I write Haskell in production.  One of the things we&#x27;ve built is a unit testing harness that denies application-level code any access to unrestricted IO.  If a proper mock is not in place, type checking fails before the tests even begin to run.<p>All tests are thus 100% reliable and parallelizable.<p>This capability of isolating and controlling side effects makes Haskell very uniquely well-suited to applications that need to withstand a lot of maintenance by a lot of people for a very long time.