From: "fxn (Xavier Noria) via ruby-core" <ruby-core@...>
Date: 2025-02-17T12:29:52+00:00
Subject: [ruby-core:121081] [Ruby master Misc#21035] Clarify or redefine Module#autoload? and Module#const_defined?

Issue #21035 has been updated by fxn (Xavier Noria).


Hey, I have pending to send examples, but I'd like to suggest that we close this ticket (could followup with examples through a different channel).

The purpose of this ticket was "clarify or redefine". Since I did not know that issuing a `require` may unregister autoloads, the behavior of `autoload?` and `const_defined?` without triggering an autoload was confusing to me. Now, the mental model is clear: autoloads can be unregistered/ignored, and `autoload?` and `const_defined?` act accordingly. Now it clicks!

Still, we have `Module#constants` and we don't have documentation covering all this. But the approach to propose that has to be worded in a different way and I believe it would more clear to start a new ticket (I'd do it myself).

Please, feel free to close!

----------------------------------------
Misc #21035: Clarify or redefine Module#autoload? and Module#const_defined?
https://bugs.ruby-lang.org/issues/21035#change-111999

* Author: fxn (Xavier Noria)
* Status: Open
----------------------------------------
The documentation for `Module#autoload?` says:

> Returns filename to be loaded if name is registered as autoload in the namespace of mod or one of its ancestors.

As a user, if I declare an autoload, I expect this API:

```ruby
module M
  autoload :Foo, 'foo'

  constants            # => [:Foo]
  const_defined?(:Foo) # => true
  autoload?(:Foo)      # => 'foo'
end
```

That it is indeed how it generally works. Even if the autoload path does not exist. But there is an edge case.

While `constants` does include always `:Foo` as far as I can tell, the return value of `const_defined?` and `autoload?` depends on the stack of features being loaded: The autoload path is resolved and if seen to be in the stack of features being loaded, the predicates return `false` and `nil`, respectively. Do you think that is intuitive?

I find that logic totally unexpected. I just defined an autoload, therefore, I think it would be natural for `autoload?` to return what I just configured. Why should `const_defined?` return nothing but `true`? And why is it not consistent with `constants`?

To me, it would make more sense that in the previous example `const_defined?` returns `true`, and `autoload?` returns `foo` unconditionally (and instantly, nowadays it takes a relative long time due to the lookup).

Now, if the autoload is triggered in a lookup **then** I would expect `Kernel#require` logic to apply. But not when calling some simple predicates.

Please, note that the present behavior is not documented, so on paper the change would not be backwards incompatible.

If, on the other side, it is preferred to keep the behavior as it is, I guess it should be documented with precision (accounting for symlinks, relative paths in `$LOAD_PATH`, etc.)




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