Several documentation systems provided the inspiration for Haddock, most notably:
and probably several others I've forgotten.
Thanks to the following people for useful feedback, discussion, patches, and moral support: Simon Peyton Jones, Mark Shields, Manuel Chakravarty, Ross Patterson, Brett Letner, the members of <haskelldoc@haskell.org>, and everyone who contributed to the many libraries that Haddock makes use of.