PLT Scheme documentation resides with the source code. The setup process that builds bytecode from Scheme source also renders HTML documentation from Scribble source. The HTML output is accompanied by cross-reference information that is used both for building more documentation when new libraries are installed and for online help in the programming environment.
Although many existing PLT Scheme tools help in building documents, the process of generating HTML is significantly different from compilation tasks. The main difference is that cyclic dependencies are common in documentation, whereas library dependencies are strictly layered. For example, the core language reference contains many pointers into the overview and a few pointers to the GUI library and other extensions; all documents, meanwhile, refer back to the core reference. Resolving mutual dependencies directly would require loading all documents into memory at once, which is impractical for the scale of the PLT Scheme documentation. The setup processes therefore builds documents one at a time, reading and writing serialized cross-reference information until it arrives at a fixed point for all documents. A fixed point usually requires two iterations, so that all documents see the information collected from all other documents. A handful of documents require a third pass, because they contain section titles from other documents, where each section title is based on still other documents (e.g., by using an identifier whose typesetting depends on whether it is documented as a procedure or syntactic form).
Another challenge in building a unified set of documentation is that individual users might augment the main installation with user-specific libraries. The main installation includes a table of contents that is the default starting point for reading documentation, and this table is updated when a new package is placed into the main installation. When a user-specific library is installed, in contrast, its document is built so that hyperlink references go into the main installation’s documentation, and a user-specific table of contents is created. When a user opens the documentation via DrScheme’s Help menu, a user-specific table of contents is opened (if it exists).
Instead of explicitly installing a library, a user can implicitly install a package from the PLaneT repository (Matthews 2006) by using a library reference of the form (planet ....). When a library is installed in this way, its documentation is installed as the library is compiled. PLaneT supports library versioning, and multiple versions of a package can be installed at a time. In that case, multiple versions of the documentation are installed; document references work with versions just as reliably as library references, since they use the same underlying module-import mechanisms to precisely identify the origin of a binding.