts ruby code as a string and generates javascript code which can be run in any environment. Generated code relies on the opal runtime which provides the class system and some other runtime helpers.
The compiler can be broken down into 3 separate stages:
The opal lexer is implemented in pure ruby using the StringScanner class from the opal stdlib. The source code is scanned and tokens are then provided to the parser. This process simply converts the ruby code given as a string, into a list of tokens representing the parts of the ruby code.
The opal parser is implemented using a standard bison like syntax, but relies on racc, a ruby implementation of yacc/bison which is again available in the standard library. The parser takes these tokens generated by the lexer and builds a syntax tree representing the ruby code. This syntax tree is represented by sexps. As ruby is such a complex and dynamic language, there is a lot of interaction between the parser and the lexer, namely through a preserved lex_state.
The opal compiler takes these sexps from the parser and generates ruby code from them. Each type of sexp has its own node type used to generate javascript. Each node creates an array of one or more fragments which are the concatendated together to form the final javascript. Fragments are used as they contain the generated code as well as a reference back to the original sexp which is useful for generating source maps afterwards.