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 <https://web.archive.org/web/20180621053227/http://www.cse.unsw.edu.au/~chak/haskell/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 <https://www.haskell.org/ghcup>`__, for instance.

Haddock lives in the GHC repository, which you can consult at ``https://gitlab.haskell.org/ghc/ghc``.

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.

Acknowledgements
----------------

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

-  `IDoc <https://web.archive.org/web/20180621053227/http://www.cse.unsw.edu.au/~chak/haskell/idoc/>`__

-  `HDoc <https://mail.haskell.org/pipermail/haskelldoc/2001-April/000067.html>`__

-  `Doxygen <https://www.doxygen.nl/index.html>`__

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.
