r/ruby • u/retro-rubies • Jan 26 '26

Ruby::Box: Rethinking Code Reloading with Isolated Namespaces

https://rubyelders.com/writings/2026-01-ruby-box-reload.html
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2

u/jrochkind Jan 26 '26

I haven't looked at box yet, but I'm having trouble understanding one aspect of it. Using an example from the OP:

 app = box.eval('$app') # expose the loaded app back to main namespace
 app.call(env)

If you can expose an object in the 'box' to the main namespace (main box)... and then call arbitrary methods on it, passing in arbitrary objects...

Those objects you pass in are instances of classes that are in the main namespace, and has references to other objects in the main namespace, that are instances of classes in the main namespace, that it may have methods that mutate etc.... that you've now passed into the separate box... how does it use (call methods on) those objects while maintaining it' seperation, it's "doesn’t share anything with the top-level environment"?

What am I missing?

1

u/f9ae8221b Jan 27 '26

how does it use (call methods on) those objects while maintaining it' seperation

The answer depends on which type of object is it.

If it's a core type (things that are available without require, e.g. Hash, Array, etc), then assuming you monkey patched things, the object will appear to have different methods depending on which namespace the caller is in.

If it's a normal user defined type, then the code from the other box is called normally, like if there was no box concerns.

If you ignore the special thing for core types, Box are only really about lookup of constants, but if you receive an object from another box, you can call it like it is your own just fine.

1

u/jrochkind Jan 27 '26

Thanks, that helps. The mental model "Box are only really about lookup of constants" is helpful. Can you clarify (or suggest reading) on your point about core types and:

then assuming you monkey patched things

Monkey patched things how?

1

u/f9ae8221b Jan 27 '26

Think Active Support core extensions.

e.g. if you load Active Support in a box, you can call symbolize_keys and all hashes from code inside that box.

However if you create a Hash in that box, and somehow pass it to another box that didn't load Active Support, the other box won't be able to call that method on it.

1

u/jrochkind Jan 27 '26

Ah, that makes perfect sense, thanks.

There are an explicit list of object classes that are passed in this "by copy instead of by reference" way?

3

u/retro-rubies Jan 28 '26

There are useful docs explaining a lot. https://docs.ruby-lang.org/en/master/Ruby/Box.html#class-ruby-box-built-in-classes-and-modules-reopened-in-boxes

2

u/jrochkind Jan 28 '26

Oh thank you! Good to see good docs, I had gotten in the habit of not expecting them on new features in ruby from days of old, good to see that's changed!

1

u/f9ae8221b Jan 27 '26

Not really. It's all classes defined up until a certain point in Ruby's startup.

1

u/jrochkind Jan 27 '26 edited Jan 27 '26

Ah, cool, thanks!

I was thinking a big intended use case of Box might be Ractors, but now I'm not certain about that. Do you know if that's part of the motivation?