This trick comes from Matthew Flatt via the Racket Discord.

Usually, when creating a Racket #lang, the reader needs to produce a module form like

(module <name> <initial-import-module>
  body)

and the <initial-import-module> is best resolved as a reference to an installed collection. That is, instead of "mylang/expander.rkt, mylang/expander is preferred: the former is resolved relative to the PWD of the module being expanded!

To make this work typically requires an installed package or linked files. This is not a big hurdle for most Racket hackers, but might be confusing for students.

Instead, we can use a trick to transform the relative module path, which is otherwise natural, into something that works regardless of location. (You’ll still need non-relative paths to make documentation work everywhere, I think.)

Here’s the code.

(module reader syntax/module-reader
  #:language (collapse-module-path-index
              (module-path-index-join "mylang/expander.rkt"
                                      (variable-reference->module-path-index
                                       (#%variable-reference))))
  (require syntax/modcollapse))

Matthew says:

I’m not sure this solves any problem for you, but you can make a module path within syntax/module-reader relative by using #:language, which has an expression afterward instead of a literal. (It’s something I’ve wanted often enough that rhombus/reader treats a relative path like this, instead of like syntax/module-reader).

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