I definitely prefer the "==" form for stating laws the interface follows.
Doctest came from the Python tradition, which is a very imperative outlook on life. I wonder whether doctest could be modified to better match the programming, documentation, and testing style of Haskell. In particular, I'd expect it to recognize "=="-style statements as things to test.
Please read: http://www.haskell.org/pipermail/libraries/2011-December/017382.html
The only real issue is whether those statements should be considered unit tests or properties (testable by QuickCheck and SmallCheck), since those two categories have very different methods of being verified.
I agree. P.S. I have extended test-framework-th to handle doctest (HUnit) also. I will probably release this in this week. --Kazu