[Jeff_Brown]

Not an API, but the documentation for Haskell libraries is unbelievably good. Here's an example[1]. This documentation is generated automatically from source code, optionally decorated with comments using a simple syntax called Haddock[2]. Any package, whether it's built into the Haskell Platform or contributed by a user, has the same kind of documentation. Even if the author includes no Haddock comments, the documentation will include the type signatures, the hyperlinking, the instance lists, and other goodies like the minimal set of functions needed to define an instance of a class.

[1] http://hackage.haskell.org/package/base-4.11.1.0/docs/Data-L... [2] https://www.haskell.org/haddock/