Feature #21000: A way to avoid loading constant required by a type check

• Status: Open
• Assignee: -
• Target version: -

[Dan0042 (Daniel DeLorme)]

There is this pattern I encounter sometimes:

if defined?(NameSpace::ClassName) and obj.is_a?(NameSpace::ClassName)

Searching in gems, the pattern is fairly common: https://pastebin.com/VGfjRWNu

I would like a way to avoid the repetition of NameSpace::ClassName above. I can think of a number of ways to approach the issue, each with different tradeoffs...

Pattern match ignores uninitialized constant

Pattern match like obj in XYZ could return false if XYZ is not defined. The danger here is that a typo could go undetected and just silently ignore the error even when the constants is expected to be defined.

Pattern match has special syntax to ignore uninitialized constant

Pattern match such as obj in XYZ? (or some other syntax) could return false if XYZ is not defined. The downside is that we're adding yet more new syntax. But it could be obj in defined?(XYZ) and then it doesn't really feel like new syntax.

Do not autoload constants required by pattern match

If we have autoload :XYZ, "xyz" then obj in XYZ could skip the autoload and return false. There is a possibility that XYZ might be defined as a regexp or other matcher that return true, but in general autoload is only used for classes/modules. And if the class/module is not yet loaded, obviously an object of that type cannot exist so we can avoid loading it. But this would only work for autoloaded constants, so can't be used to check a library that might not be loaded, ex: obj in ActiveRecord::Base

defined?(mod) returns mod if it's a class/module

If XYZ is a module, defined?(XYZ) could return XYZ instead of returning "constant". So it can be used in expressions like

case obj
when nil
when defined?(XYZ)

if obj and defined?(XYZ) === obj

if defined?(Gem::Specification)&.respond_to?(:each)

Very versatile, with the downside that it's a small backward incompatibiliy.

[nobu (Nobuyoshi Nakada)]

Dan0042 (Daniel DeLorme) wrote:

defined?(mod) returns mod if it's a class/module

If XYZ is a module, defined?(XYZ) could return XYZ instead of returning "constant". So it can be used in expressions like

case obj
when nil
when defined?(XYZ)

if obj and defined?(XYZ) === obj

if defined?(Gem::Specification)&.respond_to?(:each)

Very versatile, with the downside that it's a small backward incompatibiliy.

I like this, but we will need to consider the incompatibility and inconsistency with other constants.

[Eregon (Benoit Daloze)]

defined? so far always returned nil or frozen strings, so I think it's not so great to use for this.

Also defined? is particularly error-prone for constants & autoloading, for instance:

$DEBUG = true # to show swallowed exceptions

autoload :Foo, "foo"

p defined?(Foo) # nil

p defined?(Foo::Bar)
# Exception 'LoadError' at /home/eregon/.rubies/ruby-3.4.1/lib/ruby/3.4.0/rubygems.rb:1369 - cannot load such file -- rubygems/defaults/operating_system
# Exception 'LoadError' at /home/eregon/.rubies/ruby-3.4.1/lib/ruby/3.4.0/rubygems.rb:1386 - cannot load such file -- rubygems/defaults/ruby
# Exception 'LoadError' at <internal:/home/eregon/.rubies/ruby-3.4.1/lib/ruby/3.4.0/rubygems/core_ext/kernel_require.rb>:136 - cannot load such file -- foo
# Exception 'LoadError' at <internal:/home/eregon/.rubies/ruby-3.4.1/lib/ruby/3.4.0/rubygems/core_ext/kernel_require.rb>:144 - cannot load such file -- foo
nil

So defined?(Foo::Bar) will actually try to load Foo.

[Dan0042 (Daniel DeLorme)]

Eregon (Benoit Daloze) wrote in #note-2:

So defined?(Foo::Bar) will actually try to load Foo.

It would be a good idea to fix that right?

[Eregon (Benoit Daloze)]

I'm unsure, I suppose it could be a compatibility concern.
If you want to know if Foo::Bar exists and Foo is an autoload the only way to know is to load Foo and see if it has a constant Bar.
So while surprising these semantics do make sense since defined? doesn't have a "it's an autoload (not yet loaded), I don't know" return value, it's either nil (doesn't exist) or "constant" (exists).

That said, p defined?(Foo) # nil is weird though.

[Dan0042 (Daniel DeLorme)]

While these semantics make a certain sense, the opposite also makes just as much sense. If Foo is not yet autoloaded, that means neither Foo nor Foo::Bar are yet defined, and it makes sense to return falsy for both. At the very least defined?(Foo) and defined?(Foo::Bar) should be consistent.