Behavior of 'return' keyword within a ruby block

Question

Can someone explain the behavior of the following

def iterate
  return yield
  return "end of iterate"
end

def test_iterate
  assert_equal( "end of iterate",  iterate { return "end of block" } )
  assert_equal( "end of block",  iterate { "end of block" } )
end

I understand that Procs ( Which is what blocks are ) should return within the scope they are called. ( Unlike lambdas ) With this in mind, shouldn't both calls in the tests return "end of block"?

This test passes on 'ruby 1.8.7 (2009-06-12 patchlevel 174) [universal-darwin10.0]' ( OSX 10.6.7 )

Doesn't that mean that the `return` returns from your test function, making it pass without reaching the assertions at all? — hammar, Apr 20 '11 at 16:12
It does indeed. '1 tests, 0 assertions, 0 failures, 0 errors' — Derrick J Wippler, Apr 20 '11 at 16:25
I tested both 1.8.7 and 1.9.2 and both don't reach first `assert_equal`, which is expected, as `return` in your first block returns from the method. — Mladen Jablanović, Apr 20 '11 at 17:42
Correct, This was a simple oversight on my part. Hammar correctly stated the return statement in the first test exits the 'test_iterate' block. Altho it appeared that the test passed, infact the test never completed. — Derrick J Wippler, Apr 25 '11 at 17:04

Pedro Rolo · Accepted Answer · 2011-04-24T23:18:05.540

The return keyword returns from the lexicaly enclosing method. That is, *test_iterate*.

To return a certain value from a block in a dynamically scoped fashion, you should use the break keyword instead.

In your case:

iterate { break("end of block") }

The test will fail. because the second return statement of the iterate method will never run.

The intended semantics should be accomplished by using exceptions:

def iterate
  return yield
  rescue :exception
    return "end of iterate"
end

def test_iterate
  assert_equal( "end of iterate",  iterate { raise :exception } )
  assert_equal( "end of block",  iterate { "end of block" } )
end

This is an acceptable way of allowing the tests to pass. Answer is accepted. — Derrick J Wippler, Apr 25 '11 at 17:06

Behavior of 'return' keyword within a ruby block

1 Answers1