Ruby's `super` Keyword Weirdness

04 Sep 2017

The other day, my colleague Benoît was faced with a strange problem in his Ruby code. Here is a greatly simplified version of the code that caused the problem:

class A
    def foo
        p :A
    end
end

class B < A
end

module F
    def foo
        super
        p :F
    end
end

method = F.instance_method(:foo)
B.send(:define_method, :foo, lambda { |*args|
    method.bind(self).call(*args)
})

B.new.foo

What do you think the last line does?

It seems like it should print :F, then :A. B#foo becomes bounds to a lambda, inside which we call F#foo with self bound to self from the lambda.

Since the lambda is then bound to B#foo, it would stand to reason to think that the super from F#foo would invoke A#foo. But here's the rub: it invokes B#foo recursively.

And so it turns out that the last line fails with SystemStackError: stack level too deep.

Of course, the code is actually needlessly complicated. You could do this:

method = F.instance_method(:foo)
B.send(:define_method, :foo, method)

And — plot twist — this actually works fine.

This also works:

B.send(:define_method, :foo, lambda { |*args| super(*args); p :F })

That's interesting and wasn't covered in the Ruby's Dark Corners series can explain it. And of course, Ruby doesn't have a specification.

I have an hypothesis: when a "callable" (unbound method, lambda, ...) is directly bound to a method, then any super reference in the callable are correctly bound to that method. However, if super is called through at least one level of indirection (in our example above, we have a lambda that calls a method), then super simply calls the regular method (that is the "lowest" one in the class hierarchy), no matter what method it was reached from.

(This was tested with Ruby 2.3 on Windows and Ruby 2.4 on Mac OS)

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Ruby's `super` Keyword Weirdness