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Cabal is the standard package system for
Haskell_ software. It helps people to
configure, build and install Haskell software and to distribute it
easily to other users and developers.

There is a command line tool called ``cabal`` for working with Cabal
packages. It helps with installing existing packages and also helps
people developing their own packages. It can be used to work with local
packages or to install packages from online package archives, including
automatically installing dependencies. By default it is configured to
use Hackage_ which is Haskell's central
package archive that contains thousands of libraries and applications in
the Cabal package format.

What Cabal does
===============

Cabal is a package system for Haskell software. The point of a package
system is to enable software developers and users to easily distribute,
use and reuse software. A package system makes it easier for developers
to get their software into the hands of users. Equally importantly, it
makes it easier for software developers to be able to reuse software
components written by other developers.

Packaging systems deal with packages and with Cabal we call them *Cabal
packages*. The Cabal package is the unit of distribution. Every Cabal
package has a name and a version number which are used to identify the
package, e.g. ``filepath-1.0``.

Cabal packages can depend on other Cabal packages. There are tools to
enable automated package management. This means it is possible for
developers and users to install a package plus all of the other Cabal
packages that it depends on. It also means that it is practical to make
very modular systems using lots of packages that reuse code written by
many developers.

Cabal packages are source based and are typically (but not necessarily)
portable to many platforms and Haskell implementations. The Cabal
package format is designed to make it possible to translate into other
formats, including binary packages for various systems.

When distributed, Cabal packages use the standard compressed tarball
format, with the file extension ``.tar.gz``, e.g.
``filepath-1.0.tar.gz``.

Note that packages are not part of the Haskell language, rather they are
a feature provided by the combination of Cabal and GHC (and several
other Haskell implementations).

A tool for working with packages
--------------------------------

There is a command line tool, called "``cabal``", that users and
developers can use to build and install Cabal packages. It can be used
for both local packages and for packages available remotely over the
network. It can automatically install Cabal packages plus any other
Cabal packages they depend on.

Developers can use the tool with packages in local directories, e.g.

::

    $ cd foo/
    $ cabal install

While working on a package in a local directory, developers can run the
individual steps to configure and build, and also generate documentation
and run test suites and benchmarks.

It is also possible to install several local packages at once, e.g.

::

    $ cabal install foo/ bar/

Developers and users can use the tool to install packages from remote
Cabal package archives. By default, the ``cabal`` tool is configured to
use the central Haskell package archive called
Hackage_ but it is possible to use it
with any other suitable archive.

::

    $ cabal install xmonad

This will install the ``xmonad`` package plus all of its dependencies.

In addition to packages that have been published in an archive,
developers can install packages from local or remote tarball files, for
example

::

    $ cabal install foo-1.0.tar.gz
    $ cabal install http://example.com/foo-1.0.tar.gz

Cabal provides a number of ways for a user to customise how and where a
package is installed. They can decide where a package will be installed,
which Haskell implementation to use and whether to build optimised code
or build with the ability to profile code. It is not expected that users
will have to modify any of the information in the ``.cabal`` file.

Note that ``cabal`` is not the only tool for working with Cabal
packages. Due to the standardised format and a library for reading
``.cabal`` files, there are several other special-purpose tools.

What's in a package
-------------------

A Cabal package consists of:

-  Haskell software, including libraries, executables and tests
-  metadata about the package in a standard human and machine readable
   format (the "``.cabal``" file)
-  a standard interface to build the package (the "``Setup.hs``" file)

The ``.cabal`` file contains information about the package, supplied by
the package author. In particular it lists the other Cabal packages that
the package depends on.

For full details on what goes in the ``.cabal`` and ``Setup.hs`` files,
and for all the other features provided by the build system, see the
section on :doc:`How to package Haskell code <how-to-package-haskell-code>`.

Cabal featureset
----------------

Cabal and its associated tools and websites covers:

-  a software build system
-  software configuration
-  packaging for distribution
-  automated package management

   -  natively using the ``cabal`` command line tool; or
   -  by translation into native package formats such as RPM or deb

-  web and local Cabal package archives

   -  central Hackage website with 1000's of Cabal packages

Some parts of the system can be used without others. In particular the
built-in build system for simple packages is optional: it is possible to
use custom build systems.

Similar systems
---------------

The Cabal system is roughly comparable with the system of Python Eggs,
Ruby Gems or Perl distributions. Each system has a notion of
distributable packages, and has tools to manage the process of
distributing and installing packages.

Hackage is an online archive of Cabal packages, roughly comparable to
CPAN.

Cabal is often compared with autoconf and automake and there is some
overlap in functionality. The most obvious similarity is that the
command line interface for actually configuring and building packages
follows the same steps and has many of the same configuration
parameters.

::

    $ ./configure --prefix=...
    $ make
    $ make install

compared to

::

    $ cabal configure --prefix=...
    $ cabal build
    $ cabal install

Cabal's build system for simple packages is considerably less flexible
than make/automake, but has builtin knowledge of how to build Haskell
code and requires very little manual configuration. Cabal's simple build
system is also portable to Windows, without needing a Unix-like
environment such as cygwin/mingwin.

Compared to autoconf, Cabal takes a somewhat different approach to
package configuration. Cabal's approach is designed for automated
package management. Instead of having a configure script that tests for
whether dependencies are available, Cabal packages specify their
dependencies. There is some scope for optional and conditional
dependencies. By having package authors specify dependencies it makes it
possible for tools to install a package and all of its dependencies
automatically. It also makes it possible to translate (in a
mostly-automatically way) into another package format like RPM or deb
which also have automatic dependency resolution.


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