Hygienic Literate Programming: Lessons from ChezWEB

Literate programming systems are a class of domain specific languages designed to encourage writing programs specifically to be read as essays or books by humans instead of by machines. Systems like CWEB, WEB, and ChezWEB allow the user to associate arbitrary code bodies with a concise but natural language description. That description may then be referred to in other sections of the program source, and the code body associated with the description is substituted for the reference in the program source. Using Scheme macros, we describe how this substitution is performed unhygienically. We then describe and implement an innovation unique to ChezWEB that extends the notion of code reordering to provide hygiene and referential transparency guarantees for section references and code substitutions in the system. Our implementation strategy serves as a design pattern for implementing other language forms for which the programmer desires similar hygienic guarantees.