This trick comes from Matthew Flatt via the Racket Discord.

Usually, when creating a Racket #lang and customizing the reader, 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", we should use mylang/expander: 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.

When using #lang s-exp, of course, you can provide a relative path to an expander module. This works if you don’t need a custom reader. If you do, the language syntax is restricted so that #lang "mylang.rkt" will never work.

We can use a trick so that our reader can output a variant on the original relative module path, though. That means our custom reader can yield an expander module that isn’t in an installed package, like the title suggests.

We need to create a module path that works regardless of location, but derive it from the relative path. (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).

Categories: Blog