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
– hammar, Commented Apr 20, 2011 at 16:12
It does indeed. '1 tests, 0 assertions, 0 failures, 0 errors' — Derrick J Wippler
– Derrick J Wippler, Commented Apr 20, 2011 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
– Mladen Jablanović, Commented Apr 20, 2011 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
– Derrick J Wippler, Commented Apr 25, 2011 at 17:04

Pedro Rolo · Accepted Answer · 2011-04-24 23:18:05Z

4

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

edited Apr 24, 2011 at 23:18

answered Apr 23, 2011 at 10:31

Pedro Rolo

30.3k13 gold badges63 silver badges95 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Derrick J Wippler Over a year ago

This is an acceptable way of allowing the tests to pass. Answer is accepted.

Collectives™ on Stack Overflow

Behavior of 'return' keyword within a ruby block

1 Answer 1

1 Comment

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Related