Another interesting approach is 'success typing' in Erlang, as described in this elegant paper:

http://user.it.uu.se/~tobiasl/publications/contracts.pdf

Success typing is optionally refined through typespecs in the code and checked by the Dialyzer tool:

http://www.erlang.org/doc/reference_manual/typespec.html

http://learnyousomeerlang.com/dialyzer

Success typing is open to gradual typing of legacy code, as more typespecs are added, constraints gradually tighten, and Dialyzer finds the inconsistencies.