"Imagine an approach to programming where you write down some description of what your code should do, then before running your code you run some automatic tool to see if the code matches the description. That's Test-driven development, you say! Actually, this is what you are doing when you use static types in most languages too. Types are a description of the code's inputs and outputs, and the check ensures that inputs and outputs match up and are used consistently. Modern type systems - such as in Haskell or above - are very flexible, and allow these descriptions to be quite detailed; plus they are not too obtrusive in use and often very helpful."