This is Haddock, a tool for automatically generating documentation from annotated Haskell source code. It is primary intended for documenting library interfaces, but it should be useful for any kind of Haskell code.

Haddock lets you write documentation annotations next to the definitions of functions and types in the source code, in a syntax that is easy on the eye when writing the source code (no heavyweight mark-up).

Haddock understands Haskell's module system, so you can structure your code however you like without worrying that internal structure will be exposed in the generated documentation. For example, it is common to implement a library in several modules, but define the external API by having a single module which re-exports parts of these implementation modules. Using Haddock, you can still write documentation annotations next to the actual definitions of the functions and types in the library, but the documentation annotations from the implementation will be propagated to the external API when the documentation is generated. Abstract types and classes are handled correctly. In fact, even without any documentation annotations, Haddock can generate useful documentation from your source code.