If what you're thinking is the so called Design by Contract (using preconditions and postconditions), there are plenty of languages with that feature: The orig

If what you're thinking is the so called Design by Contract (using preconditions and postconditions), there are plenty of languages with that feature: The original one is Eiffel, if I'm not mistaken: http://en.wikipedia.org/wiki/Precondition#Eiffel_example http://en.wikipedia.org/wiki/Precondition#Eiffel_example Clojure: http://blog.fogus.me/2009/12/21/clojures-pre-and-post/ http://blog.fogus.me/2009/12/21/clojures-pre-and-post/ Ada 2012, apparently: http://www.adacore.com/uploads/technical-papers/Ada2012_Rationale_Chp1_contracts_and_aspects.pdf http://www.adacore.com/uploads/technical-papers/Ada2012_Rati... The slightly lesser known Cobra: http://cobra-language.com/trac/cobra/wiki/Contracts http://cobra-language.com/trac/cobra/wiki/Contracts (of these, I only ever wrote Clojure, and I've used a couple preconditions in production, I definitely like the concept)