Issue #17881 has been updated by byroot (Jean Boussier).


> What I had in mind in the conversation in the Zeitwerk ticket was not const_added. It was a callback that gets called when a new class/module is created and after it gets its name set if defined with the class/module keywords.

I see, seems like I missed that.

I suppose the "ideal" API for an autoloader like Zeitwerk would be something like `Module#module_defined(name, module)`:

  - Would trigger on classic `module Foo::Bar` first opening.
  - Would trigger on `Foo::Bar = Class.new`. (This one being a current limitation with TracePoint if I'm not mistaken).
  - Would **not** trigger on `Foo.autoload(:Bar, "bar")`.
  - Would not trigger for non `Module` or `Class` constants.


----------------------------------------
Feature #17881: Add a Module#const_added callback
https://bugs.ruby-lang.org/issues/17881#change-92137

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
----------------------------------------
### Use case

Autoloaders like `zeitwerk` need a callback when a new class or module is registered in the constant table.

Currently this is implemented with TracePoint's `:class` event. It works, but it is a bit unfortunate to have to use an API intended for debugging to implement production features. It doesn't feel "conceptually clean". 

It also [doesn't play well with MJIT](https://k0kubun.medium.com/ruby-3-jit-can-make-rails-faster-756310f235a), even though it's more of an MJIT limitation.

Additionally this usage of TracePoint cause [some incompatibilities with some debuggers like `byebug`](https://github.com/deivid-rodriguez/byebug/issues/564) (even though others don't have this issue). 

### Proposal

I believe that if Ruby was to call `Module#const_added` when a constant is registered, Zeitwerk could get rid of TracePoint.

For now I implemented it as: `const_added(const_name)` for similarity with `method_added`. But maybe it could make sense to have the signature be `const_added(const_name, const_value)`.

Also since `method_removed` exists, maybe `const_removed` would need to be added for consistency.

### Links

Patch: https://github.com/ruby/ruby/pull/4521
Zeitwerk side discussion: https://github.com/fxn/zeitwerk/issues/135

cc @k0kubun



-- 
https://bugs.ruby-lang.org/

Unsubscribe: <mailto:ruby-core-request / ruby-lang.org?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>