Debugging Super Methods with Ruby 2.2

Summary

Ruby exposes method metadata and caller-stack information, but before Ruby 2.2 it was difficult to identify the method ultimately reached by super, especially in metaprogrammed code. The example defines Dog#bark and SchneemsDog#bark, then extends an instance with DoubleBark, whose bark method calls super twice; inspecting the class hierarchy incorrectly reports Dog#bark because the module is not an ancestor of the instance's class. Ruby 2.2 adds Method#super_method, a feature proposed in request #9781 and accepted into the Ruby trunk, which returns the method Ruby will invoke for super, including the correct SchneemsDog#bark in this case. Calling source_location on that returned Method provides the location of the actual implementation and avoids the custom metaprogramming previously needed to trace it.

Takeaways

1. 1

   Method#super_method returns the method Ruby will call through super, rather than requiring callers to reconstruct the lookup path.

2. 2

   Extending an instance with DoubleBark can make class-based inspection report Dog#bark even though super from DoubleBark#bark refers to SchneemsDog#bark.

3. 3

   Calling source_location on the Method returned by super_method yields the location of the implementation that super will actually invoke.