> And in fact, its very existence defies another implicit principle of > mine, that is, the "principle of partial definition": Defining a new > type or instance can only break a previously typechecking program by > making it ambiguous. The idea behind that is that at some time you > may realize that oen of your types already obeys another type, and > declare that it conforms to that interface. But you don't go the > other way around, undeclaring that an interface holds, without your > program having been erroneous in the first place. Declaring that a > new interface holds (so long as it actually does) shouldn't break > anything that was already correct. > > The principle also has strong implications with library code: > including a new library but doing nothing with it shouldn't start > randomly breaking stuff. (Unless, of course, it breaks the rules and > does crazy stuff, in which case anything goes)
Is this better expressed as side-effect-free programming or loose coupling/tight cohesion? Rob
