You can always test for nil explicitly. Compare:

  unless foo
    # ... foo can be false or nil to reach this branch
  end

  unless foo == false
    # ... this branch is reached unless foo == false
  end

Since terseness is considered such a cardinal virtue amongst Ruby programmers (including myself), however, this kind of issue is often ignored, and difficult to detect unless your tests explicitly check for "truthy" and "non-truthy" values in addition to the true/false literals.

Thanks.

tell me in detail

Haskell obviously isn't Ruby, but its Maybe monad, where you have Just the login, or Nothing, looks pretty appealing.