Author here. I do practice TDD, which is where I think this thought came from. Types aren't exactly like tests, but we can use them for some of the same goals,

Author here. I do practice TDD, which is where I think this thought came from. Types aren't exactly like tests, but we can use them for some of the same goals, such as constraining and influencing our implementations. The example here may also be an example of development by "wishful thinking" (writing our implementations as if we had certain abstractions available), that is talked about in the SICP lectures.