# Introduction

This is Haddock, a tool for automatically generating documentation from annotated Haskell source code. Haddock was designed with several goals in mind:

- When documenting APIs, it is desirable to keep the documentation close to the actual interface or implementation of the API, preferably in the same file, to reduce the risk that the two become out of sync. Haddock therefore lets you write the documentation for an entity (function, type, or class) next to the definition of the entity in the source code.

- There is a tremendous amount of useful API documentation that can be extracted from just the bare source code, including types of exported functions, definitions of data types and classes, and so on. Haddock can therefore generate documentation from a set of straight Haskell 98 modules, and the documentation will contain precisely the interface that is available to a programmer using those modules.

- Documentation annotations in the source code should be easy on the eye when editing the source code itself, so as not to obscure the code and to make reading and writing documentation annotations easy. The easier it is to write documentation, the more likely the programmer is to do it. Haddock therefore uses lightweight markup in its annotations, taking several ideas from IDoc. In fact, Haddock can understand IDoc-annotated source code.

- The documentation should not expose any of the structure of the implementation, or to put it another way, the implementer of the API should be free to structure the implementation however he or she wishes, without exposing any of that structure to the consumer. In practical terms, this means that while an API may internally consist of several Haskell modules, we often only want to expose a single module to the user of the interface, where this single module just re-exports the relevant parts of the implementation modules.

  Haddock therefore understands the Haskell module system and can generate documentation which hides not only non-exported entities from the interface, but also the internal module structure of the interface. A documentation annotation can still be placed next to the implementation, and it will be propagated to the external module in the generated documentation.

- Being able to move around the documentation by following hyperlinks is essential. Documentation generated by Haddock is therefore littered with hyperlinks: every type and class name is a link to the corresponding definition, and user-written documentation annotations can contain identifiers which are linked automatically when the documentation is generated.

- We might want documentation in multiple formats - online and printed, for example. Haddock comes with HTML, LaTeX, and Hoogle backends, and it is structured in such a way that adding new backends is straightforward.

## Obtaining Haddock

Haddock is distributed with GHC distributions, and will automatically be provided if you use ghcup, for instance.

Up-to-date sources can also be obtained from our public GitHub repository. The Haddock sources are at `https://github.com/haskell/haddock`.

## License

The following license covers this documentation, and the Haddock source code, except where otherwise indicated.

> Copyright (c) 2002-2010, Simon Marlow All rights reserved.

> Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

> 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

> 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

> THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS “AS IS” AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

## Contributors

A list of contributors to the project can be seen at `https://github.com/haskell/haddock/graphs/contributors`.

## Acknowledgements

Several documentation systems provided the inspiration for Haddock, most notably:

- IDoc

- HDoc

- Doxygen

and probably several others I’ve forgotten.

Thanks to the the members of haskelldoc@haskell.org, haddock@projects.haskell.org and everyone who contributed to the many libraries that Haddock makes use of.