# 1. Package Description — <package>.cabal File

The package description file, commonly known as “the Cabal file”, describes the contents of a package. The Cabal package is the unit of distribution. When installed, its purpose is to make available one or more:

- Haskell programs (executables); and/or

- libraries, exposing a number of Haskell modules.

Public library components can be depended upon by other Cabal packages and all library components (both public and private) can be depended upon by other components of the same package.

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`”.

Note

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.

## 1.1. 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:

`package-name.cabal`

  a Unicode UTF-8 text file containing a package description. For details of the syntax of this file, see the section on package descriptions.

`Setup.hs`

  a single-module Haskell program to perform various setup tasks (with the interface described in the section on Setup.hs Commands). 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 `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 demonstrated in the section Building the package.

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

    library
      default-language: Haskell2010
      build-depends:    base >= 4 && < 5
      exposed-modules:  Foo
      extensions:       ForeignFunctionInterface
      ghc-options:      -Wall
      if os(windows)
        build-depends: Win32 >= 2.1 && < 2.6

### 1.1.1. Example: A package containing a simple library

The HUnit package contains a file `HUnit.cabal` containing:

    cabal-version:  3.0
    name:           HUnit
    version:        1.1.1
    synopsis:       A unit testing framework for Haskell
    homepage:       http://hunit.sourceforge.net/
    category:       Testing
    author:         Dean Herington
    license:        BSD-3-Clause
    license-file:   LICENSE
    build-type:     Simple

    library
      build-depends:      base >= 2 && < 4
      exposed-modules:    Test.HUnit.Base, Test.HUnit.Lang,
                          Test.HUnit.Terminal, Test.HUnit.Text, Test.HUnit
      default-extensions: CPP
      default-language:   Haskell2010

and the following `Setup.hs`:

    import Distribution.Simple
    main = defaultMain

### 1.1.2. Example: A package containing executable programs

    cabal-version:  3.0
    name:           TestPackage
    version:        0.0
    synopsis:       Small package with two programs
    author:         Angela Author
    license:        BSD-3-Clause
    build-type:     Simple

    executable program1
      build-depends:    HUnit >= 1.1.1 && < 1.2
      main-is:          main.hs
      hs-source-dirs:   prog1
      default-language: Haskell2010

    executable program2
      -- A different main.hs because of hs-source-dirs.
      main-is:          main.hs
      build-depends:    HUnit >= 1.1.1 && < 1.2
      hs-source-dirs:   prog2
      other-modules:    Utils
      default-language: Haskell2010

with `Setup.hs` the same as above.

### 1.1.3. Example: A package containing a library and executable programs

    cabal-version:   3.0
    name:            TestPackage
    version:         0.0
    synopsis:        Package with library and two programs
    license:         BSD-3-Clause
    author:          Angela Author
    build-type:      Simple

    library
      build-depends:    HUnit >= 1.1.1 && < 1.2
      hs-source-dirs:   lib
      exposed-modules:  A, B, C
      default-language: Haskell2010

    executable program1
      main-is:          main.hs
      hs-source-dirs:   prog1
      other-modules:    D, E
      default-language: Haskell2010

    executable program2
      -- A different main.hs because of hs-source-dirs.
      main-is:          main.hs
      -- No bound on a library provided by the same package.
      build-depends:    TestPackage
      hs-source-dirs:   prog2
      other-modules:    Utils
      default-language: Haskell2010

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.

## 1.2. 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 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.

- Before Cabal 3.0, 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.

identifier

  A letter followed by zero or more alphanumerics or underscores.

compiler

  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`.

### 1.2.1. Modules and preprocessors

Haskell module names listed in the `library:exposed-modules` and `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:

- `.chs` (c2hs)

- `.hsc` (hsc2hs)

- `.y` and `.ly` (happy)

- `.x` (alex)

- `.cpphs` (cpphs)

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 `build-tools` or `build-tool-depends` fields.

Some fields take lists of values, which are optionally separated by commas, except for the `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.

### 1.2.2. Package properties

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

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 `package-name.cabal`.

  A valid package name comprises an alphanumeric ‘word’; or two or more such words separated by a hyphen character (`-`). A word cannot be comprised only of the digits `0` to `9`.

  An alphanumeric character belongs to one of the Unicode Letter categories (Lu (uppercase), Ll (lowercase), Lt (titlecase), Lm (modifier), or Lo (other)) or Number categories (Nd (decimal), Nl (letter), or No (other)).

  Package names are case-sensitive.

  Expressed as a regular expression:

  `[0-9]*[\p{L}\p{N}-[0-9]][\p{L}\p{N}]*(-[0-9]*[\p{L}\p{N}-[0-9]][\p{L}\p{N}]*)*`

  Expressed in ABNF:

      package-name      = package-name-part *("-" package-name-part)
      package-name-part = *DIGIT UALPHANUM-NOT-DIGIT *UALNUM

      DIGIT = %x30-39 ; 0-9

      UALNUM = UALPHANUM-NOT-DIGIT / DIGIT
      UALPHANUM-NOT-DIGIT = ... ; set of Unicode code-points in Letter or
                                ; Number categories, other than the DIGIT
                                ; code-points

  Note

  Hackage will not accept package names that use alphanumeric characters other than `A` to `Z`, `a` to `z`, and `0` to `9` (the ASCII subset).

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:

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

cabal-version: x.y[.z]

  The version of the Cabal specification that this package description uses. The Cabal specification does slowly evolve (see also Package Description Format Specification History), introducing new features and occasionally changing the meaning of existing features. Specifying which version of the specification you are using 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 `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

      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

      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.

build-type: identifier
  Default value:

    `Custom` or `Simple`

  The type of build used by this package. Build types are the constructors of the BuildType type. This field is optional and when missing, its default value is inferred according to the following rules:

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

  > - For lower `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:

      import Distribution.Simple
      main = defaultMain

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

      import Distribution.Simple
      import SetupHooks (setupHooks)
      main = defaultMainWithSetupHooks setupHooks

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

      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:

      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. For more exotic needs, the `Hooks` build type is recommended; see Hooks.

license: SPDX expression
  Default value:

    `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

      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 `cabal-version` value as defined in the following table:

  Cabal specification version | SPDX license list 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 specification.

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

license-file: filename

  See `license-files`.

license-files: filename list
  Since:

    Cabal 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 package.

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

author: freeform

  The original author of the package.

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

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.

stability: freeform

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

homepage: URL

  The package homepage.

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 system.

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

      bug-reports: https://github.com/haskell/cabal/issues

package-url: URL

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

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.

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 runhaskell Setup.hs haddock and thus may contain the same markup as Haddock documentation comments.

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.

tested-with: compiler list

  A list of compilers and versions against which the package has been tested (or at least built). The value of this field is not used by Cabal and is rather intended as extra metadata for use by third party tooling, such as e.g. CI tooling.

  Here’s a typical usage example:

      tested-with: GHC == 9.10.1, GHC == 9.8.2, GHC == 9.6.5

  The same can be spread over several lines, for instance:

      tested-with: GHC == 9.10.1
                 , GHC == 9.8.2
                 , GHC == 9.6.5

  The separating comma can also be dropped altogether:

      tested-with:
        GHC == 9.10.1
        GHC == 9.8.2
        GHC == 9.6.5

  However, this alternative might disappear in the future.

  Starting with `cabal-version` 3.0, there are further conveniences.

  1. A preceding `,` is allowed, so a bullet-list style is possible (recommended):

     >     tested-with:
     >       , GHC == 9.10.1
     >       , GHC == 9.8.2
     >       , GHC == 9.6.5

  2. A concise set notation syntax is available:

         tested-with: GHC == { 9.10.1, 9.8.2, 9.6.5 }

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.

  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 subdirectories.

  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 allowed.

  - 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).

  - Prior to Cabal 3.8, if a `**` wildcard is used, then the file name must include a `*` wildcard (e.g., `data/**/README.rst` was not allowed). As of `cabal-version: 3.8` or greater, this restriction is lifted.

  - 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 and bug-free since Cabal 3.0.

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.

extra-source-files: filename list

  A list of additional files to be included in source distributions built with runhaskell Setup.hs sdist. As with `data-files` it can use a limited form of `*` wildcards in file names. Files listed here are tracked by `cabal build`; changes in these files cause (partial) rebuilds.

extra-doc-files: filename list
  Since:

    Cabal 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 `data-files` it can use a limited form of `*` wildcards in file names.

extra-tmp-files: filename list

  A list of additional files or directories to be removed by runhaskell Setup.hs clean. These would typically be additional files created by additional hooks, such as the scheme described in the section on system-dependent parameters.

extra-files: filename list

  A list of additional files to be included in source distributions built with runhaskell Setup.hs sdist. As with `data-files` it can use a limited form of `*` wildcards in file names.

### 1.2.3. Library

library name

  Build information for libraries.

  A package can include zero or more library components. A library can be unnamed or named (using the `name` argument). It can also be depended upon only by components in the same package (private) or by those components and components in other packages (public). A package can have no more than one unnamed library.

  Note

  The ‘cabal’ executable provided by the ‘cabal-install’ package will not accept dependencies on sublibraries of packages with no unnamed library.

  This guide refers to an unnamed library as the main library and a named library as a sublibrary (such components may be considered as subidiary, or ancillary, to the main library). It refers to a private sublibrary as an internal library.

  A sublibrary cannot have the same name as its package.

  Note

  Before version 3.4 of the Cabal specification, a private sublibrary could shadow a dependency on the main library of another package, if their names clashed.

  A main library is always public and a sublibrary is private by default. See the `library:visibility` field for setting a sublibrary as public.

  Being able to include more than one public library in a package allows the separation of the unit of distribution (the package) from the unit of buildable code (the library). This is useful for Haskell projects with many libraries that are distributed together as it avoids duplication and potential inconsistencies.

  Note

  Before version 3.0 of the Cabal specification, all sublibraries were internal libraries. Before version 2.0, a package could not include sublibraries.

  See Sublibraries - Examples for examples.

A library section should contain the following fields:

visibility: visibility specifiers
  Since:

    3.0

  Default value:

    `private` for sublibraries. Cannot be set for the main library, which is always public.

  Can be set to `private` or `public`. A `private` library component can only be depended on by other components of the same package. A `public` component can be depended on by those components and by components of other packages.

  See the `build-depends` field for the syntax to specify a dependency on a library component.

exposed-modules: identifier list
  Required:

    if this package contains a library

  A list of modules added by this package.

virtual-modules: identifier list
  Since:

    Cabal 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 database.

exposed: boolean
  Default value:

    `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.

reexported-modules: exportlist
  Since:

    Cabal 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.

signatures: signature list
  Since:

    Cabal 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 How to use Backpack modules 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).

Sublibraries - Examples

An example of the use of a private sublibrary (an internal library) is a test suite that needs access to some internal modules in the package’s main library, which you do not otherwise want to expose. You could put those modules in an internal library, which the main library and the test suite `build-depends` upon. Your Cabal file might then look something like this:

    cabal-version:  3.4
    name:           foo
    version:        0.1.0.0
    license:        BSD-3-Clause
    license-file:   LICENSE
    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
        default-language: Haskell2010

    library
        exposed-modules:  Foo.Public
        build-depends:    foo:foo-internal, base >= 4.3 && < 5
        default-language: Haskell2010

    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:foo-internal, base
        default-language: Haskell2010

Another example of the use of internal libraries is a package that includes one or more executables but does not include a public library.

Internal libraries can be used to incorporate (vendor or bundle) an external dependency into a package, effectively simulating private dependencies. Below is an example:

    cabal-version: 3.4
    name: haddock-library
    version: 1.6.0
    license: BSD-3-Clause

    library
      build-depends:
        , base         ^>= 4.19.0.0
        , bytestring   ^>= 0.12.0.0
        , containers   ^>= 0.6.8 || ^>= 0.7.0
        , transformers ^>= 0.6.1.0

      hs-source-dirs:       src

      -- internal sub-lib
      build-depends:        haddock-library:attoparsec

      exposed-modules:
        Documentation.Haddock

      default-language: Haskell2010

    library attoparsec
      build-depends:
        , base         ^>= 4.19.0.0
        , bytestring   ^>= 0.12.0.0
        , deepseq      ^>= 1.5.0.0

      hs-source-dirs:       vendor/attoparsec-0.13.1.0

      -- NB: haddock-library needs only small part of lib:attoparsec
      --     internally, so we only bundle that subset here
      exposed-modules:
        Data.Attoparsec.ByteString
        Data.Attoparsec.Combinator

      other-modules:
        Data.Attoparsec.Internal

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

      default-language: Haskell2010

### 1.2.4. Executables

A package description can contain multiple executable sections. The documentation of the cabal run command contains detailed information on how to run an executable.

executable name

  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).

main-is: filename (required)

  By convention, a Haskell program must have a module called `Main` which exports an IO action named `main`. When the program is executed, the action is performed. This field specifies the name of the `.hs` or `.lhs` source file containing that module. It is the `.hs` filename that must be listed, even if that file is generated using a preprocessor. The file must be relative to one of the directories listed in `hs-source-dirs`.

  Further, while the name of the source file may vary, if the convention is being followed, the module itself must be named `Main` and export `main`.

  However, GHC’s `-main-is` option can be used to change the name of the relevant IO action. For example, if source file `MyMainSourceFile.hs` contains a module named `MyMainModule` exporting `myMainFunc` and that is to be the relevant IO action, you can specify:

      executable my-app
        main-is: MyMainSourceFile.hs
        ghc-options: -main-is MyMainModule.myMainFunc
        build-depends:
            base
        default-language: Haskell2010

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

scope: token
  Since:

    Cabal 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.

### 1.2.5. Test suites

A package description can contain multiple test suite sections. The documentation of the cabal test command contains detailed information on how to run test suites.

test-suite name

  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).

type: interface (required until ``cabal-version`` 3.8)

  The interface type and version of the test suite. Cabal supports two test suite interfaces, called `exitcode-stdio-1.0` (default since `cabal-version` 3.8) 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` (default since `cabal-version` 3.8) 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.

main-is: filename
  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 `hs-source-dirs`. This field is analogous to the `main-is` field of an executable section; see that documentation for further information.

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 `test-module` field.

test-module: identifier
  Required:

    `detailed-0.9`

  Disallowed:

    `exitcode-stdio-1.0`

  The module exporting the `tests` symbol.

code-generators:

  > An optional list of preprocessors which can generate new modules for use in the test-suite.

  A list of executables (possibly brought into scope by `build-tool-depends`) that are run after all other preprocessors. These executables are invoked as so: `exe-name TARGETDIR [SOURCEDIRS] -- [GHCOPTIONS]`. The arguments are, in order a target dir for output, a sequence of all source directories with source files of local lib components that the given test stanza depends on, and following a double dash, all options cabal would pass to ghc for a build. They are expected to output a newline-separated list of generated modules which have been written to the targetdir (excepting, if written, the main module). This can be used for driving doctests and other discover-style tests generated from source code.

#### 1.2.5.1. 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.

foo.cabal

    Cabal-Version:  3.0
    Name:           foo
    Version:        1.0
    License:        BSD-3-Clause
    Build-Type:     Simple

    Test-Suite test-foo
        type:             exitcode-stdio-1.0
        main-is:          test-foo.hs
        build-depends:    base >= 4 && < 5
        default-language: Haskell2010

test-foo.hs

    module Main where

    import System.Exit (exitFailure)

    main = do
        putStrLn "This test always fails!"
        exitFailure

#### 1.2.5.2. 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.

bar.cabal

    Cabal-Version:  3.0
    Name:           bar
    Version:        1.0
    License:        BSD-3-Clause
    Build-Type:     Simple

    Test-Suite test-bar
        type:             detailed-0.9
        test-module:      Bar
        -- To keep this (untested) example working, we rely on the test type
        -- version rather than an upper bound on Cabal. In real code, PVP
        -- compliance may require including the upper bound.
        build-depends:    base >= 4 && < 5, Cabal >= 1.9.2
        default-language: Haskell2010

Bar.hs

    module Bar ( tests ) where

    import Distribution.TestSuite

    tests :: IO [Test]
    tests = return [ Test succeeds, Test fails ]
      where
        succeeds = TestInstance
            { run = return $ Finished Pass
            , name = "succeeds"
            , tags = []
            , options = []
            , setOption = \_ _ -> Right succeeds
            }
        fails = TestInstance
            { run = return $ Finished $ Fail "Always fails!"
            , name = "fails"
            , tags = []
            , options = []
            , setOption = \_ _ -> Right fails
            }

### 1.2.6. Benchmarks

A package description can contain multiple benchmark sections. The documentation of the cabal bench command contains detailed information on how to run benchmarks.

benchmark name
  Since:

    Cabal 1.9.2

  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).

type: interface (required until ``cabal-version`` 3.8)

  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` (default since `cabal-version` 3.8) 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.

main-is: filename

  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 `hs-source-dirs`. This field is analogous to the `main-is` field of an executable section; see that documentation for further information.

#### 1.2.6.1. Example:

foo.cabal

    Cabal-Version:  3.0
    Name:           foo
    Version:        1.0
    License:        BSD-3-Clause
    Build-Type:     Simple

    Benchmark bench-foo
        type:             exitcode-stdio-1.0
        main-is:          bench-foo.hs
        build-depends:    base >= 4 && < 5, time >= 1.1 && < 1.7
        default-language: Haskell2010

bench-foo.hs

    {-# LANGUAGE BangPatterns #-}
    module Main where

    import Data.Time.Clock

    fib 0 = 1
    fib 1 = 1
    fib n = fib (n-1) + fib (n-2)

    main = do
        start <- getCurrentTime
        let !r = fib 20
        end <- getCurrentTime
        putStrLn $ "fib 20 took " ++ show (diffUTCTime end start)

### 1.2.7. Build information

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.

build-depends: library list

  Declares the dependencies on library components required to build the current package component. See `build-tool-depends` for declaring dependencies on build-time tools. Dependencies on libraries from another package should be annotated with a version constraint.

  Library Names

  A library is identified by the name of its package, optionally followed by a colon and the library’s name (for example, `my-package:my-library`). If a library name is omitted, the package’s main library will be used. To refer expressly to a package’s main library, use the name of the package as the library name (for example, `my-package:my-package`). More than one library from the same package can be specified with the shorthand syntax `my-package:{my-library1,my-library2}`.

  Note

  The `build-depends` field of an executable section (from version 1.8 of the Cabal specification), test suite section (from version 1.10) or benchmark section (from version 1.14) can specify a library in the same package by the name of the library (without a version constraint). Cabal then treats the executable, test suite or benchmark as if it were in another package that depended on the package.

  Note

  Before version 3.4 of the Cabal specification, from version 2.0, a private sublibrary (an internal library) was identified by only the name of the sublibrary. An internal library could shadow a dependency on the main library of another package, if the names clashed.

  See the section on `library` for information about how a package can specify library components.

  Version Constraints

  Version constraints use the operators `==, >=, >, <, <=` and a version number. Multiple constraints can be combined using `&&` or `||`.

  Note

  Even though there is no `/=` operator, by combining operators we can skip over one or more versions, to skip a deprecated version or to skip versions that narrow the constraint solving more than we’d like.

  For example, the `time =1.12.*` series depends on `base >=4.13 && <5` but `time-1.12.3` bumps the lower bound on base to `>=4.14`. If we still want to compile with a `ghc-8.8.*` version of GHC that ships with `base-4.13` and with later GHC versions, then we can use `time >=1.12 && (<1.12.3 || >1.12.3)`.

  Hackage shows deprecated and preferred versions for packages, such as for containers and aeson for example. Deprecating package versions is not the same deprecating a package as a whole, for which Hackage keeps a deprecated packages list.

  If no version constraint is specified, any version is assumed to be acceptable. For example:

      library
        build-depends:
          base >= 2,
          foo >= 1.2.3 && < 1.3,
          bar

  Dependencies like `foo >= 1.2.3 && < 1.3` turn out to be very common because it is recommended practice 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:

      build-depends:
        foo ^>= 1.2.3.4,
        bar ^>= 1

  This allows to assert the positive knowledge that this package is known to be semantically compatible with the releases `foo-1.2.3.4` 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 ^>= 1.2.3.4` asserts the positive information that “`foo-1.2.3.4` 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 >= 1.2.3.4 && < 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-1.2.3.4` 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 >= 1.2.3.4` as the initially inferred approximation for the lower bound resulting from the assertion `foo ^>= 1.2.3.4`. 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, `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 expect 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

      build-depends:
        foo >= 1.2.3.4 && < 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 `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.

  Starting with Cabal 3.0, a set notation for the `==` and `^>=` operator is available. For instance,

      tested-with: GHC == 8.6.3, GHC == 8.4.4, GHC == 8.2.2, GHC == 8.0.2,
                   GHC == 7.10.3, GHC == 7.8.4, GHC == 7.6.3, GHC == 7.4.2

      build-depends: network ^>= 2.6.3.6 || ^>= 2.7.0.2 || ^>= 2.8.0.0 || ^>= 3.0.1.0

  can be then written in a more convenient and concise form

      tested-with: GHC == { 8.6.3, 8.4.4, 8.2.2, 8.0.2, 7.10.3, 7.8.4, 7.6.3, 7.4.2 }

      build-depends: network ^>= { 2.6.3.6, 2.7.0.2, 2.8.0.0, 3.0.1.0 }

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 `other-modules`, `library:exposed-modules` or `executable:main-is` fields.

hs-source-dir: directory list
  Removed:

    Cabal 3.0

  Deprecated:

    Cabal 2.0

  Default value:

    `.`

  Root directories for the module hierarchy.

  Deprecated in favor of `hs-source-dirs`.

hs-source-dirs: directory list
  Default value:

    `.`

  Root directories for the module hierarchy.

  Note

  Components can share source directories but modules found there will be recompiled even if other components already built them, i.e., if a library and an executable share a source directory and the executable depends on the library and imports its `Foo` module, `Foo` will be compiled twice, once as part of the library and again for the executable.

default-extensions: identifier list
  Since:

    Cabal 1.10

  > 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 type. For example, `CPP` specifies that Haskell source files are to be preprocessed with a C preprocessor.

other-extensions: identifier list
  Since:

    Cabal 1.10

  > 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.

  >     {-# LANGUAGE CPP, MultiParamTypeClasses #-}

  > In Cabal-1.24 the dependency solver will use this and `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 `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 `other-extensions` declarations.

default-language: identifier
  Since:

    Cabal 1.10

  > Specifies a language standard or a group of language extensions to be activated for the project. In the case of GHC, see here for details.

  > The possible values are:

  > - `GHC2024` (only available for GHC version `9.10` or later)

  > - `GHC2021` (only available for GHC version `9.2` or later)

  > - `Haskell2010`

  > - `Haskell98`

other-languages: identifier
  Since:

    Cabal 1.10

  > Specifies a language standard used by some (but not necessarily all) modules.

extensions: identifier list
  Removed:

    Cabal 3.0

  Deprecated:

    Cabal 1.10

  Deprecated in favor of `default-extensions`.

build-tool-depends: package:executable list
  Since:

    Cabal 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 `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 `build-depends` dependencies.

  2. Internal dependencies 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 README.md 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:

  1. For Nix-style local builds, both internal and external dependencies.

  2. For old-style builds, only for internal dependencies [1]. It’s up to the user to provide needed executables in this case under `PATH`.

  Note

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

build-tools: program list
  Removed:

    Cabal 3.0

  Deprecated:

    Cabal 2.0

  Deprecated in favor of `build-tool-depends`, but see below for backwards compatibility information.

  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 hard-coded set of common tools

  > 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 `build-tool-depends` entry. In the first case, the entry is desugared into a `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 `build-tool-depends` to understand the desugared field’s meaning, along with restrictions on version bounds.

  Backward Compatibility

  Although this field is deprecated in favor of `build-tool-depends`, there are some situations where you may prefer to use `build-tools` in cases (1) and (2), as it is supported by more versions of Cabal. In case (3), `build-tool-depends` is better for backwards-compatibility, as it will be ignored by old versions of Cabal; if you add the executable to `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`.

  Set of Known Tool Names

  Identifiers specified in `build-tools` are desugared into their respective equivalent `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

  `build-tools` identifier | desugared `build-tool-depends` identifier | Note
  --- | --- | ---
  `alex` | `alex:alex` |
  `c2hs` | `c2hs:c2hs` |
  `cpphs` | `cpphs:cpphs` |
  `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 use of the Hooks build type .

buildable: boolean
  Default value:

    `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.

ghc-options: token list

  Additional options for GHC.

  If specifying extensions (via `-X<Extension>` flags) one can often achieve the same effect using the `default-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`.

ghc-prof-options: token list

  Additional options for GHC when the package is built with profiling enabled.

  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 `ghc-prof-options` field: use `-fno-prof-auto` or one of the other `-fprof-auto*` flags.

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 `ghc-options`, and are passed to GHC during both the compile and link phases.

ghc-prof-shared-options: token list

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

  Note that if any `ghc-shared-options` are set, the `-dynamic-too` option will never be passed to GHC, leading to all modules being compiled twice (once to generate the ``.o` files and another to generate the `.dyn_o` files).

ghcjs-options: token list

  Like `ghc-options` but applies to GHCJS

ghcjs-prof-options: token list

  Like `ghc-prof-options` but applies to GHCJS

ghcjs-shared-options: token list

  Like `ghc-shared-options` but applies to GHCJS

ghcjs-prof-shared-options: token list

  Like `ghc-prof-shared-options` but applies to GHCJS

includes: filename list
  Deprecated:

    Cabal 2.0

  Since:

    Cabal 1.0

  From GHC 6.10.1, `includes` has no effect when compiling with GHC. From Cabal 2.0, support for GHC versions before GHC 6.12 was removed.

  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 `include-dirs`.

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

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 `install-includes` should be found in relative to the top of the source tree or relative to one of the directories listed in `include-dirs`.

  `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 `install-includes` to be usable when compiling the package itself, they need to be listed in the `includes` field as well.

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 `includes` and `install-includes`.

  Directories here will be passed as `-I<dir>` flags to GHC.

c-sources: filename list

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

cxx-sources: filename list
  Since:

    Cabal 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 `cc-options` and the `cxx-options` fields. The files listed in the `cxx-sources` can reference files listed in the `c-sources` field and vice-versa. The object files will be linked appropriately.

asm-sources: filename list
  Since:

    Cabal 3.0

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

cmm-sources: filename list
  Since:

    Cabal 3.0

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

js-sources: filename list

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

extra-libraries: token list

  A list of extra libraries to link with (when not linking fully static executables). Libraries will be passed as `-optl-l<lib>` flags to GHC.

extra-libraries-static: token list

  A list of extra libraries to link with (when linking fully static executables).

extra-ghci-libraries: token list

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

extra-bundled-libraries: token list
  Since:

    Cabal 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. Library names must either be prefixed with “HS” or “C” and corresponding library file names must match:

  > - Libraries with name “HS<library-name>”:

  >     - libHS<library-name>.a

  >     - libHS<library-name>-ghc<ghc-flavour><ghc-version>.<dyn-library-extension>*

  > - Libraries with name “C<library-name>”:

  >     - libC<library-name>.a

  >     - lib<library-name>.<dyn-library-extension>*

extra-lib-dirs: directory list

  A list of directories to search for libraries (when not linking fully static executables). Directories will be passed as `-optl-L<dir>` flags to GHC.

extra-lib-dirs-static: directory list

  A list of directories to search for libraries (when linking fully static executables).

extra-library-flavours: notsure

  TBW

extra-dynamic-library-flavours: notsure

  TBW

cc-options: token list

  Command-line arguments to be passed to the Haskell compiler for the C compiling phase (as `-optc` flags for GHC). Since the arguments are compiler-dependent, this field is more useful with the setup described in the section on system-dependent parameters.

jspp-options: token list

  Command-line arguments for pre-processing JS code. Applies to pre-processed Haskell source like .js. Flags here will be passed as `-optJSP` flags to GHC.

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. Flags here will be passed as `-optP` flags to GHC.

cxx-options: token list
  Since:

    Cabal 2.2

  Command-line arguments to be passed to the Haskell compiler for the C++ compiling phase (as `-optcxx` flags for GHC). The C++ sources to which these command-line arguments should be applied can be specified with the `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 `c-sources` and `cxx-sources` fields respectively, and providing different command-line arguments with the `cc-options` and the `cxx-options` fields.

cmm-options: token list
  Since:

    Cabal 3.0

  Command-line arguments to be passed to the Haskell compiler when compiling C– code. See also `cmm-sources`.

asm-options: token list
  Since:

    Cabal 3.0

  Command-line arguments to be passed to the Haskell compiler (as `-opta` flags for GHC) when compiling assembler code. See also `asm-sources`.

ld-options: token list

  Command-line arguments to be passed to the Haskell compiler (as `-optl` flags for GHC) for the linking phase. Note that only executables (including test-suites and benchmarks) are linked so this has no effect in libraries. Since the arguments are compiler-dependent, this field is more useful with the setup described in the section on system-dependent parameters.

hsc2hs-options: token list
  Since:

    Cabal 3.6

  Command-line arguments to be passed to `hsc2hs`.

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_CONFIG_PATH`.

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.

extra-framework-dirs: directory list
  Since:

    Cabal 1.24

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

mixins: mixin list
  Since:

    Cabal 2.0

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

      Mixin ::= PackageName IncludeRenaming
      IncludeRenaming ::= ModuleRenaming { "requires" ModuleRenaming }
      ModuleRenaming ::=
          {- empty -}
        | "(" Renaming "," ... "," Renaming ")"
        | "hiding" "(" ModuleName "," ... "," ModuleName ")"
      Renaming ::=
          ModuleName
        | ModuleName "as" ModuleName

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

      library
        build-depends:
          foo ^>= 1.2.3
        mixins:
          foo

  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:

      library
        mixins:
          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:

      library:
        mixins:
          foo hiding (Foo.Bar)

  Hiding modules exposes everything that is not explicitly hidden.

  Note

  Cabal files with `cabal-version` < 3.0 suffer from an infelicity in how the entries of `mixins` are parsed: an entry will fail to parse if the provided renaming clause has whitespace after the opening parenthesis.

  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:

      library
        mixins:
          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:

      library
        mixins:
          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:

      library
        mixins:
          sigonly requires (SigOnly.SomeSig as AnotherSigOnly.SomeSig)

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

  Mixin packages are part of the How to use Backpack modules extension to the Haskell module system.

  The matching of the module signatures required by a `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

  How to use Backpack modules has the limitation that implementation modules that instantiate signatures required by a `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.

### 1.2.8. 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
                           MyForeignLib.SomeOtherModule
      build-depends:       base >=4.7 && <4.9
      hs-source-dirs:      src
      c-sources:           csrc/MyForeignLibWrapper.c
      default-language:    Haskell2010

foreign-library name
  Since:

    Cabal 2.0

  Build information for foreign libraries.

type: foreign library type

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

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.

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.

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.

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 `libfoo.so.4.2.3` to be built with SONAME `libfoo.so.4`, and appropriate symlinks `libfoo.so.4` and `libfoo.so` to be installed.

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){
      hs_exit();
    }

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 `~/.local/lib`.

### 1.2.9. Configurations

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 a certain feature, that can be switched on or off by the package user. Here is an example package description file using configurations:

#### 1.2.9.1. Example: A package containing a library and executable programs

    Cabal-Version: 3.0
    Name: Test1
    Version: 0.0.1
    License: BSD-3-Clause
    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
      -- assign automatically while searching for a solution

    Library
      Build-Depends:      base >= 4.2 && < 4.9
      Exposed-Modules:    Testing.Test1
      Default-Extensions: CPP
      Default-Language:   Haskell2010

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

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

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

    Executable test1
      Main-is:          T1.hs
      Build-Depends:
        , base >= 4.2 && < 4.9
        , Test1
      Default-Language: Haskell2010

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

#### 1.2.9.2. Layout

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:

#### 1.2.9.3. Example: Using explicit braces rather than indentation for layout

    Cabal-Version: 3.0
    Name: Test1
    Version: 0.0.1
    License: BSD-3-Clause
    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
      Default-Extensions:  CPP
      Default-language:    Haskell2010
      if flag(debug) {
        CPP-Options: -DDEBUG
        if !os(windows) {
          CC-Options: "-DDEBUG"
        } else {
          CC-Options: "-DNDEBUG"
        }
      }
    }

#### 1.2.9.4. Configuration Flags

flag name

  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:

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

      UALNUM = UALPHA / DIGIT
      UALPHA = ... ; set of alphabetic Unicode code-points

  Note

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

description: freeform

  The description of this flag.

default: boolean
  Default value:

    `True`

  The default value of this flag.

  Note

  This value may be overridden in several ways. 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.

manual: boolean
  Default value:

    `False`

  Since:

    1.6

    By default, Cabal tries to resolve dependencies using the flag’s `default` value. If that fails, it tries again with the negated default value. However, if the flag is marked as `manual`, Cabal will only use the default value and will not retry with the negated default. Note that the default value can still be overridden using command-line flags.

### 1.2.10. Conditional Blocks

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

    if condition
       property-descriptions-or-conditionals

or

    if condition
         property-descriptions-or-conditionals
    else
         property-descriptions-or-conditionals

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
         property-descriptions-or-conditionals
    elif condition2
         property-descriptions-or-conditionals
    else
         property-descriptions-or-conditionals

#### 1.2.10.1. Conditions

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.

`os(name)`

  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.

`arch(name)`

  Tests if the current architecture is name. name should be the name of one of the nullary constructors of `Distribution.System.Arch` (e.g. `x86_64`, `aarch64` or `i386`), otherwise it will be treated as an ‘other architecture’ of the given name. It will be compared with `Distribution.System.buildArch`, which is derived from `System.Info.arch` (certain architectures are treated as synonymous; e.g. `aarch64` / `arm64` or `powerpc64` / `powerpc64le` are not distinguished). For a match, this test evaluates to true, otherwise false. The match is case-insensitive.

`impl(compiler)`

  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.

`flag(name)`

  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.

`true`

  Constant value true.

`false`

  Constant value false.

#### 1.2.10.2. 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 `build-depends` field in conditional blocks that determine if a particular flag assignment is satisfiable (`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.

### 1.2.11. 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 is declared 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
      else
        Main-is: Main.hs

### 1.2.12. Common stanzas

common name
  Since:

    Cabal 2.2

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

    common deps
      build-depends: base ^>= 4.18
      ghc-options: -Wall

    common test-deps
      build-depends: tasty ^>= 1.4

    library
      import:           deps
      exposed-modules:  Foo
      default-language: Haskell2010

    test-suite tests
      import:           deps, test-deps
      type:             exitcode-stdio-1.0
      main-is:          Tests.hs
      build-depends:    foo
      default-language: Haskell2010

- 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.

Note

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

import: token-list

  TBW

## 1.3. Source code repository marker

source-repository
  Since:

    Cabal 1.6

A marker that points to the source code for this package within a source code repository.

There are two kinds. You can specify one or the other or both at once:

- 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.

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:     git
      location: https://github.com/haskell/cabal

    source-repository this
      type:     git
      location: https://github.com/haskell/cabal
      tag:      1.6.1

The cabal get command uses the kind of repository with its `--source-repository` option, if provided.

The VCS fields of `source-repository` are:

type: VCS kind

  This field is required.

  Cabal supports specifying different information for various common source control systems. This is the name of the source control system used for a repository. The currently recognised types are:

  - `darcs`

  - `git`

  - `svn`

  - `cvs`

  - `mercurial` (or alias `hg`)

  - `bazaar` (or alias `bzr`)

  - `arch`

  - `monotone`

  - `pijul`

  The VCS kind will determine what other fields are appropriate to specify for a particular version control system.

location: VCS location

  This field is required.

  The location of the repository, usually a URL but the exact form of this field depends on the repository type. For example:

  - for Darcs: `http://code.haskell.org/foo/`

  - for Git: `https://github.com/foo/bar.git`

  - for CVS: `anoncvs@cvs.foo.org:/cvs`

module: token

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

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

branch: VCS branch

  This field is optional.

  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 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.

tag: VCS tag

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

  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.

  A tag identifies a particular state of a source repository. The exact form of the tag depends on the repository type.

subdir: VCS subdirectory

  This field is optional but, if given, specifies a single subdirectory.

  A field of this type is always optional because it defaults to empty, which corresponds to the root directory of the repository and is the same as specifying `.` explicitly.

  Some projects put the sources for multiple packages inside a single VCS 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` file.

## 1.4. Hooks

The `Hooks` build type allows customising the configuration and the building of a package using a collection of hooks into the build system.

Introduced in Cabal 3.14, this build type provides an alternative to Custom setups which integrates better with the rest of the Haskell ecosystem.

To use this build type in your package, you need to:

> - Declare a `cabal-version` of at least 3.14 in your `.cabal` file.

> - Declare `build-type: Hooks` in your `.cabal` file.

> - Include a `custom-setup` stanza in your `.cabal` file, which declares the version of the Hooks API your package is using.

> - Define a `SetupHooks.hs` module next to your `.cabal` file. It must export a value `setupHooks :: SetupHooks`.

More specifically, your `.cabal` file should resemble the following:

>     cabal-version: 3.14
>     build-type: Hooks

>     custom-setup:
>       setup-depends:
>         base        >= 4.18 && < 5,
>         Cabal-hooks >= 0.1  && < 0.2

while a basic `SetupHooks.hs` file might look like the following:

>     module SetupHooks where
>     import Distribution.Simple.SetupHooks ( SetupHooks, noSetupHooks )

>     setupHooks :: SetupHooks
>     setupHooks =
>      noSetupHooks
>        { configureHooks = myConfigureHooks
>        , buildHooks = myBuildHooks }

>     -- ...

Refer to the Hackage documentation for the Distribution.Simple.SetupHooks module for an overview of the `Hooks` API. Further motivation and a technical overview of the design is available in Haskell Tech Proposal #60 .

## 1.5. Custom setup scripts

Deprecated since Cabal 3.14: prefer using the Hooks build type instead.

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 to this article for more details.

As of Cabal library version 3.0, `defaultMain*` variants implement support for response files. Custom `Setup.hs` files that do not use one of these main functions are required to implement their own support, such as by using `GHC.ResponseFile.getArgsWithResponseFiles`.

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

custom-setup
  Since:

    Cabal 1.24

  A `custom-setup` stanza is required for `Custom` and `Hooks` `build-type`, and will be ignored (with a warning) for other build types.

  The stanza contains information needed for the compilation of custom `Setup.hs` scripts, and of `SetupHooks.hs` hooks. For example:

    custom-setup
      setup-depends:
        base   >= 4.18 && < 5,
        Cabal  >= 3.10

setup-depends: package list
  Since:

    Cabal 1.24

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

  If the field is not specified the implicit package set will be used. The package set contains packages bundled with GHC (i.e. `base`, `bytestring`) and specifically `Cabal`. The specific bounds are put on `Cabal` dependency: lower-bound is inferred from `cabal-version`, and the upper-bound is `< 1.25`.

  `Cabal` version is additionally restricted by GHC, with absolute minimum being `1.20`, and for example `Custom` builds with GHC-8.10 require at least `Cabal-3.2`.

### 1.5.1. 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` stanza has been declared (or `cabal 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:

    {-# LANGUAGE CPP #-}
    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
    #else
    # 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.
    #endif

    main = ...

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

    {-# LANGUAGE CPP #-}
    import Distribution.Simple

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

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

    main = ...

## 1.6. Autogenerated modules and includes

Modules that are built automatically at setup, created with a custom setup script, must appear on `other-modules` for the library, executable, test-suite or benchmark stanzas or also on `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 `autogen-modules` field of the stanza that is using them, besides `other-modules` or `library:exposed-modules`. With this there is no need to create complex build hooks for this poweruser case.

autogen-modules: module list
  Since:

    Cabal 2.0

  Todo

  document autogen-modules field

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

    Library
        default-language: Haskell2010
        build-depends: base
        exposed-modules:
            MyLibrary
            MyLibHelperModule
        other-modules:
            MyLibModule
        autogen-modules:
            MyLibHelperModule

    Executable Exe
        default-language: Haskell2010
        main-is: Dummy.hs
        build-depends: base
        other-modules:
            MyExeModule
            MyExeHelperModule
        autogen-modules:
            MyExeHelperModule

autogen-includes: filename list
  Since:

    Cabal 3.0

  A list of header files from this package which are autogenerated (e.g. by a `configure` script). Autogenerated header files are not packaged by `sdist` command.

## 1.7. Accessing data files from package code

The placement on the target system of files listed in the `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 `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, the result is the name of a corresponding file on the system on which the program is running, if the filename were listed in the `data-files` field. No check is performed that the given filename is listed in that field.

Note

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

The `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 `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.

### 1.7.1. Accessing the package version

The auto generated `PackageInfo_pkgname` module exports the constant `version ::` Version which is defined as the version of your package as specified in the `version` field.

### 1.7.2. Accessing package-related information

The auto generated `PackageInfo_pkgname` module exports the following package-related constants:

    name :: String
    version :: Version
    synopsis :: String
    copyright :: String
    homepage :: String

Unlike `Paths_pkgname` (see <#accessing-data-files-from-package-code>), `PackageInfo_pkgname` is system- and path-independent. It aims to be easier to work with for hash-based tools such as Nix.

## 1.8. 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 `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.

Note that the package’s `extra-source-files` are available to the `configure` script when it is executed. In typical `autoconf` fashion, `--host` flag will be passed to the `configure` script to indicate the host platform when cross-compiling. Moreover, various bits of build configuration will be passed via environment variables:

> - `CC` will reflect the path to the C compiler

> - `CXX` will reflect the path to the C++ compiler (if available, it always will be if ghc >= 9.4)

> - `CFLAGS` will reflect the flags to the C compiler

> - `CXXFLAGS` will reflect the flags to the C++ compiler (if available)

> - `CABAL_FLAGS` will contain the Cabal flag assignment of the current package using traditional Cabal flag syntax (e.g. `+flagA -flagB`)

> - `CABAL_FLAG_<flag>` will be set to either `0` or `1` depending upon whether flag `<flag>` is enabled. Note that any any non-alpha-numeric characters in the flag name are replaced with `_`.

The build information file should have the following structure:

> buildinfo

> `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`.

### 1.8.1. Example: Using autoconf

This example is for people familiar with the autoconf tools.

In the X11 package, the file `configure.ac` contains:

    AC_INIT([Haskell X11 package], [1.1], [libraries@haskell.org], [X11])

    # Safety check: Ensure that we are in the correct source directory.
    AC_CONFIG_SRCDIR([X11.cabal])

    # Header file to place defines in
    AC_CONFIG_HEADERS([include/HsX11Config.h])

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

    # Build the package if we found X11 stuff
    if test "$no_x" = yes
    then BUILD_PACKAGE_BOOL=False
    else BUILD_PACKAGE_BOOL=True
    fi
    AC_SUBST([BUILD_PACKAGE_BOOL])

    AC_CONFIG_FILES([X11.buildinfo])
    AC_OUTPUT

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 `X11.buildinfo.in`:

    buildable: @BUILD_PACKAGE_BOOL@
    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.

Note

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 `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 `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 `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 `HsX11Config.h.in`, 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.

## 1.9. 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 `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 ...
    #else
    ... code that works with base-3 ...
    #endif

In general, Cabal supplies a macro `MIN_VERSION_```package```_(A,B,C)` for each package depended on via `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.

## 1.10. More complex packages

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

- By using the `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 `custom-setup` stanza with a `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 `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 `build-type` `Custom`, you can also write your own setup script from scratch, 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.

Footnotes

[ 1 ]

Some packages (ab)use `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.