Question

I’d like to write the method (define_variables) which can get a block and use the variables defined in it. Is it possible? For example, I’d like to get 5 in output:

module A
  def self.define_variables
    yield
    puts a # not 5 :(
  end
end

A::define_variables do
  a = 5
end

Maybe there is some tricks with eval, but haven’t found anyone yet.

Answer 1

In short, no. After you’ve called yield those variables defined in the block are gone (sort of, as we shall see), except for what is returned—that’s just how scope works. In your example, the 5 is still there in that it is returned by the block, and thus puts yield would print 5. Using this you could return a hash from the block {:a => 5}, and then access multiple “variables” that way. In Ruby 1.8 (in IRb only) you can do:

eval "a = 5"
a # => 5

Though I don’t know of anyway to eval the contents of a block. Regardless, in Ruby 1.9 the scope of eval was isolated and this will give you a NameError. You can do an eval within the context of a Binding though:

def foo
  b = yield
  eval(a, b) + 2
end

foo do
  a = 5
  binding
end # => 7

It seems to me that what you’re trying to do is emulate macros in Ruby, which is just not possible (at least not pure Ruby), and I discourage the use of any of the “workarounds” I’ve mentioned above.