CLJS-2143

readme.md

Option 1.

https://github.com/Deraen/clojurescript/commit/bb94e566a8827d0356740491e2506fba02faa109

• Load all js_transform.clj files in classpath
• Problems:
  • files are not real namespaces (no ns)
  • load-reader will eval the code in file each time load-js-transforms! is called, which is each time cljs.closure/build is called
  • a js_transform.clj file is loaded even if js-transforms method from it is not needed
    • for example, if any lib in classpath has dep on cljsjs/babel-standalone

Option 2.

https://github.com/Deraen/clojurescript/commit/9866a33c78ebf882e5e01d07316d00c7794e8edc

• If js-transforms method is not found, but the value is namespaced keyword, try to load namespace pointed by the namespace part of the keyword and run js-transforms again in hopes that the namespace provided the method
• If namespace doesn't provide the method, there will be a warning about unsupported preprocess value
• Benefits vs. Option 1:
  • Real namespaces
  • Works with current cljsjs/babel-standalone
  • Minimal change

Option 3?

• :preprocess value should be a symbol
• js-transforms multimethod is removed
• symbol will point directly at a function, the namespace is required if needed
• Benefits:
  • Using a symbol to refer to a function is better than refering to a multimethod using namespaced keyword

Other ideas

• :preprocess should be allowed to be vector of transformations to run?
  • Would be handled at process-js-modules, will work with options 2 and 3

Currently have a hard time coming up with use-cases for this since it may sometimes be required to process multiple files in one pass instead of one-by-one. If only for performance reasons ...

Besides that I would probably suggest using something middleware-ish although I hate the term. Something along the lines of http://pedestal.io/reference/interceptors.

:preprocess
[{:enter some.ns/enter
  :leave some.ns/leave
  :other-opt "foo"}]

Not sure :enter and :leave are required, could just make it :fn and take map itself as the opts.

All "simpler" entries could just desugar into the above

• :preprocess some.ns
• :preprocess some.ns/process-js-module
• :preprocess [{:fn some.ns/process-js-module}]

Option 3 sounds good to me. I don't think we need to remove js-transforms immediately to proceed though. Supporting vectors sounds OK but it seems like an easy thing to add after the main work is done. @thheller I'm not seeing the value of the interceptor model here, since that was done for asynchrony. Is there something you have in mind?

It should be easy to leave the multimethod version around and use that when the value is keyword).

I agree that vector support can be added later, if needed.