Is `respond_to_missing?`'s second argument use

Question

When using method_missing in Ruby, it's almost always a good idea to define respond_to_missing? as well.

respond_to_missing? takes two arguments; the name of the method we're checking (symbol), and a boolean which indicates whether we should include private methods in our check (include_all).

Now here's what I'm confused about: method_missing doesn't take any arguments which might indicate to it whether it should call private methods or not, as respond_to_missing? does. Furthermore, method_missing gets called regardless of whether the original method call was in a public or private context, and regardless of what respond_to_missing? returns for the given method in the appropriate context. So all operations of method_missing are accessible publicly.

If that's the case, then what purpose does the second argument to respond_to_missing? (include_all) serve? Whether the given object responds to a missing method cannot be affected by the context in which the missing method was called, so why even have this argument at all?

Answer 1

method_missing doesn't take any arguments which might indicate to it whether it should call private methods or not

method_missing() does not call public or private methods. method_missing() is called at the very end of the method lookup path when a method cannot be found. So comparing method_missing() and respond_to* is non-sensical. Why do you think that they have to operate the same way? respond_to?() never was symmetrical with method_missing() in the first place.

Answer 2

I think that respond_to_missing? has a second argument for the same reason that respond_to? does. In both cases, it allows code to ask an object what methods it responds to in a way that respects method privacy. If used properly, it can help you encapsulate your objects better.

You have pointed out a missing feature in method_missing, namely that it should have an argument that says whether the method was called in a public or private context. Maybe method_missing will have that feature some day. Until then, all functionality of the object that is implemented through method_missing will effectively be public, but you can still discourage people from using it in your documentation and via respond_to_missing?.