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Lets assume we have created a project directory and already have a Haskell module or two.

Every project needs a name, we'll call this example "proglet".

$ cd proglet/
$ ls

It is assumed that (apart from external dependencies) all the files that make up a package live under a common project root directory. This simple example has all the project files in one directory, but most packages will use one or more subdirectories.

To turn this into a Cabal package we need two extra files in the project's root directory:

  • proglet.cabal: containing package metadata and build information.
  • Setup.hs: usually containing a few standardized lines of code, but can be customized if necessary.

We can create both files manually or we can use cabal init to create them for us.

Using "cabal init"

The cabal init command is interactive. It asks us a number of questions starting with the package name and version.

$ cabal init
Package name [default "proglet"]?
Package version [default "0.1"]?

It also asks questions about various other bits of package metadata. For a package that you never intend to distribute to others, these fields can be left blank.

One of the important questions is whether the package contains a library or an executable. Libraries are collections of Haskell modules that can be re-used by other Haskell libraries and programs, while executables are standalone programs.

What does the package build:
   1) Library
   2) Executable
Your choice?

For the moment these are the only choices. For more complex packages (e.g. a library and multiple executables or test suites) the .cabal file can be edited afterwards.

Finally, cabal init creates the initial proglet.cabal and Setup.hs files, and depending on your choice of license, a LICENSE file as well.

Generating LICENSE...
Generating Setup.hs...
Generating proglet.cabal...

You may want to edit the .cabal file and add a Description field.

As this stage the proglet.cabal is not quite complete and before you are able to build the package you will need to edit the file and add some build information about the library or executable.

Editing the .cabal file

Load up the .cabal file in a text editor. The first part of the .cabal file has the package metadata and towards the end of the file you will find the :pkg-section:`executable` or :pkg-section:`library` section.

You will see that the fields that have yet to be filled in are commented out. Cabal files use "--" Haskell-style comment syntax. (Note that comments are only allowed on lines on their own. Trailing comments on other lines are not allowed because they could be confused with program options.)

If you selected earlier to create a library package then your .cabal file will have a section that looks like this:

  exposed-modules:     Proglet
  -- other-modules:
  -- build-depends:

Alternatively, if you selected an executable then there will be a section like:

executable proglet
  -- main-is:
  -- other-modules:
  -- build-depends:

The build information fields listed (but commented out) are just the few most important and common fields. There are many others that are covered later in this chapter.

Most of the build information fields are the same between libraries and executables. The difference is that libraries have a number of "exposed" modules that make up the public interface of the library, while executables have a file containing a Main module.

The name of a library always matches the name of the package, so it is not specified in the library section. Executables often follow the name of the package too, but this is not required and the name is given explicitly.

Modules included in the package

For a library, cabal init looks in the project directory for files that look like Haskell modules and adds all the modules to the :pkg-field:`library:exposed-modules` field. For modules that do not form part of your package's public interface, you can move those modules to the :pkg-field:`other-modules` field. Either way, all modules in the library need to be listed.

For an executable, cabal init does not try to guess which file contains your program's Main module. You will need to fill in the :pkg-field:`executable:main-is` field with the file name of your program's Main module (including .hs or .lhs extension). Other modules included in the executable should be listed in the :pkg-field:`other-modules` field.

Modules imported from other packages

While your library or executable may include a number of modules, it almost certainly also imports a number of external modules from the standard libraries or other pre-packaged libraries. (These other libraries are of course just Cabal packages that contain a library.)

You have to list all of the library packages that your library or executable imports modules from. Or to put it another way: you have to list all the other packages that your package depends on.

For example, suppose the example Proglet module imports the module Data.Map. The Data.Map module comes from the containers package, so we must list it:

  exposed-modules:     Proglet
  build-depends:       containers, base == 4.*

In addition, almost every package also depends on the base library package because it exports the standard Prelude module plus other basic modules like Data.List.

You will notice that we have listed base == 4.*. This gives a constraint on the version of the base package that our package will work with. The most common kinds of constraints are:

  • pkgname >= n
  • pkgname ^>= n (since Cabal 2.0)
  • pkgname >= n && < m
  • pkgname == n.* (since Cabal 1.6)

The last is just shorthand, for example base == 4.* means exactly the same thing as base >= 4 && < 5. Please refer to the documentation on the :pkg-field:`build-depends` field for more information.

Building the package

For simple packages that's it! We can now try configuring and building the package:

$ cabal configure
$ cabal build

Assuming those two steps worked then you can also install the package:

$ cabal install

For libraries this makes them available for use in GHCi or to be used by other packages. For executables it installs the program so that you can run it (though you may first need to adjust your system's $PATH).

Next steps

What we have covered so far should be enough for very simple packages that you use on your own system.

The next few sections cover more details needed for more complex packages and details needed for distributing packages to other people.

The previous chapter covers building and installing packages -- your own packages or ones developed by other people.

Package concepts

Before diving into the details of writing packages it helps to understand a bit about packages in the Haskell world and the particular approach that Cabal takes.

The point of packages

Packages are a mechanism for organising and distributing code. Packages are particularly suited for "programming in the large", that is building big systems by using and re-using code written by different people at different times.

People organise code into packages based on functionality and dependencies. Social factors are also important: most packages have a single author, or a relatively small team of authors.

Packages are also used for distribution: the idea is that a package can be created in one place and be moved to a different computer and be usable in that different environment. There are a surprising number of details that have to be got right for this to work, and a good package system helps to simply this process and make it reliable.

Packages come in two main flavours: libraries of reusable code, and complete programs. Libraries present a code interface, an API, while programs can be run directly. In the Haskell world, library packages expose a set of Haskell modules as their public interface. Cabal packages can contain a library or executables or both.

Some programming languages have packages as a builtin language concept. For example in Java, a package provides a local namespace for types and other definitions. In the Haskell world, packages are not a part of the language itself. Haskell programs consist of a number of modules, and packages just provide a way to partition the modules into sets of related functionality. Thus the choice of module names in Haskell is still important, even when using packages.

Package names and versions

All packages have a name, e.g. "HUnit". Package names are assumed to be unique. Cabal package names may contain letters, numbers and hyphens, but not spaces and may also not contain a hyphened section consisting of only numbers. The namespace for Cabal packages is flat, not hierarchical.

Packages also have a version, e.g "1.1". This matches the typical way in which packages are developed. Strictly speaking, each version of a package is independent, but usually they are very similar. Cabal package versions follow the conventional numeric style, consisting of a sequence of digits such as "1.0.1" or "2.0". There are a range of common conventions for "versioning" packages, that is giving some meaning to the version number in terms of changes in the package, such as e.g. SemVer; however, for packages intended to be distributed via Hackage Haskell's `Package Versioning Policy`_ applies (see also the PVP/SemVer FAQ section).

The combination of package name and version is called the package ID and is written with a hyphen to separate the name and version, e.g. "HUnit-1.1".

For Cabal packages, the combination of the package name and version uniquely identifies each package. Or to put it another way: two packages with the same name and version are considered to be the same.

Strictly speaking, the package ID only identifies each Cabal source package; the same Cabal source package can be configured and built in different ways. There is a separate installed package ID that uniquely identifies each installed package instance. Most of the time however, users need not be aware of this detail.

Kinds of package: Cabal vs GHC vs system

It can be slightly confusing at first because there are various different notions of package floating around. Fortunately the details are not very complicated.

Cabal packages

Cabal packages are really source packages. That is they contain Haskell (and sometimes C) source code.

Cabal packages can be compiled to produce GHC packages. They can also be translated into operating system packages.

GHC packages

This is GHC's view on packages. GHC only cares about library packages, not executables. Library packages have to be registered with GHC for them to be available in GHCi or to be used when compiling other programs or packages.

The low-level tool ghc-pkg is used to register GHC packages and to get information on what packages are currently registered.

You never need to make GHC packages manually. When you build and install a Cabal package containing a library then it gets registered with GHC automatically.

Haskell implementations other than GHC have essentially the same concept of registered packages. For the most part, Cabal hides the slight differences.

Operating system packages

On operating systems like Linux and Mac OS X, the system has a specific notion of a package and there are tools for installing and managing packages.

The Cabal package format is designed to allow Cabal packages to be translated, mostly-automatically, into operating system packages. They are usually translated 1:1, that is a single Cabal package becomes a single system package.

It is also possible to make Windows installers from Cabal packages, though this is typically done for a program together with all of its library dependencies, rather than packaging each library separately.

Unit of distribution

The Cabal package is the unit of distribution. What this means is that each Cabal package can be distributed on its own in source or binary form. Of course there may dependencies between packages, but there is usually a degree of flexibility in which versions of packages can work together so distributing them independently makes sense.

It is perhaps easiest to see what being "the unit of distribution" means by contrast to an alternative approach. Many projects are made up of several interdependent packages and during development these might all be kept under one common directory tree and be built and tested together. When it comes to distribution however, rather than distributing them all together in a single tarball, it is required that they each be distributed independently in their own tarballs.

Cabal's approach is to say that if you can specify a dependency on a package then that package should be able to be distributed independently. Or to put it the other way round, if you want to distribute it as a single unit, then it should be a single package.

Explicit dependencies and automatic package management

Cabal takes the approach that all packages dependencies are specified explicitly and specified in a declarative way. The point is to enable automatic package management. This means tools like cabal can resolve dependencies and install a package plus all of its dependencies automatically. Alternatively, it is possible to mechanically (or mostly mechanically) translate Cabal packages into system packages and let the system package manager install dependencies automatically.

It is important to track dependencies accurately so that packages can reliably be moved from one system to another system and still be able to build it there. Cabal is therefore relatively strict about specifying dependencies. For example Cabal's default build system will not even let code build if it tries to import a module from a package that isn't listed in the .cabal file, even if that package is actually installed. This helps to ensure that there are no "untracked dependencies" that could cause the code to fail to build on some other system.

The explicit dependency approach is in contrast to the traditional "./configure" approach where instead of specifying dependencies declaratively, the ./configure script checks if the dependencies are present on the system. Some manual work is required to transform a ./configure based package into a Linux distribution package (or similar). This conversion work is usually done by people other than the package author(s). The practical effect of this is that only the most popular packages will benefit from automatic package management. Instead, Cabal forces the original author to specify the dependencies but the advantage is that every package can benefit from automatic package management.

The "./configure" approach tends to encourage packages that adapt themselves to the environment in which they are built, for example by disabling optional features so that they can continue to work when a particular dependency is not available. This approach makes sense in a world where installing additional dependencies is a tiresome manual process and so minimising dependencies is important. The automatic package management view is that packages should just declare what they need and the package manager will take responsibility for ensuring that all the dependencies are installed.

Sometimes of course optional features and optional dependencies do make sense. Cabal packages can have optional features and varying dependencies. These conditional dependencies are still specified in a declarative way however and remain compatible with automatic package management. The need to remain compatible with automatic package management means that Cabal's conditional dependencies system is a bit less flexible than with the "./configure" approach.


GNU autoconf places restrictions on paths, including the path that the user builds a package from. Package authors using build-type: configure should be aware of these restrictions; because users may be unexpectedly constrained and face mysterious errors, it is recommended that build-type: configure is only used where strictly necessary.


One of the purposes of Cabal is to make it easier to build packages on different platforms (operating systems and CPU architectures), with different compiler versions and indeed even with different Haskell implementations. (Yes, there are Haskell implementations other than GHC!)

Cabal provides abstractions of features present in different Haskell implementations and wherever possible it is best to take advantage of these to increase portability. Where necessary however it is possible to use specific features of specific implementations.

For example a package author can list in the package's .cabal what language extensions the code uses. This allows Cabal to figure out if the language extension is supported by the Haskell implementation that the user picks. Additionally, certain language extensions such as Template Haskell require special handling from the build system and by listing the extension it provides the build system with enough information to do the right thing.

Another similar example is linking with foreign libraries. Rather than specifying GHC flags directly, the package author can list the libraries that are needed and the build system will take care of using the right flags for the compiler. Additionally this makes it easier for tools to discover what system C libraries a package needs, which is useful for tracking dependencies on system libraries (e.g. when translating into Linux distribution packages).

In fact both of these examples fall into the category of explicitly specifying dependencies. Not all dependencies are other Cabal packages. Foreign libraries are clearly another kind of dependency. It's also possible to think of language extensions as dependencies: the package depends on a Haskell implementation that supports all those extensions.

Where compiler-specific options are needed however, there is an "escape hatch" available. The developer can specify implementation-specific options and more generally there is a configuration mechanism to customise many aspects of how a package is built depending on the Haskell implementation, the operating system, computer architecture and user-specified configuration flags.

Developing packages

The Cabal package is the unit of distribution. When installed, its purpose is to make available:

  • One or more Haskell programs.
  • At most one library, exposing a number of Haskell modules.

However having both a library and executables in a package does not work very well; if the executables depend on the library, they must explicitly list all the modules they directly or indirectly import from that library. Fortunately, starting with Cabal, executables can also declare the package that they are in as a dependency, and Cabal will treat them as if they were in another package that depended on the library.

Internally, the package may consist of much more than a bunch of Haskell modules: it may also have C source code and header files, source code meant for preprocessing, documentation, test cases, auxiliary tools etc.

A package is identified by a globally-unique package name, which consists of one or more alphanumeric words separated by hyphens. To avoid ambiguity, each of these words should contain at least one letter. Chaos will result if two distinct packages with the same name are installed on the same system. A particular version of the package is distinguished by a version number, consisting of a sequence of one or more integers separated by dots. These can be combined to form a single text string called the package ID, using a hyphen to separate the name from the version, e.g. "HUnit-1.1".


Packages are not part of the Haskell language; they simply populate the hierarchical space of module names. In GHC 6.6 and later a program may contain multiple modules with the same name if they come from separate packages; in all other current Haskell systems packages may not overlap in the modules they provide, including hidden modules.

Creating a package

Suppose you have a directory hierarchy containing the source files that make up your package. You will need to add two more files to the root directory of the package:

a Unicode UTF-8 text file containing a package description. For details of the syntax of this file, see the section on package descriptions.
a single-module Haskell program to perform various setup tasks (with the interface described in the section on :ref:`installing-packages`). This module should import only modules that will be present in all Haskell implementations, including modules of the Cabal library. The content of this file is determined by the :pkg-field:`build-type` setting in the .cabal file. In most cases it will be trivial, calling on the Cabal library to do most of the work.

Once you have these, you can create a source bundle of this directory for distribution. Building of the package is discussed in the section on :ref:`installing-packages`.

One of the purposes of Cabal is to make it easier to build a package with different Haskell implementations. So it provides abstractions of features present in different Haskell implementations and wherever possible it is best to take advantage of these to increase portability. Where necessary however it is possible to use specific features of specific implementations. For example one of the pieces of information a package author can put in the package's .cabal file is what language extensions the code uses. This is far preferable to specifying flags for a specific compiler as it allows Cabal to pick the right flags for the Haskell implementation that the user picks. It also allows Cabal to figure out if the language extension is even supported by the Haskell implementation that the user picks. Where compiler-specific options are needed however, there is an "escape hatch" available. The developer can specify implementation-specific options and more generally there is a configuration mechanism to customise many aspects of how a package is built depending on the Haskell implementation, the Operating system, computer architecture and user-specified configuration flags.

name:     Foo
version:  1.0

  build-depends:   base >= 4 && < 5
  exposed-modules: Foo
  extensions:      ForeignFunctionInterface
  ghc-options:     -Wall
  if os(windows)
    build-depends: Win32 >= 2.1 && < 2.6

Example: A package containing a simple library

The HUnit package contains a file HUnit.cabal containing:

name:           HUnit
version:        1.1.1
synopsis:       A unit testing framework for Haskell
category:       Testing
author:         Dean Herington
license:        BSD3
license-file:   LICENSE
cabal-version:  1.12
build-type:     Simple

  build-depends:      base >= 2 && < 4
  exposed-modules:    Test.HUnit.Base, Test.HUnit.Lang,
                      Test.HUnit.Terminal, Test.HUnit.Text, Test.HUnit
  default-extensions: CPP

and the following Setup.hs:

import Distribution.Simple
main = defaultMain

Example: A package containing executable programs

name:           TestPackage
version:        0.0
synopsis:       Small package with two programs
author:         Angela Author
license:        BSD3
build-type:     Simple
cabal-version:  >= 1.8

executable program1
  build-depends:  HUnit >= 1.1.1 && < 1.2
  main-is:        Main.hs
  hs-source-dirs: prog1

executable program2
  main-is:        Main.hs
  build-depends:  HUnit >= 1.1.1 && < 1.2
  hs-source-dirs: prog2
  other-modules:  Utils

with Setup.hs the same as above.

Example: A package containing a library and executable programs

name:            TestPackage
version:         0.0
synopsis:        Package with library and two programs
license:         BSD3
author:          Angela Author
build-type:      Simple
cabal-version:   >= 1.8

  build-depends:   HUnit >= 1.1.1 && < 1.2
  exposed-modules: A, B, C

executable program1
  main-is:         Main.hs
  hs-source-dirs:  prog1
  other-modules:   A, B

executable program2
  main-is:         Main.hs
  hs-source-dirs:  prog2
  other-modules:   A, C, Utils

with Setup.hs the same as above. Note that any library modules required (directly or indirectly) by an executable must be listed again.

The trivial setup script used in these examples uses the simple build infrastructure provided by the Cabal library (see Distribution.Simple). The simplicity lies in its interface rather that its implementation. It automatically handles preprocessing with standard preprocessors, and builds packages for all the Haskell implementations.

The simple build infrastructure can also handle packages where building is governed by system-dependent parameters, if you specify a little more (see the section on system-dependent parameters). A few packages require more elaborate solutions.

Package descriptions

The package description file must have a name ending in ".cabal". It must be a Unicode text file encoded using valid UTF-8. There must be exactly one such file in the directory. The first part of the name is usually the package name, and some of the tools that operate on Cabal packages require this; specifically, Hackage rejects packages which don't follow this rule.

In the package description file, lines whose first non-whitespace characters are "--" are treated as comments and ignored.

This file should contain of a number global property descriptions and several sections.

  • The package properties describe the package as a whole, such as name, license, author, etc.
  • Optionally, a number of configuration flags can be declared. These can be used to enable or disable certain features of a package. (see the section on configurations).
  • The (optional) library section specifies the library properties and relevant build information.
  • Following is an arbitrary number of executable sections which describe an executable program and relevant build information.

Each section consists of a number of property descriptions in the form of field/value pairs, with a syntax roughly like mail message headers.

  • Case is not significant in field names, but is significant in field values.
  • To continue a field value, indent the next line relative to the field name.
  • Field names may be indented, but all field values in the same section must use the same indentation.
  • Tabs are not allowed as indentation characters due to a missing standard interpretation of tab width.
  • To get a blank line in a field value, use an indented "."

The syntax of the value depends on the field. Field types include:

token, filename, directory
Either a sequence of one or more non-space non-comma characters, or a quoted string in Haskell 98 lexical syntax. The latter can be used for escaping whitespace, for example: ghc-options: -Wall "-with-rtsopts=-T -I1". Unless otherwise stated, relative filenames and directories are interpreted from the package root directory.
freeform, URL, address
An arbitrary, uninterpreted string.
A letter followed by zero or more alphanumerics or underscores.
A compiler flavor (one of: GHC, UHC or LHC) followed by a version range. For example, GHC ==6.10.3, or LHC >=0.6 && <0.8.

Modules and preprocessors

Haskell module names listed in the :pkg-field:`library:exposed-modules` and :pkg-field:`library:other-modules` fields may correspond to Haskell source files, i.e. with names ending in ".hs" or ".lhs", or to inputs for various Haskell preprocessors. The simple build infrastructure understands the extensions:

When building, Cabal will automatically run the appropriate preprocessor and compile the Haskell module it produces. For the c2hs and hsc2hs preprocessors, Cabal will also automatically add, compile and link any C sources generated by the preprocessor (produced by hsc2hs's #def feature or c2hs's auto-generated wrapper functions). Dependencies on pre-processors are specified via the :pkg-field:`build-tools` or :pkg-field:`build-tool-depends` fields.

Some fields take lists of values, which are optionally separated by commas, except for the :pkg-field:`build-depends` field, where the commas are mandatory.

Some fields are marked as required. All others are optional, and unless otherwise specified have empty default values.

Package properties

These fields may occur in the first top-level properties section and describe the package as a whole:

.. pkg-field:: name: package-name (required)

    The unique name of the package, without the version number.

    As pointed out in the section on `package descriptions`_, some
    tools require the package-name specified for this field to match
    the package description's file-name :file:`{package-name}.cabal`.

    Package names are case-sensitive and must match the regular expression
    (i.e. alphanumeric "words" separated by dashes; each alphanumeric
    word must contain at least one letter):

    Or, expressed in ABNF_:

    .. code-block:: abnf

        package-name      = package-name-part *("-" package-name-part)
        package-name-part = *DIGIT UALPHA *UALNUM

        UALPHA = ... ; set of alphabetic Unicode code-points

    .. note::

        Hackage restricts package names to the ASCII subset.

.. pkg-field:: version: numbers (required)

    The package version number, usually consisting of a sequence of
    natural numbers separated by dots, i.e. as the regular
    expression ``[0-9]+([.][0-9]+)*`` or expressed in ABNF_:

    .. code-block:: abnf

        package-version = 1*DIGIT *("." 1*DIGIT)

.. pkg-field:: cabal-version: x.y[.z]

    The version of the Cabal specification that this package
    description uses. The Cabal specification does slowly evolve (see
    also :ref:`spec-history`), introducing new features and
    occasionally changing the meaning of existing features. By
    specifying which version of the specification you are using it
    enables programs which process the package description to know
    what syntax to expect and what each part means.

    The version number you specify will affect both compatibility and
    behaviour. Most tools (including the Cabal library and the ``cabal``
    program) understand a range of versions of the Cabal specification.
    Older tools will of course only work with older versions of the
    Cabal specification that was known at the time. Most of the time,
    tools that are too old will recognise this fact and produce a
    suitable error message. Likewise, ``cabal check`` will tell you
    whether the version number is sufficiently high for the features
    you use in the package description.

    As for behaviour, new versions of the Cabal specification can change the
    meaning of existing syntax. This means if you want to take advantage
    of the new meaning or behaviour then you must specify the newer
    Cabal version. Tools are expected to use the meaning and behaviour
    appropriate to the version given in the package description.

    In particular, the syntax of package descriptions changed
    significantly with Cabal version 1.2 and the :pkg-field:`cabal-version`
    field is now required. Files written in the old syntax are still
    recognized, so if you require compatibility with very old Cabal
    versions then you may write your package description file using the
    old syntax. Please consult the user's guide of an older Cabal
    version for a description of that syntax.

    Starting with ``cabal-version: 2.2`` this field is only valid if
    fully contained in the very first line of a package description
    and ought to adhere to the ABNF_ grammar

    .. code-block:: abnf

        newstyle-spec-version-decl = "cabal-version" *WS ":" *WS newstyle-spec-version *WS

        newstyle-spec-version      = NUM "." NUM [ "." NUM ]

        NUM    = DIGIT0 / DIGITP 1*DIGIT0
        DIGIT0 = %x30-39
        DIGITP = %x31-39
        WS     = %20

    .. note::

        For package descriptions using a format prior to
        ``cabal-version: 1.12`` the legacy syntax resembling a version
        range syntax

        .. code-block:: cabal

            cabal-version: >= 1.10

        needs to be used.

        This legacy syntax is supported up until ``cabal-version: >=
        2.0`` it is however strongly recommended to avoid using the
        legacy syntax. See also :issue:`4899`.

.. pkg-field:: build-type: identifier

    :default: ``Custom`` or ``Simple``

    The type of build used by this package. Build types are the
    constructors of the
    `BuildType <../release/cabal-latest/doc/API/Cabal/Distribution-PackageDescription.html#t:BuildType>`__
    type. This field is optional and when missing, its default value
    is inferred according to the following rules:

     - When :pkg-field:`cabal-version` is set to ``2.2`` or higher,
       the default is ``Simple`` unless a :pkg-section:`custom-setup`
       exists, in which case the inferred default is ``Custom``.

     - For lower :pkg-field:`cabal-version` values, the default is
       ``Custom`` unconditionally.

    If the build type is anything other than ``Custom``, then the
    ``Setup.hs`` file *must* be exactly the standardized content
    discussed below. This is because in these cases, ``cabal`` will
    ignore the ``Setup.hs`` file completely, whereas other methods of
    package management, such as ``runhaskell Setup.hs [CMD]``, still
    rely on the ``Setup.hs`` file.

    For build type ``Simple``, the contents of ``Setup.hs`` must be:

    .. code-block:: haskell

        import Distribution.Simple
        main = defaultMain

    For build type ``Configure`` (see the section on `system-dependent
    parameters`_ below), the contents of
    ``Setup.hs`` must be:

    .. code-block:: haskell

        import Distribution.Simple
        main = defaultMainWithHooks autoconfUserHooks

    For build type ``Make`` (see the section on `more complex packages`_ below),
    the contents of ``Setup.hs`` must be:

    .. code-block:: haskell

        import Distribution.Make
        main = defaultMain

    For build type ``Custom``, the file ``Setup.hs`` can be customized,
    and will be used both by ``cabal`` and other tools.

    For most packages, the build type ``Simple`` is sufficient.

.. pkg-field:: license: SPDX expression

    :default: ``NONE``

    The type of license under which this package is distributed.

    Starting with ``cabal-version: 2.2`` the ``license`` field takes a
    (case-sensitive) SPDX expression such as

    .. code-block:: cabal

        license: Apache-2.0 AND (MIT OR GPL-2.0-or-later)

    See `SPDX IDs: How to use <>`__ for more
    examples of SPDX expressions.

    The version of the
    `list of SPDX license identifiers <>`__
    is a function of the :pkg-field:`cabal-version` value as defined
    in the following table:

    | Cabal specification      | SPDX license list  |
    | version                  | version            |
    |                          |                    |
    | ``cabal-version: 2.2``   | ``3.0 2017-12-28`` |
    | ``cabal-version: 2.4``   | ``3.2 2018-07-10`` |

    **Pre-SPDX Legacy Identifiers**

    The license identifier in the table below are defined for
    ``cabal-version: 2.0`` and previous versions of the Cabal

    | :pkg-field:`license`     | Note            |
    | identifier               |                 |
    |                          |                 |
    | ``GPL``                  |                 |
    | ``GPL-2``                |                 |
    | ``GPL-3``                |                 |
    | ``LGPL``                 |                 |
    | ``LGPL-2.1``             |                 |
    | ``LGPL-3``               |                 |
    | ``AGPL``                 | since 1.18      |
    | ``AGPL-3``               |                 |
    | ``BSD2``                 | since 1.20      |
    | ``BSD3``                 |                 |
    | ``MIT``                  |                 |
    | ``ISC``                  | since 1.22      |
    | ``MPL-2.0``              | since 1.20      |
    | ``Apache``               |                 |
    | ``Apache-2.0``           |                 |
    | ``PublicDomain``         |                 |
    | ``AllRightsReserved``    |                 |
    | ``OtherLicense``         |                 |

.. pkg-field:: license-file: filename

    See :pkg-field:`license-files`.

.. pkg-field:: license-files: filename list
    :since: 1.20

    The name of a file(s) containing the precise copyright license for
    this package. The license file(s) will be installed with the

    If you have multiple license files then use the :pkg-field:`license-files`
    field instead of (or in addition to) the :pkg-field:`license-file` field.

.. pkg-field:: copyright: freeform

    The content of a copyright notice, typically the name of the holder
    of the copyright on the package and the year(s) from which copyright
    is claimed. For example::

      copyright: (c) 2006-2007 Joe Bloggs

.. pkg-field:: author: freeform

    The original author of the package.

    Remember that ``.cabal`` files are Unicode, using the UTF-8

.. pkg-field:: maintainer: address

    The current maintainer or maintainers of the package. This is an
    e-mail address to which users should send bug reports, feature
    requests and patches.

.. pkg-field:: stability: freeform

    The stability level of the package, e.g. ``alpha``,
    ``experimental``, ``provisional``, ``stable``.

.. pkg-field:: homepage: URL

    The package homepage.

.. pkg-field:: bug-reports: URL

    The URL where users should direct bug reports. This would normally
    be either:

    -  A ``mailto:`` URL, e.g. for a person or a mailing list.

    -  An ``http:`` (or ``https:``) URL for an online bug tracking

    For example Cabal itself uses a web-based bug tracking system



.. pkg-field:: package-url: URL

    The location of a source bundle for the package. The distribution
    should be a Cabal package.

.. pkg-field:: synopsis: freeform

    A very short description of the package, for use in a table of
    packages. This is your headline, so keep it short (one line) but as
    informative as possible. Save space by not including the package
    name or saying it's written in Haskell.

.. pkg-field:: description: freeform

    Description of the package. This may be several paragraphs, and
    should be aimed at a Haskell programmer who has never heard of your
    package before.

    For library packages, this field is used as prologue text by
    :ref:`setup-haddock` and thus may contain the same markup as Haddock_
    documentation comments.

.. pkg-field:: category: freeform

    A classification category for future use by the package catalogue
    Hackage_. These categories have not
    yet been specified, but the upper levels of the module hierarchy
    make a good start.

.. pkg-field:: tested-with: compiler list

    A list of compilers and versions against which the package has been
    tested (or at least built).

.. pkg-field:: data-files: filename list

    A list of files to be installed for run-time use by the package.
    This is useful for packages that use a large amount of static data,
    such as tables of values or code templates. Cabal provides a way to
    `find these files at run-time <#accessing-data-files-from-package-code>`_.

    A limited form of ``*`` wildcards in file names, for example
    ``data-files: images/*.png`` matches all the ``.png`` files in the
    ``images`` directory. ``data-files: audio/**/*.mp3`` matches all
    the ``.mp3`` files in the ``audio`` directory, including

    The specific limitations of this wildcard syntax are

    - ``*`` wildcards are only allowed in place of the file name, not
      in the directory name or file extension. It must replace the
      whole file name (e.g., ``*.html`` is allowed, but
      ``chapter-*.html`` is not). If a wildcard is used, it must be
      used with an extension, so ``data-files: data/*`` is not

    - Prior to Cabal 2.4, when matching a wildcard plus extension, a
      file's full extension must match exactly, so ``*.gz`` matches
      ``foo.gz`` but not ``foo.tar.gz``. This restriction has been
      lifted when ``cabal-version: 2.4`` or greater so that ``*.gz``
      does match ``foo.tar.gz``

    - ``*`` wildcards will not match if the file name is empty (e.g.,
      ``*.html`` will not match ``foo/.html``).

    - ``**`` wildcards can only appear as the final path component
      before the file name (e.g., ``data/**/images/*.jpg`` is not
      allowed). If a ``**`` wildcard is used, then the file name must
      include a ``*`` wildcard (e.g., ``data/**/README.rst`` is not

    - A wildcard that does not match any files is an error.

    The reason for providing only a very limited form of wildcard is to
    concisely express the common case of a large number of related files
    of the same file type without making it too easy to accidentally
    include unwanted files.

    On efficiency: if you use ``**`` patterns, the directory tree will
    be walked starting with the parent directory of the ``**``. If
    that's the root of the project, this might include ``.git/``,
    ``dist-newstyle/``, or other large directories! To avoid this
    behaviour, put the files that wildcards will match against in
    their own folder.

    ``**`` wildcards are available starting in Cabal 2.4.

.. pkg-field:: data-dir: directory

    The directory where Cabal looks for data files to install, relative
    to the source directory. By default, Cabal will look in the source
    directory itself.

.. pkg-field:: extra-source-files: filename list

    A list of additional files to be included in source distributions
    built with :ref:`setup-sdist`. As with :pkg-field:`data-files` it can use
    a limited form of ``*`` wildcards in file names.

.. pkg-field:: extra-doc-files: filename list
    :since: 1.18

    A list of additional files to be included in source distributions,
    and also copied to the html directory when Haddock documentation is
    generated. As with :pkg-field:`data-files` it can use a limited form of
    ``*`` wildcards in file names.

.. pkg-field:: extra-tmp-files: filename list

    A list of additional files or directories to be removed by
    :ref:`setup-clean`. These  would typically be additional files created by
    additional hooks, such as the scheme described in the section on
    `system-dependent parameters`_


.. pkg-section:: library name
    :synopsis: Library build information.

    Build information for libraries.

    Currently, there can only be one publicly exposed library in a
    package, and its name is the same as package name set by global
    :pkg-field:`name` field. In this case, the ``name`` argument to
    the :pkg-section:`library` section must be omitted.

    Starting with Cabal 2.0, private internal sub-library components
    can be defined by using setting the ``name`` field to a name
    different from the current package's name; see section on
    :ref:`Internal Libraries <sublibs>` for more information.

The library section should contain the following fields:

.. pkg-field:: exposed-modules: identifier list

    :required: if this package contains a library

    A list of modules added by this package.

.. pkg-field:: virtual-modules: identifier list
    :since: 2.2

    A list of virtual modules provided by this package.  Virtual modules
    are modules without a source file.  See for example the ``GHC.Prim``
    module from the ``ghc-prim`` package.  Modules listed here will not be
    built, but still end up in the list of ``exposed-modules`` in the
    installed package info when the package is registered in the package

.. pkg-field:: exposed: boolean

    :default: ``True``

    Some Haskell compilers (notably GHC) support the notion of packages
    being "exposed" or "hidden" which means the modules they provide can
    be easily imported without always having to specify which package
    they come from. However this only works effectively if the modules
    provided by all exposed packages do not overlap (otherwise a module
    import would be ambiguous).

    Almost all new libraries use hierarchical module names that do not
    clash, so it is very uncommon to have to use this field. However it
    may be necessary to set ``exposed: False`` for some old libraries
    that use a flat module namespace or where it is known that the
    exposed modules would clash with other common modules.

.. pkg-field:: visibility: visibilty specifiers

    :since 3.0

    :default: ``private`` for internal libraries. Cannot be set for public library.

    Cabal recognizes ``public`` and ``private`` here...

    Multiple public libraries...

.. pkg-field:: reexported-modules: exportlist
    :since: 1.22

    Supported only in GHC 7.10 and later. A list of modules to
    *reexport* from this package. The syntax of this field is
    ``orig-pkg:Name as NewName`` to reexport module ``Name`` from
    ``orig-pkg`` with the new name ``NewName``. We also support
    abbreviated versions of the syntax: if you omit ``as NewName``,
    we'll reexport without renaming; if you omit ``orig-pkg``, then we
    will automatically figure out which package to reexport from, if
    it's unambiguous.

    Reexported modules are useful for compatibility shims when a package
    has been split into multiple packages, and they have the useful
    property that if a package provides a module, and another package
    reexports it under the same name, these are not considered a
    conflict (as would be the case with a stub module.) They can also be
    used to resolve name conflicts.

.. pkg-field:: signatures: signature list
    :since: 2.0

    Supported only in GHC 8.2 and later. A list of `module signatures <>`__ required by this package.

    Module signatures are part of the Backpack_ extension to
    the Haskell module system.

    Packages that do not export any modules and only export required signatures
    are called "signature-only packages", and their signatures are subjected to
    `signature thinning

The library section may also contain build information fields (see the section on build information).

Internal Libraries

Cabal 2.0 and later support "internal libraries", which are extra named libraries (as opposed to the usual unnamed library section). For example, suppose that your test suite needs access to some internal modules in your library, which you do not otherwise want to export. You could put these modules in an internal library, which the main library and the test suite :pkg-field:`build-depends` upon. Then your Cabal file might look something like this:

cabal-version:  2.0
name:           foo
license:        BSD3
build-type:     Simple

library foo-internal
    exposed-modules: Foo.Internal
    -- NOTE: no explicit constraints on base needed
    --       as they're inherited from the 'library' stanza
    build-depends: base

    exposed-modules: Foo.Public
    build-depends: foo-internal, base >= 4.3 && < 5

test-suite test-foo
    type:       exitcode-stdio-1.0
    main-is:    test-foo.hs
    -- NOTE: no constraints on 'foo-internal' as same-package
    --       dependencies implicitly refer to the same package instance
    build-depends: foo-internal, base

Internal libraries are also useful for packages that define multiple executables, but do not define a publically accessible library. Internal libraries are only visible internally in the package (so they can only be added to the :pkg-field:`build-depends` of same-package libraries, executables, test suites, etc.) Internal libraries locally shadow any packages which have the same name; consequently, don't name an internal library with the same name as an external dependency if you need to be able to refer to the external dependency in a :pkg-field:`build-depends` declaration.

Shadowing can be used to vendor an external dependency into a package and thus emulate private dependencies. Below is an example based on a real-world use case:

cabal-version: 2.2
name: haddock-library
version: 1.6.0

    , base         ^>=
    , bytestring   ^>=
    , containers   ^>= || ^>=
    , transformers ^>=

  hs-source-dirs:       src

  -- internal sub-lib
  build-depends:        attoparsec


library attoparsec
    , base         ^>=
    , bytestring   ^>=
    , deepseq      ^>=

  hs-source-dirs:       vendor/attoparsec-

  -- NB: haddock-library needs only small part of lib:attoparsec
  --     internally, so we only bundle that subset here


  ghc-options: -funbox-strict-fields -Wall -fwarn-tabs -O2

Opening an interpreter session

While developing a package, it is often useful to make its code available inside an interpreter session. This can be done with the repl command:

$ cabal repl

The name comes from the acronym REPL, which stands for "read-eval-print-loop". By default cabal repl loads the first component in a package. If the package contains several named components, the name can be given as an argument to repl. The name can be also optionally prefixed with the component's type for disambiguation purposes. Example:

$ cabal repl foo
$ cabal repl exe:foo
$ cabal repl test:bar
$ cabal repl bench:baz
Freezing dependency versions

If a package is built in several different environments, such as a development environment, a staging environment and a production environment, it may be necessary or desirable to ensure that the same dependency versions are selected in each environment. This can be done with the freeze command:

$ cabal freeze

The command writes the selected version for all dependencies to the cabal.config file. All environments which share this file will use the dependency versions specified in it.

Generating dependency version bounds

Cabal also has the ability to suggest dependency version bounds that conform to `Package Versioning Policy`_, which is a recommended versioning system for publicly released Cabal packages. This is done by running the gen-bounds command:

$ cabal gen-bounds

For example, given the following dependencies specified in :pkg-field:`build-depends`:

  foo == 0.5.2
  bar == 1.1

gen-bounds will suggest changing them to the following:

  foo >= 0.5.2 && < 0.6
  bar >= 1.1 && < 1.2
Listing outdated dependency version bounds

Manually updating dependency version bounds in a .cabal file or a freeze file can be tedious, especially when there's a lot of dependencies. The cabal outdated command is designed to help with that. It will print a list of packages for which there is a new version on Hackage that is outside the version bound specified in the build-depends field. The outdated command can also be configured to act on the freeze file (both old- and new-style) and ignore major (or all) version bumps on Hackage for a subset of dependencies.

The following flags are supported by the outdated command:

Read dependency version bounds from the freeze file (cabal.config) instead of the package description file ($PACKAGENAME.cabal). --v1-freeze-file is an alias for this flag starting in Cabal 2.4.
Read dependency version bounds from the new-style freeze file (by default, cabal.project.freeze) instead of the package description file. --v2-freeze-file is an alias for this flag starting in Cabal 2.4.
--project-file PROJECTFILE

Read dependendency version bounds from the new-style freeze file related to the named project file (i.e., $PROJECTFILE.freeze) instead of the package desctription file. If multiple --project-file flags are provided, only the final one is considered. This flag must only be passed in when --new-freeze-file is present.

Print only the names of outdated dependencies, one per line.
Exit with a non-zero exit code when there are outdated dependencies.
-q, --quiet
Don't print any output. Implies -v0 and --exit-code.
Don't warn about outdated dependency version bounds for the packages in this list.
Ignore major version bumps for these packages. E.g. if there's a version 2.0 of a package pkg on Hackage and the freeze file specifies the constraint pkg == 1.9, cabal outdated --freeze --minor=pkg will only consider the pkg outdated when there's a version of pkg on Hackage satisfying pkg > 1.9 && < 2.0. --minor can also be used without arguments, in that case major version bumps are ignored for all packages.


$ cd /some/package
$ cabal outdated
Outdated dependencies:
haskell-src-exts <1.17 (latest: 1.19.1)
language-javascript <0.6 (latest:
unix == (latest:

$ cabal outdated --simple-output

$ cabal outdated --ignore=haskell-src-exts
Outdated dependencies:
language-javascript <0.6 (latest:
unix == (latest:

$ cabal outdated --ignore=haskell-src-exts,language-javascript,unix
All dependencies are up to date.

$ cabal outdated --ignore=haskell-src-exts,language-javascript,unix -q
$ echo $?

$ cd /some/other/package
$ cabal outdated --freeze-file
Outdated dependencies:
HTTP ==4000.3.3 (latest: 4000.3.4)
HUnit == (latest:

$ cabal outdated --freeze-file --ignore=HTTP --minor=HUnit
Outdated dependencies:
HUnit == (latest:


.. pkg-section:: executable name
    :synopsis: Executable build info section.

    Executable sections (if present) describe executable programs contained
    in the package and must have an argument after the section label, which
    defines the name of the executable. This is a freeform argument but may
    not contain spaces.

The executable may be described using the following fields, as well as build information fields (see the section on build information).

.. pkg-field:: main-is: filename (required)

    The name of the ``.hs`` or ``.lhs`` file containing the ``Main``
    module. Note that it is the ``.hs`` filename that must be listed,
    even if that file is generated using a preprocessor. The source file
    must be relative to one of the directories listed in
    :pkg-field:`hs-source-dirs`. Further, while the name of the file may
    vary, the module itself must be named ``Main``.

    Starting with ``cabal-version: 1.18`` this field supports
    specifying a C, C++, or objC source file as the main entry point.

.. pkg-field:: scope: token
    :since: 2.0

    Whether the executable is ``public`` (default) or ``private``, i.e. meant to
    be run by other programs rather than the user. Private executables are
    installed into `$libexecdir/$libexecsubdir`.

Running executables

You can have Cabal build and run your executables by using the run command:


This command will configure, build and run the executable EXECUTABLE. The double dash separator is required to distinguish executable flags from run's own flags. If there is only one executable defined in the whole package, the executable's name can be omitted. See the output of cabal help run for a list of options you can pass to cabal run.

Test suites

.. pkg-section:: test-suite name
    :synopsis: Test suite build information.

    Test suite sections (if present) describe package test suites and must
    have an argument after the section label, which defines the name of the
    test suite. This is a freeform argument, but may not contain spaces. It
    should be unique among the names of the package's other test suites, the
    package's executables, and the package itself. Using test suite sections
    requires at least Cabal version 1.9.2.

The test suite may be described using the following fields, as well as build information fields (see the section on build information).

.. pkg-field:: type: interface (required)

    The interface type and version of the test suite. Cabal supports two
    test suite interfaces, called ``exitcode-stdio-1.0`` and
    ``detailed-0.9``. Each of these types may require or disallow other
    fields as described below.

Test suites using the exitcode-stdio-1.0 interface are executables that indicate test failure with a non-zero exit code when run; they may provide human-readable log information through the standard output and error channels. The exitcode-stdio-1.0 type requires the main-is field.

.. pkg-field:: main-is: filename
    :synopsis: Module containing tests main function.

    :required: ``exitcode-stdio-1.0``
    :disallowed: ``detailed-0.9``

    The name of the ``.hs`` or ``.lhs`` file containing the ``Main``
    module. Note that it is the ``.hs`` filename that must be listed,
    even if that file is generated using a preprocessor. The source file
    must be relative to one of the directories listed in
    :pkg-field:`hs-source-dirs`. This field is analogous to the ``main-is`` field
    of an executable section.

Test suites using the detailed-0.9 interface are modules exporting the symbol tests :: IO [Test]. The Test type is exported by the module Distribution.TestSuite provided by Cabal. For more details, see the example below.

The detailed-0.9 interface allows Cabal and other test agents to inspect a test suite's results case by case, producing detailed human- and machine-readable log files. The detailed-0.9 interface requires the :pkg-field:`test-module` field.

.. pkg-field:: test-module: identifier

    :required: ``detailed-0.9``
    :disallowed: ``exitcode-stdio-1.0``

    The module exporting the ``tests`` symbol.

Example: Package using exitcode-stdio-1.0 interface

The example package description and executable source file below demonstrate the use of the exitcode-stdio-1.0 interface.

Example: Package using detailed-0.9 interface

The example package description and test module source file below demonstrate the use of the detailed-0.9 interface. The test module also develops a simple implementation of the interface set by Distribution.TestSuite, but in actual usage the implementation would be provided by the library that provides the testing facility.

Running test suites

You can have Cabal run your test suites using its built-in test runner:

$ cabal configure --enable-tests
$ cabal build
$ cabal test

See the output of cabal help test for a list of options you can pass to cabal test.


.. pkg-section:: benchmark name
    :since: 1.9.2
    :synopsis: Benchmark build information.

    Benchmark sections (if present) describe benchmarks contained in the
    package and must have an argument after the section label, which defines
    the name of the benchmark. This is a freeform argument, but may not
    contain spaces. It should be unique among the names of the package's
    other benchmarks, the package's test suites, the package's executables,
    and the package itself. Using benchmark sections requires at least Cabal
    version 1.9.2.

The benchmark may be described using the following fields, as well as build information fields (see the section on build information).

.. pkg-field:: type: interface (required)

    The interface type and version of the benchmark. At the moment Cabal
    only support one benchmark interface, called ``exitcode-stdio-1.0``.

Benchmarks using the exitcode-stdio-1.0 interface are executables that indicate failure to run the benchmark with a non-zero exit code when run; they may provide human-readable information through the standard output and error channels.

.. pkg-field:: main-is: filename

    :required: ``exitcode-stdio-1.0``

    The name of the ``.hs`` or ``.lhs`` file containing the ``Main``
    module. Note that it is the ``.hs`` filename that must be listed,
    even if that file is generated using a preprocessor. The source file
    must be relative to one of the directories listed in
    :pkg-field:`hs-source-dirs`. This field is analogous to the ``main-is``
    field of an executable section. Further, while the name of the file may
    vary, the module itself must be named ``Main``.

Example: Package using exitcode-stdio-1.0 interface

The example package description and executable source file below demonstrate the use of the exitcode-stdio-1.0 interface.

Running benchmarks

You can have Cabal run your benchmark using its built-in benchmark runner:

$ cabal configure --enable-benchmarks
$ cabal build
$ cabal bench

See the output of cabal help bench for a list of options you can pass to cabal bench.

Foreign libraries

Foreign libraries are system libraries intended to be linked against programs written in C or other "foreign" languages. They come in two primary flavours: dynamic libraries (.so files on Linux, .dylib files on OSX, .dll files on Windows, etc.) are linked against executables when the executable is run (or even lazily during execution), while static libraries (.a files on Linux/OSX, .lib files on Windows) get linked against the executable at compile time.

Foreign libraries only work with GHC 7.8 and later.

A typical stanza for a foreign library looks like

foreign-library myforeignlib
  type:                native-shared
  lib-version-info:    6:3:2

  if os(Windows)
    options: standalone
    mod-def-file: MyForeignLib.def

  other-modules:       MyForeignLib.SomeModule
  build-depends:       base >=4.7 && <4.9
  hs-source-dirs:      src
  c-sources:           csrc/MyForeignLibWrapper.c
  default-language:    Haskell2010
.. pkg-section:: foreign-library name
    :since: 2.0
    :synopsis: Foriegn library build information.

    Build information for `foreign libraries`_.

.. pkg-field:: type: foreign library type

   Cabal recognizes ``native-static`` and ``native-shared`` here, although
   we currently only support building `native-shared` libraries.

.. pkg-field:: options: foreign library option list

   Options for building the foreign library, typically specific to the
   specified type of foreign library. Currently we only support
   ``standalone`` here. A standalone dynamic library is one that does not
   have any dependencies on other (Haskell) shared libraries; without
   the ``standalone`` option the generated library would have dependencies
   on the Haskell runtime library (``libHSrts``), the base library
   (``libHSbase``), etc. Currently, ``standalone`` *must* be used on Windows
   and *must not* be used on any other platform.

.. pkg-field:: mod-def-file: filename

   This option can only be used when creating dynamic Windows libraries
   (that is, when using ``native-shared`` and the ``os`` is ``Windows``). If
   used, it must be a path to a *module definition file*. The details of
   module definition files are beyond the scope of this document; see the
   `GHC <>`_
   manual for some details and some further pointers.

.. pkg-field:: lib-version-info: current:revision:age

   This field is currently only used on Linux.

   This field specifies a Libtool-style version-info field that sets
   an appropriate ABI version for the foreign library. Note that the
   three numbers specified in this field do not directly specify the
   actual ABI version: ``6:3:2`` results in library version ``4.2.3``.

   With this field set, the SONAME of the library is set, and symlinks
   are installed.

   How you should bump this field on an ABI change depends on the
   breakage you introduce:

   -  Programs using the previous version may use the new version as
      drop-in replacement, and programs using the new version can also
      work with the previous one. In other words, no recompiling nor
      relinking is needed. In this case, bump ``revision`` only, don't
      touch current nor age.
   -  Programs using the previous version may use the new version as
      drop-in replacement, but programs using the new version may use
      APIs not present in the previous one. In other words, a program
      linking against the new version may fail with "unresolved
      symbols" if linking against the old version at runtime: set
      revision to 0, bump current and age.
   -  Programs may need to be changed, recompiled, and relinked in
      order to use the new version. Bump current, set revision and age
      to 0.

   Also refer to the Libtool documentation on the version-info field.

.. pkg-field:: lib-version-linux: version

   This field is only used on Linux.

   Specifies the library ABI version directly for foreign libraries
   built on Linux: so specifying ``4.2.3`` causes a library
   ```` to be built with SONAME ````, and
   appropriate symlinks ```` and ```` to be

Note that typically foreign libraries should export a way to initialize and shutdown the Haskell runtime. In the example above, this is done by the csrc/MyForeignLibWrapper.c file, which might look something like

#include <stdlib.h>
#include "HsFFI.h"

HsBool myForeignLibInit(void){
  int argc = 2;
  char *argv[] = { "+RTS", "-A32m", NULL };
  char **pargv = argv;

  // Initialize Haskell runtime
  hs_init(&argc, &pargv);

  // do any other initialization here and
  // return false if there was a problem
  return HS_BOOL_TRUE;

void myForeignLibExit(void){

With modern ghc regular libraries are installed in directories that contain package keys. This isn't usually a problem because the package gets registered in ghc's package DB and so we can figure out what the location of the library is. Foreign libraries however don't get registered, which means that we'd have to have a way of finding out where a platform library got installed (other than by searching the lib/ directory). Instead, we install foreign libraries in ~/.cabal/lib, much like we install executables in ~/.cabal/bin.

Build information

.. pkg-section:: None

The following fields may be optionally present in a library, executable, test suite or benchmark section, and give information for the building of the corresponding library or executable. See also the sections on system-dependent parameters and configurations for a way to supply system-dependent values for these fields.

.. pkg-field:: build-depends: library list

    Declares the *library* dependencies required to build the current
    package component; see :pkg-field:`build-tool-depends` for
    declaring build-time *tool* dependencies. External library
    dependencies should be annotated with a version constraint.

    **Library Names**

    External libraries are identified by the package's name they're
    provided by (currently a package can only publically expose its
    main library compeonent; in future, packages with multiple exposed
    public library components will be supported and a syntax for
    referring to public sub-libraries will be provided).

    In order to specify an intra-package dependency on an internal
    library component you can use the unqualified name of the
    component library component. Note that locally defined sub-library
    names shadow external package names of the same name. See section on
    :ref:`Internal Libraries <sublibs>` for examples and more information.

    **Version Constraints**

    Version constraints use the operators ``==, >=, >, <, <=`` and a
    version number. Multiple constraints can be combined using ``&&`` or
    ``||``. If no version constraint is specified, any version is
    assumed to be acceptable. For example:


            base >= 2,
            foo >= 1.2.3 && < 1.3,

    Dependencies like ``foo >= 1.2.3 && < 1.3`` turn out to be very
    common because it is recommended practise for package versions to
    correspond to API versions (see PVP_).

    Since Cabal 1.6, there is a special wildcard syntax to help with
    such ranges


        build-depends: foo ==1.2.*

    It is only syntactic sugar. It is exactly equivalent to
    ``foo >= 1.2 && < 1.3``.

    .. Warning::

       A potential pitfall of the wildcard syntax is that the
       constraint ``nats == 1.0.*`` doesn't match the release
       ``nats-1`` because the version ``1`` is lexicographically less
       than ``1.0``. This is not an issue with the caret-operator
       ``^>=`` described below.

    Starting with Cabal 2.0, there's a new version operator to express
    PVP_-style major upper bounds conveniently, and is inspired by similar
    syntactic sugar found in other language ecosystems where it's often
    called the "Caret" operator:


          foo ^>=,
          bar ^>= 1

    This allows to assert the positive knowledge that this package is
    *known* to be semantically compatible with the releases
    ``foo-`` and ``bar-1`` respectively. The information
    encoded via such ``^>=``-assertions is used by the cabal solver to
    infer version constraints describing semantically compatible
    version ranges according to the PVP_ contract (see below).

    Another way to say this is that ``foo < 1.3`` expresses *negative*
    information, i.e. "``foo-1.3`` or ``foo-1.4.2`` will *not* be
    compatible"; whereas ``foo ^>=`` asserts the *positive*
    information that "``foo-`` is *known* to be compatible" and (in
    the absence of additional information) according to the PVP_
    contract we can (positively) infer right away that all versions
    satisfying ``foo >= && < 1.3`` will be compatible as well.

    .. Note::

       More generally, the PVP_ contract implies that we can safely
       relax the lower bound to ``>= 1.2``, because if we know that
       ``foo-`` is semantically compatible, then so is
       ``foo-1.2`` (if it typechecks). But we'd need to perform
       additional static analysis (i.e. perform typechecking) in order
       to know if our package in the role of an API consumer will
       successfully typecheck against the dependency ``foo-1.2``.  But
       since we cannot do this analysis during constraint solving and
       to keep things simple, we pragmatically use ``foo >=``
       as the initially inferred approximation for the lower bound
       resulting from the assertion ``foo ^>=``. If further
       evidence becomes available that e.g. ``foo-1.2`` typechecks,
       one can simply revise the dependency specification to include
       the assertion ``foo ^>= 1.2``.

    The subtle but important difference in signaling allows tooling to
    treat explicitly expressed ``<``-style constraints and inferred
    (``^>=``-style) upper bounds differently.  For instance,
    :option:`--allow-newer`'s ``^``-modifier allows to relax only
    ``^>=``-style bounds while leaving explicitly stated
    ``<``-constraints unaffected.

    Ignoring the signaling intent, the default syntactic desugaring rules are

    - ``^>= x`` == ``>= x && < x.1``
    - ``^>= x.y`` == ``>= x.y && < x.(y+1)``
    - ``^>= x.y.z`` == ``>= x.y.z && < x.(y+1)``
    - ``^>= x.y.z.u`` == ``>= x.y.z.u && < x.(y+1)``
    - etc.

    .. Note::

       One might expected the desugaring to truncate all version
       components below (and including) the patch-level, i.e.
       ``^>= x.y.z.u`` == ``>= x.y.z && < x.(y+1)``,
       as the major and minor version components alone are supposed to
       uniquely identify the API according to the PVP_.  However, by
       designing ``^>=`` to be closer to the ``>=`` operator, we avoid
       the potentially confusing effect of ``^>=`` being more liberal
       than ``>=`` in the presence of patch-level versions.

    Consequently, the example declaration above is equivalent to


          foo >= && < 1.3,
          bar >= 1 && < 1.1

    .. Note::

       Prior to Cabal 1.8, ``build-depends`` specified in each
       section were global to all sections. This was unintentional, but
       some packages were written to depend on it, so if you need your
       :pkg-field:`build-depends` to be local to each section, you must specify
       at least ``Cabal-Version: >= 1.8`` in your ``.cabal`` file.

    .. Note::

       Cabal 1.20 experimentally supported module thinning and
       renaming in ``build-depends``; however, this support has since been
       removed and should not be used.

.. pkg-field:: other-modules: identifier list

    A list of modules used by the component but not exposed to users.
    For a library component, these would be hidden modules of the
    library. For an executable, these would be auxiliary modules to be
    linked with the file named in the ``main-is`` field.

    .. Note::

       Every module in the package *must* be listed in one of
       :pkg-field:`other-modules`, :pkg-field:`library:exposed-modules` or
       :pkg-field:`executable:main-is` fields.

.. pkg-field:: hs-source-dirs: directory list

    :default: ``.``

    Root directories for the module hierarchy.

    For backwards compatibility, the old variant ``hs-source-dir`` is
    also recognized.

.. pkg-field:: default-extensions: identifier list

    A list of Haskell extensions used by every module. These determine
    corresponding compiler options enabled for all files. Extension
    names are the constructors of the
    `Extension <../release/cabal-latest/doc/API/Cabal/Language-Haskell-Extension.html#t:Extension>`__
    type. For example, ``CPP`` specifies that Haskell source files are
    to be preprocessed with a C preprocessor.

.. pkg-field:: other-extensions: identifier list

    A list of Haskell extensions used by some (but not necessarily all)
    modules. From GHC version 6.6 onward, these may be specified by
    placing a ``LANGUAGE`` pragma in the source files affected e.g.

    .. code-block:: haskell

        {-# LANGUAGE CPP, MultiParamTypeClasses #-}

    In Cabal-1.24 the dependency solver will use this and
    :pkg-field:`default-extensions` information. Cabal prior to 1.24 will abort
    compilation if the current compiler doesn't provide the extensions.

    If you use some extensions conditionally, using CPP or conditional
    module lists, it is good to replicate the condition in
    :pkg-field:`other-extensions` declarations:


        other-extensions: CPP
        if impl(ghc >= 7.5)
          other-extensions: PolyKinds

    You could also omit the conditionally used extensions, as they are
    for information only, but it is recommended to replicate them in
    :pkg-field:`other-extensions` declarations.

.. pkg-field:: extensions: identifier list
   :deprecated: 1.12
   :removed: 3.0

   Deprecated in favor of :pkg-field:`default-extensions`.

.. pkg-field:: build-tool-depends: package:executable list
    :since: 2.0

    A list of Haskell executables needed to build this component. Executables are provided
    during the whole duration of the component, so this field can be used for executables
    needed during :pkg-section:`test-suite` as well.

    Each is specified by the package containing the executable and the name of the
    executable itself, separated by a colon, and optionally followed by a version bound.

    All executables defined in the given Cabal file are termed as *internal* dependencies
    as opposed to the rest which are *external* dependencies.

    Each of the two is handled differently:

    1. External dependencies can (and should) contain a version bound like conventional
       :pkg-field:`build-depends` dependencies.
    2. Internal depenedencies should not contain a version bound, as they will be always
       resolved within the same configuration of the package in the build plan.
       Specifically, version bounds that include the package's version will be warned for
       being extraneous, and version bounds that exclude the package's version will raise
       an error for being impossible to follow.

    For example (1) using a test-suite to make sure Haskell snippets are tested using
    `markdown-unlit <>`__:


        build-tool-depends: markdown-unlit:markdown-unlit >= 0.5.0 && < 0.6

    For example (2) using a test-suite to test executable behaviour in the same package:


        build-tool-depends: mypackage:executable

    Cabal tries to make sure that all specified programs are atomically built and prepended
    on the ``$PATH`` shell variable before building the component in question, but can only do
    so for Nix-style builds. Specifically:

    a) For Nix-style local builds, both internal and external dependencies.
    b) For old-style builds, only for internal dependencies [#old-style-build-tool-depends]_.
       It's up to the user to provide needed executables in this case under `$PATH.`

    .. note::

      :pkg-field:`build-tool-depends` was added in Cabal 2.0, and it will
      be ignored (with a warning) with old versions of Cabal.  See
      :pkg-field:`build-tools` for more information about backwards

.. pkg-field:: build-tools: program list
    :deprecated: 2.0
    :removed: 3.0

    Deprecated in favor of :pkg-field:`build-tool-depends`, but :ref:`see below for backwards compatibility information <buildtoolsbc>`.

    A list of Haskell programs needed to build this component.
    Each may be followed by an optional version bound.
    Confusingly, each program in the list either refer to one of three things:

      1. Another executables in the same package (supported since Cabal 1.12)

      2. Tool name contained in Cabal's :ref:`hard-coded set of common tools <buildtoolsmap>`

      3. A pre-built executable that should already be on the ``PATH``
         (supported since Cabal 2.0)

    These cases are listed in order of priority:
    an executable in the package will override any of the hard-coded packages with the same name,
    and a hard-coded package will override any executable on the ``PATH``.

    In the first two cases, the list entry is desugared into a :pkg-field:`build-tool-depends` entry.
    In the first case, the entry is desugared into a :pkg-field:`build-tool-depends` entry by prefixing with ``$pkg:``.
    In the second case, it is desugared by looking up the package and executable name in a hard-coded table.
    In either case, the optional version bound is passed through unchanged.
    Refer to the documentation for :pkg-field:`build-tool-depends` to understand the desugared field's meaning, along with restrictions on version bounds.

    .. _buildtoolsbc:

    **Backward Compatiblity**

    Although this field is deprecated in favor of :pkg-field:`build-tool-depends`, there are some situations where you may prefer to use :pkg-field:`build-tools` in cases (1) and (2), as it is supported by more versions of Cabal.
    In case (3), :pkg-field:`build-tool-depends` is better for backwards-compatibility, as it will be ignored by old versions of Cabal; if you add the executable to :pkg-field:`build-tools`, a setup script built against old Cabal will choke.
    If an old version of Cabal is used, an end-user will have to manually arrange for the requested executable to be in your ``PATH``.

    .. _buildtoolsmap:

    **Set of Known Tool Names**

    Identifiers specified in :pkg-field:`build-tools` are desugared into their respective equivalent :pkg-field:`build-tool-depends` form according to the table below. Consequently, a legacy specification such as::

        build-tools: alex >= 3.2.1 && < 3.3, happy >= 1.19.5 && < 1.20

    is simply desugared into the equivalent specification::

        build-tool-depends: alex:alex >= 3.2.1 && < 3.3, happy:happy >= 1.19.5 && < 1.20

    | :pkg-field:`build-tools` | desugared                         | Note            |
    | identifier               | :pkg-field:`build-tool-depends`   |                 |
    |                          | identifier                        |                 |
    | ``alex``                 | ``alex:alex``                     |                 |
    | ``c2hs``                 | ``c2hs:c2hs``                     |                 |
    | ``cpphs``                | ``cpphs:cpphs``                   |                 |
    | ``greencard``            | ``greencard:greencard``           |                 |
    | ``haddock``              | ``haddock:haddock``               |                 |
    | ``happy``                | ``happy:happy``                   |                 |
    | ``hsc2hs``               | ``hsc2hs:hsc2hs``                 |                 |
    | ``hscolour``             | ``hscolour:hscolour``             |                 |
    | ``hspec-discover``       | ``hspec-discover:hspec-discover`` | since Cabal 2.0 |

    This built-in set can be programmatically extended via ``Custom`` setup scripts; this, however, is of limited use since the Cabal solver cannot access information injected by ``Custom`` setup scripts.

.. pkg-field:: buildable: boolean

    :default: ``True``

    Is the component buildable? Like some of the other fields below,
    this field is more useful with the slightly more elaborate form of
    the simple build infrastructure described in the section on
    `system-dependent parameters`_.

.. pkg-field:: ghc-options: token list

    Additional options for GHC. You can often achieve the same effect
    using the :pkg-field:`extensions` field, which is preferred.

    Options required only by one module may be specified by placing an
    ``OPTIONS_GHC`` pragma in the source file affected.

    As with many other fields, whitespace can be escaped by using
    Haskell string syntax. Example:
    ``ghc-options: -Wcompat "-with-rtsopts=-T -I1" -Wall``.

.. pkg-field:: ghc-prof-options: token list

    Additional options for GHC when the package is built with profiling

    Note that as of Cabal-1.24, the default profiling detail level
    defaults to ``exported-functions`` for libraries and
    ``toplevel-functions`` for executables. For GHC these correspond to
    the flags ``-fprof-auto-exported`` and ``-fprof-auto-top``. Prior to
    Cabal-1.24 the level defaulted to ``none``. These levels can be
    adjusted by the person building the package with the
    ``--profiling-detail`` and ``--library-profiling-detail`` flags.

    It is typically better for the person building the package to pick
    the profiling detail level rather than for the package author. So
    unless you have special needs it is probably better not to specify
    any of the GHC ``-fprof-auto*`` flags here. However if you wish to
    override the profiling detail level, you can do so using the
    :pkg-field:`ghc-prof-options` field: use ``-fno-prof-auto`` or one of the
    other ``-fprof-auto*`` flags.

.. pkg-field:: ghc-shared-options: token list

    Additional options for GHC when the package is built as shared
    library. The options specified via this field are combined with the
    ones specified via :pkg-field:`ghc-options`, and are passed to GHC during
    both the compile and link phases.

.. pkg-field:: includes: filename list

    A list of header files to be included in any compilations via C.
    This field applies to both header files that are already installed
    on the system and to those coming with the package to be installed.
    The former files should be found in absolute paths, while the latter
    files should be found in paths relative to the top of the source
    tree or relative to one of the directories listed in

    These files typically contain function prototypes for foreign
    imports used by the package. This is in contrast to
    :pkg-field:`install-includes`, which lists header files that are intended
    to be exposed to other packages that transitively depend on this

.. pkg-field:: install-includes: filename list

    A list of header files from this package to be installed into
    ``$libdir/includes`` when the package is installed. Files listed in
    :pkg-field:`install-includes` should be found in relative to the top of the
    source tree or relative to one of the directories listed in

    :pkg-field:`install-includes` is typically used to name header files that
    contain prototypes for foreign imports used in Haskell code in this
    package, for which the C implementations are also provided with the
    package. For example, here is a ``.cabal`` file for a hypothetical
    ``bindings-clib`` package that bundles the C source code for ``clib``::

        include-dirs:     cbits
        c-sources:        clib.c
        install-includes: clib.h

    Now any package that depends (directly or transitively) on the
    ``bindings-clib`` library can use ``clib.h``.

    Note that in order for files listed in :pkg-field:`install-includes` to be
    usable when compiling the package itself, they need to be listed in
    the :pkg-field:`includes` field as well.

.. pkg-field:: include-dirs: directory list

    A list of directories to search for header files, when preprocessing
    with ``c2hs``, ``hsc2hs``, ``cpphs`` or the C preprocessor, and also
    when compiling via C. Directories can be absolute paths (e.g., for
    system directories) or paths that are relative to the top of the
    source tree. Cabal looks in these directories when attempting to
    locate files listed in :pkg-field:`includes` and

.. pkg-field:: c-sources: filename list

    A list of C source files to be compiled and linked with the Haskell

.. pkg-field:: cxx-sources: filename list
    :since: 2.2

    A list of C++ source files to be compiled and linked with the Haskell
    files. Useful for segregating C and C++ sources when supplying different
    command-line arguments to the compiler via the :pkg-field:`cc-options`
    and the :pkg-field:`cxx-options` fields. The files listed in the
    :pkg-field:`cxx-sources` can reference files listed in the
    :pkg-field:`c-sources` field and vice-versa. The object files will be linked

.. pkg-field:: asm-sources: filename list
    :since: 2.2

    A list of assembly source files to be compiled and linked with the
    Haskell files.

.. pkg-field:: cmm-sources: filename list

    A list of C-- source files to be compiled and linked with the Haskell

.. pkg-field:: js-sources: filename list

    A list of JavaScript source files to be linked with the Haskell
    files (only for JavaScript targets).

.. pkg-field:: extra-libraries: token list

    A list of extra libraries to link with.

.. pkg-field:: extra-ghci-libraries: token list

    A list of extra libraries to be used instead of 'extra-libraries'
    when the package is loaded with GHCi.

.. pkg-field:: extra-bundled-libraries: token list
   :since: 2.2

   A list of libraries that are supposed to be copied from the build
   directory alongside the produced Haskell libraries.  Note that you
   are under the obligation to produce those libraries in the build
   directory (e.g. via a custom setup).  Libraries listed here will
   be included when ``copy``-ing packages and be listed in the
   ``hs-libraries`` of the package configuration in the package database.

.. pkg-field:: extra-lib-dirs: directory list

    A list of directories to search for libraries.

.. pkg-field:: cc-options: token list

    Command-line arguments to be passed to the C compiler. Since the
    arguments are compiler-dependent, this field is more useful with the
    setup described in the section on `system-dependent parameters`_.

.. pkg-field:: cpp-options: token list

    Command-line arguments for pre-processing Haskell code. Applies to
    Haskell source and other pre-processed Haskell source like .hsc
    .chs. Does not apply to C code, that's what cc-options is for.

.. pkg-field:: cxx-options: token list
    :since: 2.2

    Command-line arguments to be passed to the compiler when compiling
    C++ code. The C++ sources to which these command-line arguments
    should be applied can be specified with the :pkg-field:`cxx-sources`
    field. Command-line options for C and C++ can be passed separately to
    the compiler when compiling both C and C++ sources by segregating the C
    and C++ sources with the :pkg-field:`c-sources` and
    :pkg-field:`cxx-sources` fields respectively, and providing different
    command-line arguments with the :pkg-field:`cc-options` and the
    :pkg-field:`cxx-options` fields.

.. pkg-field:: asm-options: token list
    :since: 2.2

    Command-line arguments to be passed to the assembler when compiling
    assembler code. See also :pkg-field:`asm-sources`.

.. pkg-field:: ld-options: token list

    Command-line arguments to be passed to the linker. Since the
    arguments are compiler-dependent, this field is more useful with the
    setup described in the section on `system-dependent parameters`_.

.. pkg-field:: pkgconfig-depends: package list

    A list of
    `pkg-config <>`__
    packages, needed to build this package. They can be annotated with
    versions, e.g. ``gtk+-2.0 >= 2.10, cairo >= 1.0``. If no version
    constraint is specified, any version is assumed to be acceptable.
    Cabal uses ``pkg-config`` to find if the packages are available on
    the system and to find the extra compilation and linker options
    needed to use the packages.

    If you need to bind to a C library that supports ``pkg-config`` then
    it is much preferable to use this field rather than hard code options
    into the other fields. ``pkg-config --list-all`` will show you all
    supported libraries. Depending on your system you may need to adjust

.. pkg-field:: frameworks: token list

    On Darwin/MacOS X, a list of frameworks to link to. See Apple's
    developer documentation for more details on frameworks. This entry
    is ignored on all other platforms.

.. pkg-field:: extra-frameworks-dirs: directory list
    :since: 1.24

    On Darwin/MacOS X, a list of directories to search for frameworks.
    This entry is ignored on all other platforms.

.. pkg-field:: mixins: mixin list
    :since: 2.0

    Supported only in GHC 8.2 and later. A list of packages mentioned in the
    :pkg-field:`build-depends` field, each optionally accompanied by a list of
    module and module signature renamings.

    The simplest mixin syntax is simply the name of a package mentioned in the
    :pkg-field:`build-depends` field. For example:


            foo ^>= 1.2.3

    But this doesn't have any effect. More interesting is to use the mixin
    entry to rename one or more modules from the package, like this:


            foo (Foo.Bar as AnotherFoo.Bar, Foo.Baz as AnotherFoo.Baz)

    Note that renaming a module like this will hide all the modules
    that are not explicitly named.

    Modules can also be hidden:


            foo hiding (Foo.Bar)

    Hiding modules exposes everything that is not explicitly hidden.

    .. Note::

       The current version of Cabal suffers from an infelicity in how the
       entries of :pkg-field:`mixins` are parsed: an entry will fail to parse
       if the provided renaming clause has whitespace after the opening
       parenthesis. This will be fixed in future versions of Cabal.

       See issues :issue:`5150`, :issue:`4864`, and :issue:`5293`.

    There can be multiple mixin entries for a given package, in effect creating
    multiple copies of the dependency:


            foo (Foo.Bar as AnotherFoo.Bar, Foo.Baz as AnotherFoo.Baz),
            foo (Foo.Bar as YetAnotherFoo.Bar)

    The ``requires`` clause is used to rename the module signatures required by
    a package:


            foo (Foo.Bar as AnotherFoo.Bar) requires (Foo.SomeSig as AnotherFoo.SomeSig)

    Signature-only packages don't have any modules, so only the signatures can
    be renamed, with the following syntax:


            sigonly requires (SigOnly.SomeSig as AnotherSigOnly.SomeSig)

    See the :pkg-field:`library:signatures` field for more details.

    Mixin packages are part of the Backpack_ extension to the
    Haskell module system.

    The matching of the module signatures required by a
    :pkg-field:`build-depends` dependency with the implementation modules
    present in another dependency is triggered by a coincidence of names. When
    the names of the signature and of the implementation are already the same,
    the matching is automatic. But when the names don't coincide, or we want to
    instantiate a signature in two different ways, adding mixin entries that
    perform renamings becomes necessary.

    .. Warning::

       Backpack_ has the limitation that implementation modules that instantiate
       signatures required by a :pkg-field:`build-depends` dependency can't
       reside in the same component that has the dependency. They must reside
       in a different package dependency, or at least in a separate internal


Library and executable sections may include conditional blocks, which test for various system parameters and configuration flags. The flags mechanism is rather generic, but most of the time a flag represents certain feature, that can be switched on or off by the package user. Here is an example package description file using configurations:

Example: A package containing a library and executable programs
Name: Test1
Version: 0.0.1
Cabal-Version: >= 1.8
License: BSD3
Author:  Jane Doe
Synopsis: Test package to test configurations
Category: Example
Build-Type: Simple

Flag Debug
  Description: Enable debug support
  Default:     False
  Manual:      True

Flag WebFrontend
  Description: Include API for web frontend.
  Default:     False
  Manual:      True

Flag NewDirectory
  description: Whether to build against @directory >= 1.2@
  -- This is an automatic flag which the solver will be
  -- assign automatically while searching for a solution

  Build-Depends:   base >= 4.2 && < 4.9
  Exposed-Modules: Testing.Test1
  Extensions:      CPP

  GHC-Options: -Wall
  if flag(Debug)
    CPP-Options: -DDEBUG
    if !os(windows)
      CC-Options: "-DDEBUG"
      CC-Options: "-DNDEBUG"

  if flag(WebFrontend)
    Build-Depends: cgi >= 0.42 && < 0.44
    Other-Modules: Testing.WebStuff

    if flag(NewDirectory)
        build-depends: directory >= 1.2 && < 1.4
        Build-Depends: time >= 1.0 && < 1.9
        build-depends: directory == 1.1.*
        Build-Depends: old-time >= 1.0 && < 1.2

Executable test1
  Main-is: T1.hs
  Other-Modules: Testing.Test1
  Build-Depends: base >= 4.2 && < 4.9

  if flag(debug)
    CC-Options: "-DDEBUG"
    CPP-Options: -DDEBUG

Flags, conditionals, library and executable sections use layout to indicate structure. This is very similar to the Haskell layout rule. Entries in a section have to all be indented to the same level which must be more than the section header. Tabs are not allowed to be used for indentation.

As an alternative to using layout you can also use explicit braces {}. In this case the indentation of entries in a section does not matter, though different fields within a block must be on different lines. Here is a bit of the above example again, using braces:

Example: Using explicit braces rather than indentation for layout
Name: Test1
Version: 0.0.1
Cabal-Version: >= 1.8
License: BSD3
Author:  Jane Doe
Synopsis: Test package to test configurations
Category: Example
Build-Type: Simple

Flag Debug {
  Description: Enable debug support
  Default:     False
  Manual:      True

Library {
  Build-Depends:   base >= 4.2 && < 4.9
  Exposed-Modules: Testing.Test1
  Extensions:      CPP
  if flag(debug) {
    CPP-Options: -DDEBUG
    if !os(windows) {
      CC-Options: "-DDEBUG"
    } else {
      CC-Options: "-DNDEBUG"
Configuration Flags
.. pkg-section:: flag name
    :synopsis: Flag declaration.

    Flag section declares a flag which can be used in `conditional blocks`_.

    Flag names are case-insensitive and must match ``[[:alnum:]_][[:alnum:]_-]*``
    regular expression, or expressed as ABNF_:

    .. code-block:: abnf

       flag-name = (UALNUM / "_") *(UALNUM / "_" / "-")

       UALPHA = ... ; set of alphabetic Unicode code-points

    .. note::

        Hackage accepts ASCII-only flags, ``[a-zA-Z0-9_][a-zA-Z0-9_-]*`` regexp.

.. pkg-field:: description: freeform

    The description of this flag.

.. pkg-field:: default: boolean

    :default: ``True``

    The default value of this flag.

    .. note::

      This value may be `overridden in several
      ways <installing-packages.html#controlling-flag-assignments>`__. The
      rationale for having flags default to True is that users usually
      want new features as soon as they are available. Flags representing
      features that are not (yet) recommended for most users (such as
      experimental features or debugging support) should therefore
      explicitly override the default to False.

.. pkg-field:: manual: boolean

    :default: ``False``
    :since: 1.6

    By default, Cabal will first try to satisfy dependencies with the
    default flag value and then, if that is not possible, with the
    negated value. However, if the flag is manual, then the default
    value (which can be overridden by commandline flags) will be used.

Conditional Blocks

Conditional blocks may appear anywhere inside a library or executable section. They have to follow rather strict formatting rules. Conditional blocks must always be of the shape

if condition


if condition

Note that the if and the condition have to be all on the same line.

Since Cabal 2.2 conditional blocks support elif construct.

if condition1
elif condition2

Conditions can be formed using boolean tests and the boolean operators || (disjunction / logical "or"), && (conjunction / logical "and"), or ! (negation / logical "not"). The unary ! takes highest precedence, || takes lowest. Precedence levels may be overridden through the use of parentheses. For example, os(darwin) && !arch(i386) || os(freebsd) is equivalent to (os(darwin) && !(arch(i386))) || os(freebsd).

The following tests are currently supported.

Tests if the current operating system is name. The argument is tested against System.Info.os on the target system. There is unfortunately some disagreement between Haskell implementations about the standard values of System.Info.os. Cabal canonicalises it so that in particular os(windows) works on all implementations. If the canonicalised os names match, this test evaluates to true, otherwise false. The match is case-insensitive.
Tests if the current architecture is name. The argument is matched against System.Info.arch on the target system. If the arch names match, this test evaluates to true, otherwise false. The match is case-insensitive.

Tests for the configured Haskell implementation. An optional version constraint may be specified (for example impl(ghc >= 6.6.1)). If the configured implementation is of the right type and matches the version constraint, then this evaluates to true, otherwise false. The match is case-insensitive.

Note that including a version constraint in an impl test causes it to check for two properties:

  • The current compiler has the specified name, and
  • The compiler's version satisfied the specified version constraint

As a result, !impl(ghc >= x.y.z) is not entirely equivalent to impl(ghc < x.y.z). The test !impl(ghc >= x.y.z) checks that:

  • The current compiler is not GHC, or
  • The version of GHC is earlier than version x.y.z.
Evaluates to the current assignment of the flag of the given name. Flag names are case insensitive. Testing for flags that have not been introduced with a flag section is an error.
Constant value true.
Constant value false.
Resolution of Conditions and Flags

If a package descriptions specifies configuration flags the package user can control these in several ways. If the user does not fix the value of a flag, Cabal will try to find a flag assignment in the following way.

  • For each flag specified, it will assign its default value, evaluate all conditions with this flag assignment, and check if all dependencies can be satisfied. If this check succeeded, the package will be configured with those flag assignments.
  • If dependencies were missing, the last flag (as by the order in which the flags were introduced in the package description) is tried with its alternative value and so on. This continues until either an assignment is found where all dependencies can be satisfied, or all possible flag assignments have been tried.

To put it another way, Cabal does a complete backtracking search to find a satisfiable package configuration. It is only the dependencies specified in the :pkg-field:`build-depends` field in conditional blocks that determine if a particular flag assignment is satisfiable (:pkg-field:`build-tools` are not considered). The order of the declaration and the default value of the flags determines the search order. Flags overridden on the command line fix the assignment of that flag, so no backtracking will be tried for that flag.

If no suitable flag assignment could be found, the configuration phase will fail and a list of missing dependencies will be printed. Note that this resolution process is exponential in the worst case (i.e., in the case where dependencies cannot be satisfied). There are some optimizations applied internally, but the overall complexity remains unchanged.

Meaning of field values when using conditionals

During the configuration phase, a flag assignment is chosen, all conditionals are evaluated, and the package description is combined into a flat package descriptions. If the same field both inside a conditional and outside then they are combined using the following rules.

  • Boolean fields are combined using conjunction (logical "and").

  • List fields are combined by appending the inner items to the outer items, for example

    other-extensions: CPP
    if impl(ghc)
      other-extensions: MultiParamTypeClasses

    when compiled using GHC will be combined to

    other-extensions: CPP, MultiParamTypeClasses

    Similarly, if two conditional sections appear at the same nesting level, properties specified in the latter will come after properties specified in the former.

  • All other fields must not be specified in ambiguous ways. For example

    Main-is: Main.hs
    if flag(useothermain)
      Main-is: OtherMain.hs

    will lead to an error. Instead use

    if flag(useothermain)
      Main-is: OtherMain.hs
      Main-is: Main.hs

Common stanzas

.. pkg-section:: common name
    :since: 2.2
    :synopsis: Common build info section

Starting with Cabal-2.2 it's possible to use common build info stanzas.

common deps
  build-depends: base ^>= 4.11
  ghc-options: -Wall

common test-deps
  build-depends: tasty ^>=

  import: deps
  exposed-modules: Foo

test-suite tests
  import: deps, test-deps
  type: exitcode-stdio-1.0
  main-is: Tests.hs
  build-depends: foo
  • You can use build information fields in common stanzas.
  • Common stanzas must be defined before use.
  • Common stanzas can import other common stanzas.
  • You can import multiple stanzas at once. Stanza names must be separated by commas.
  • import must be the first field in a section. Since Cabal 3.0 imports are also allowed inside conditionals.


The name import was chosen, because there is includes field.

Source Repositories

.. pkg-section:: source-repository
    :since: 1.6

It is often useful to be able to specify a source revision control repository for a package. Cabal lets you specifying this information in a relatively structured form which enables other tools to interpret and make effective use of the information. For example the information should be sufficient for an automatic tool to checkout the sources.

Cabal supports specifying different information for various common source control systems. Obviously not all automated tools will support all source control systems.

Cabal supports specifying repositories for different use cases. By declaring which case we mean automated tools can be more useful. There are currently two kinds defined:

  • The head kind refers to the latest development branch of the package. This may be used for example to track activity of a project or as an indication to outside developers what sources to get for making new contributions.
  • The this kind refers to the branch and tag of a repository that contains the sources for this version or release of a package. For most source control systems this involves specifying a tag, id or hash of some form and perhaps a branch. The purpose is to be able to reconstruct the sources corresponding to a particular package version. This might be used to indicate what sources to get if someone needs to fix a bug in an older branch that is no longer an active head branch.

You can specify one kind or the other or both. As an example here are the repositories for the Cabal library. Note that the this kind of repository specifies a tag.

source-repository head
  type:     darcs

source-repository this
  type:     darcs
  tag:      1.6.1

The exact fields are as follows:

.. pkg-field:: type: token

    The name of the source control system used for this repository. The
    currently recognised types are:

    -  ``darcs``
    -  ``git``
    -  ``svn``
    -  ``cvs``
    -  ``mercurial`` (or alias ``hg``)
    -  ``bazaar`` (or alias ``bzr``)
    -  ``arch``
    -  ``monotone``

    This field is required.

.. pkg-field:: location: URL

    The location of the repository. The exact form of this field depends
    on the repository type. For example:

    -  for darcs: ````
    -  for git: ``git://``
    -  for CVS: ````

    This field is required.

.. pkg-field:: module: token

    CVS requires a named module, as each CVS server can host multiple
    named repositories.

    This field is required for the CVS repository type and should not be
    used otherwise.

.. pkg-field:: branch: token

    Many source control systems support the notion of a branch, as a
    distinct concept from having repositories in separate locations. For
    example CVS, SVN and git use branches while for darcs uses different
    locations for different branches. If you need to specify a branch to
    identify a your repository then specify it in this field.

    This field is optional.

.. pkg-field:: tag: token

    A tag identifies a particular state of a source repository. The tag
    can be used with a ``this`` repository kind to identify the state of
    a repository corresponding to a particular package version or
    release. The exact form of the tag depends on the repository type.

    This field is required for the ``this`` repository kind.

.. pkg-field:: subdir: directory

    Some projects put the sources for multiple packages under a single
    source repository. This field lets you specify the relative path
    from the root of the repository to the top directory for the
    package, i.e. the directory containing the package's ``.cabal``

    This field is optional. It default to empty which corresponds to the
    root directory of the repository.

Downloading a package's source

The cabal get command allows to access a package's source code - either by unpacking a tarball downloaded from Hackage (the default) or by checking out a working copy from the package's source repository.

$ cabal get [FLAGS] PACKAGES

The get command supports the following options:

-d --destdir PATH
Where to place the package source, defaults to (a subdirectory of) the current directory.
-s --source-repository [head|this|...]
Fork the package's source repository using the appropriate version control system. The optional argument allows to choose a specific repository kind.
--index-state [HEAD|@<unix-timestamp>|<iso8601-utc-timestamp>]
Use source package index state as it existed at a previous time. Accepts unix-timestamps (e.g. @1474732068), ISO8601 UTC timestamps (e.g. 2016-09-24T17:47:48Z), or HEAD (default). This determines which package versions are available as well as which .cabal file revision is selected (unless --pristine is used).
Unpack the original pristine tarball, rather than updating the .cabal file with the latest revision from the package archive.

Custom setup scripts

Since Cabal 1.24, custom Setup.hs are required to accurately track their dependencies by declaring them in the .cabal file rather than rely on dependencies being implicitly in scope. Please refer this article for more details.

Declaring a custom-setup stanza also enables the generation of MIN_VERSION_package_(A,B,C) CPP macros for the Setup component.

.. pkg-section:: custom-setup
   :synopsis: Custom Setup.hs build information.
   :since: 1.24

   The optional :pkg-section:`custom-setup` stanza contains information needed
   for the compilation of custom ``Setup.hs`` scripts,

    base  >= 4.5 && < 4.11,
    Cabal >= 1.14 && < 1.25
.. pkg-field:: setup-depends: package list
    :since: 1.24

    The dependencies needed to compile ``Setup.hs``. See the
    :pkg-field:`build-depends` field for a description of the syntax expected by
    this field.

Backward compatibility and custom-setup

Versions prior to Cabal 1.24 don't recognise custom-setup stanzas, and will behave agnostic to them (except for warning about an unknown section). Consequently, versions prior to Cabal 1.24 can't ensure the declared dependencies setup-depends are in scope, and instead whatever is registered in the current package database environment will become eligible (and resolved by the compiler) for the Setup.hs module.

The availability of the MIN_VERSION_package_(A,B,C) CPP macros inside Setup.hs scripts depends on the condition that either

  • a custom-setup section has been declared (or cabal new-build is being used which injects an implicit hard-coded custom-setup stanza if it's missing), or
  • GHC 8.0 or later is used (which natively injects package version CPP macros)

Consequently, if you need to write backward compatible Setup.hs scripts using CPP, you should declare a custom-setup stanza and use the pattern below:

import Distribution.Simple

#if defined(MIN_VERSION_Cabal)
-- version macros are available and can be used as usual
# if MIN_VERSION_Cabal(a,b,c)
-- code specific to lib:Cabal >= a.b.c
# else
-- code specific to lib:Cabal < a.b.c
# endif
# warning Enabling heuristic fall-back. Please upgrade cabal-install to 1.24 or later if Setup.hs fails to compile.

-- package version macros not available; except for exotic environments,
-- you can heuristically assume that lib:Cabal's version is correlated
-- with __GLASGOW_HASKELL__, and specifically since we can assume that
-- GHC < 8.0, we can assume that lib:Cabal is version 1.22 or older.

main = ...

The simplified (heuristic) CPP pattern shown below is useful if all you need is to distinguish Cabal < 2.0 from Cabal >= 2.0.

import Distribution.Simple

#if !defined(MIN_VERSION_Cabal)
# define MIN_VERSION_Cabal(a,b,c) 0

#if MIN_VERSION_Cabal(2,0,0)
-- code for lib:Cabal >= 2.0
-- code for lib:Cabal < 2.0

main = ...

Autogenerated modules

Modules that are built automatically at setup, created with a custom setup script, must appear on :pkg-field:`other-modules` for the library, executable, test-suite or benchmark stanzas or also on :pkg-field:`library:exposed-modules` for libraries to be used, but are not really on the package when distributed. This makes commands like sdist fail because the file is not found.

These special modules must appear again on the :pkg-field:`autogen-modules` field of the stanza that is using it, besides :pkg-field:`other-modules` or :pkg-field:`library:exposed-modules`. With this there is no need to create complex build hooks for this poweruser case.

.. pkg-field:: autogen-modules: module list
   :since: 2.0

   .. todo:: document autogen-modules field

Right now :pkg-field:`executable:main-is` modules are not supported on :pkg-field:`autogen-modules`.

    default-language: Haskell2010
    build-depends: base

Executable Exe
    default-language: Haskell2010
    main-is: Dummy.hs
    build-depends: base

Accessing data files from package code

The placement on the target system of files listed in the :pkg-field:`data-files` field varies between systems, and in some cases one can even move packages around after installation (see prefix independence). To enable packages to find these files in a portable way, Cabal generates a module called :file:`Paths_{pkgname}` (with any hyphens in pkgname replaced by underscores) during building, so that it may be imported by modules of the package. This module defines a function

getDataFileName :: FilePath -> IO FilePath

If the argument is a filename listed in the :pkg-field:`data-files` field, the result is the name of the corresponding file on the system on which the program is running.


If you decide to import the :file:`Paths_{pkgname}` module then it must be listed in the :pkg-field:`other-modules` field just like any other module in your package and on :pkg-field:`autogen-modules` as the file is autogenerated.

The :file:`Paths_{pkgname}` module is not platform independent, as any other autogenerated module, so it does not get included in the source tarballs generated by sdist.

The :file:`Paths_{pkgname}` module also includes some other useful functions and values, which record the version of the package and some other directories which the package has been configured to be installed into (e.g. data files live in getDataDir):

version :: Version

getBinDir :: IO FilePath
getLibDir :: IO FilePath
getDynLibDir :: IO FilePath
getDataDir :: IO FilePath
getLibexecDir :: IO FilePath
getSysconfDir :: IO FilePath

The actual location of all these directories can be individually overridden at runtime using environment variables of the form pkg_name_var, where pkg_name is the name of the package with all hyphens converted into underscores, and var is either bindir, libdir, dynlibdir, datadir, libexedir or sysconfdir. For example, the configured data directory for pretty-show is controlled with the pretty_show_datadir environment variable.

Accessing the package version

The aforementioned auto generated :file:`Paths_{pkgname}` module also exports the constant version :: Version which is defined as the version of your package as specified in the version field.

System-dependent parameters

For some packages, especially those interfacing with C libraries, implementation details and the build procedure depend on the build environment. The build-type Configure can be used to handle many such situations. In this case, Setup.hs should be:

import Distribution.Simple
main = defaultMainWithHooks autoconfUserHooks

Most packages, however, would probably do better using the Simple build type and configurations.

The :pkg-field:`build-type` Configure differs from Simple in two ways:

  • The package root directory must contain a shell script called configure. The configure step will run the script. This configure script may be produced by autoconf or may be hand-written. The configure script typically discovers information about the system and records it for later steps, e.g. by generating system-dependent header files for inclusion in C source files and preprocessed Haskell source files. (Clearly this won't work for Windows without MSYS or Cygwin: other ideas are needed.)
  • If the package root directory contains a file called package.buildinfo after the configuration step, subsequent steps will read it to obtain additional settings for build information fields,to be merged with the ones given in the .cabal file. In particular, this file may be generated by the configure script mentioned above, allowing these settings to vary depending on the build environment.

The build information file should have the following structure:


executable: name buildinfo

executable: name buildinfo ...

where each buildinfo consists of settings of fields listed in the section on build information. The first one (if present) relates to the library, while each of the others relate to the named executable. (The names must match the package description, but you don't have to have entries for all of them.)

Neither of these files is required. If they are absent, this setup script is equivalent to defaultMain.

Example: Using autoconf

This example is for people familiar with the autoconf tools.

In the X11 package, the file contains:

AC_INIT([Haskell X11 package], [1.1], [], [X11])

# Safety check: Ensure that we are in the correct source directory.

# Header file to place defines in

# Check for X11 include paths and libraries
AC_TRY_CPP([#include <X11/Xlib.h>],,[no_x=yes])

# Build the package if we found X11 stuff
if test "$no_x" = yes


Then the setup script will run the configure script, which checks for the presence of the X11 libraries and substitutes for variables in the file

cc-options: @X_CFLAGS@
ld-options: @X_LIBS@

This generates a file X11.buildinfo supplying the parameters needed by later stages:

buildable: True
cc-options:  -I/usr/X11R6/include
ld-options:  -L/usr/X11R6/lib

The configure script also generates a header file include/HsX11Config.h containing C preprocessor defines recording the results of various tests. This file may be included by C source files and preprocessed Haskell source files in the package.


Packages using these features will also need to list additional files such as configure, templates for .buildinfo files, files named only in .buildinfo files, header files and so on in the :pkg-field:`extra-source-files` field to ensure that they are included in source distributions. They should also list files and directories generated by configure in the :pkg-field:`extra-tmp-files` field to ensure that they are removed by setup clean.

Quite often the files generated by configure need to be listed somewhere in the package description (for example, in the :pkg-field:`install-includes` field). However, we usually don't want generated files to be included in the source tarball. The solution is again provided by the .buildinfo file. In the above example, the following line should be added to X11.buildinfo:

install-includes: HsX11Config.h

In this way, the generated HsX11Config.h file won't be included in the source tarball in addition to, but it will be copied to the right location during the install process. Packages that use custom Setup.hs scripts can update the necessary fields programmatically instead of using the .buildinfo file.

Conditional compilation

Sometimes you want to write code that works with more than one version of a dependency. You can specify a range of versions for the dependency in the :pkg-field:`build-depends`, but how do you then write the code that can use different versions of the API?

Haskell lets you preprocess your code using the C preprocessor (either the real C preprocessor, or cpphs). To enable this, add extensions: CPP to your package description. When using CPP, Cabal provides some pre-defined macros to let you test the version of dependent packages; for example, suppose your package works with either version 3 or version 4 of the base package, you could select the available version in your Haskell modules like this:

#if MIN_VERSION_base(4,0,0)
... code that works with base-4 ...
... code that works with base-3 ...

In general, Cabal supplies a macro MIN_VERSION_``package``_(A,B,C) for each package depended on via :pkg-field:`build-depends`. This macro is true if the actual version of the package in use is greater than or equal to A.B.C (using the conventional ordering on version numbers, which is lexicographic on the sequence, but numeric on each component, so for example 1.2.0 is greater than 1.0.3).

Since version 1.20, the MIN_TOOL_VERSION_``tool`` family of macros lets you condition on the version of build tools used to build the program (e.g. hsc2hs).

Since version 1.24, the macro CURRENT_COMPONENT_ID, which expands to the string of the component identifier that uniquely identifies this component. Furthermore, if the package is a library, the macro CURRENT_PACKAGE_KEY records the identifier that was passed to GHC for use in symbols and for type equality.

Since version 2.0, the macro CURRENT_PACKAGE_VERSION expands to the string version number of the current package.

Cabal places the definitions of these macros into an automatically-generated header file, which is included when preprocessing Haskell source code by passing options to the C preprocessor.

Cabal also allows to detect when the source code is being used for generating documentation. The __HADDOCK_VERSION__ macro is defined only when compiling via Haddock_ instead of a normal Haskell compiler. The value of the __HADDOCK_VERSION__ macro is defined as A*1000 + B*10 + C, where A.B.C is the Haddock version. This can be useful for working around bugs in Haddock or generating prettier documentation in some special cases.

More complex packages

For packages that don't fit the simple schemes described above, you have a few options:

  • By using the :pkg-field:`build-type` Custom, you can supply your own Setup.hs file, and customize the simple build infrastructure using hooks. These allow you to perform additional actions before and after each command is run, and also to specify additional preprocessors. A typical Setup.hs may look like this:

    import Distribution.Simple
    main = defaultMainWithHooks simpleUserHooks { postHaddock = posthaddock }
    posthaddock args flags desc info = ....

    See UserHooks in Distribution.Simple for the details, but note that this interface is experimental, and likely to change in future releases.

    If you use a custom Setup.hs file you should strongly consider adding a :pkg-section:`custom-setup` stanza with a :pkg-field:`custom-setup:setup-depends` field to ensure that your setup script does not break with future dependency versions.

  • You could delegate all the work to make, though this is unlikely to be very portable. Cabal supports this with the :pkg-field:`build-type` Make and a trivial setup library Distribution.Make, which simply parses the command line arguments and invokes make. Here Setup.hs should look like this:

    import Distribution.Make
    main = defaultMain

    The root directory of the package should contain a configure script, and, after that has run, a Makefile with a default target that builds the package, plus targets install, register, unregister, clean, dist and docs. Some options to commands are passed through as follows:

    • The --with-hc-pkg, --prefix, --bindir, --libdir, --dynlibdir, --datadir, --libexecdir and --sysconfdir options to the configure command are passed on to the configure script. In addition the value of the --with-compiler option is passed in a --with-hc option and all options specified with --configure-option= are passed on.

    • The --destdir option to the copy command becomes a setting of a destdir variable on the invocation of make copy. The supplied Makefile should provide a copy target, which will probably look like this:

      copy :
              $(MAKE) install prefix=$(destdir)/$(prefix) \
                              bindir=$(destdir)/$(bindir) \
                              libdir=$(destdir)/$(libdir) \
                              dynlibdir=$(destdir)/$(dynlibdir) \
                              datadir=$(destdir)/$(datadir) \
                              libexecdir=$(destdir)/$(libexecdir) \
                              sysconfdir=$(destdir)/$(sysconfdir) \
  • Finally, with the :pkg-field:`build-type` Custom, you can also write your own setup script from scratch. It must conform to the interface described in the section on building and installing packages, and you may use the Cabal library for all or part of the work. One option is to copy the source of Distribution.Simple, and alter it for your needs. Good luck.



Some packages (ab)use :pkg-field:`build-depends` on old-style builds, but this has a few major drawbacks:

  • using Nix-style builds it's considered an error if you depend on a exe-only package via build-depends: the solver will refuse it.
  • it may or may not place the executable on $PATH.
  • it does not ensure the correct version of the package is installed, so you might end up overwriting versions with each other.