Change Block Binding Without Eval?

Question

I realize that you can change the binding of a block using instance_eval

class Foo
  def bar &block
    instance_eval &block
  end
end

Foo.new.bar { self } # returns the instance

But some built in methods accept blocks and in that case it doesn't seem possible to change the binding of the block without messing with the internals of the built in method.

class Foo
  def enum &block
    Enumerator.new &block
  end
end

Foo.new.enum { self }.each {} # returns main!!!

Is there a way around this?

Answer 1

You can work around it this way:

class Foo
  def enum &block
    Enumerator.new do |*args|
      instance_exec *args, &block
    end
  end
end

But I'm confident that you cannot change the binding of an existing Proc short of instance_eval/instance_exec-ing it.