The point of typing is that types
are tests: they ensure logical consistency of your program, and they are checked by the compiler. This has a number of consequences.
- Some error conditions are inexpressible in your program. You cannot write classical tests to verify these, as trying to recreate the erroneous conditions will result in type errors.
- You cannot forget to "write a type" like you can forget to write a test. A type always covers all the guarantees it makes. This is especially useful when there are no tests, as you have at least some kind of helper available to you.
- Types are a form of documentation. Again, even a poorly documented, poorly named project still has types.
- Types do not make tests obsolete, but they greatly reduce the number of them you have to write and maintain.
As a result of this, most Haskell projects have abysmal test coverage compared to what you'd expect in Python or Java. Nobody would (or could!) write tests that all "instanceof" checks are covered. When you forget to implement a branch in your program, you'll get a compilation warning. When you feed a value to a well-written function, you're guaranteed to get a non-nonsense result out of it (instead of an exception). But still, you can make core changes to programs you do not understand at all - while understanding the type system in general - and have your changes work as expected.