That's typical Haskell brain wash talk. Types can be a useful part of the documentation, but if your program is the least bit interesting, you will have parts

That's typical Haskell brain wash talk. Types can be a useful part of the documentation, but if your program is the least bit interesting, you will have parts that you should properly document. This is so much more true for libraries, especially those that adhere to a more mathematical style. If you write such a library, you should also write a paper explaining the library.