Developing packages

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

However having both a library and executables in a package does not work very well; if the executables depend on the library, they must explicitly list all the modules they directly or indirectly import from that library. Fortunately, starting with Cabal 1.8.0.4, executables can also declare the package that they are in as a dependency, and Cabal will treat them as if they were in another package that dependended on the library.

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

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

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.

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.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 building and installing packages). This module should import only modules that will be present in all Haskell implementations, including modules of the Cabal library. In most cases it will be trivial, calling on the Cabal library to do most of the work.

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

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

name:     Foo
version:  1.0

library
  build-depends:   base
  exposed-modules: Foo
  extensions:      ForeignFunctionInterface
  ghc-options:     -Wall
  nhc98-options:   -K4m
  if os(windows)
    build-depends: Win32

Example: A package containing a simple library

The HUnit package contains a file HUnit.cabal containing:

name:           HUnit
version:        1.1.1
synopsis:       A unit testing framework for Haskell
homepage:       http://hunit.sourceforge.net/
category:       Testing
author:         Dean Herington
license:        BSD3
license-file:   LICENSE
cabal-version:  >= 1.10
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

and the following Setup.hs:

import Distribution.Simple
main = defaultMain

Example: A package containing executable programs

name:           TestPackage
version:        0.0
synopsis:       Small package with two programs
author:         Angela Author
license:        BSD3
build-type:     Simple
cabal-version:  >= 1.2

executable program1
  build-depends:  HUnit
  main-is:        Main.hs
  hs-source-dirs: prog1

executable program2
  main-is:        Main.hs
  build-depends:  HUnit
  hs-source-dirs: prog2
  other-modules:  Utils

with Setup.hs the same as above.

Example: A package containing a library and executable programs

name:            TestPackage
version:         0.0
synopsis:        Package with library and two programs
license:         BSD3
author:          Angela Author
build-type:      Simple
cabal-version:   >= 1.2

library
  build-depends:   HUnit
  exposed-modules: A, B, C

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

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

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

The trivial setup script used in these examples uses the simple build infrastructure provided by the Cabal library (see Distribution.Simple). The simplicity lies in its interface rather that its implementation. It automatically handles preprocessing with standard preprocessors, and builds packages for all the Haskell implementations (except nhc98, for now).

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

Package descriptions

The package description file must have a name ending in “.cabal”. It must be a Unicode text file encoded using valid UTF-8. There must be exactly one such file in the directory. The first part of the name is usually the package name, and some of the tools that operate on Cabal packages require this.

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

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

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

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. 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, NHC, YHC, Hugs, HBC, Helium, JHC, or LHC) followed by a version range. For example, GHC ==6.10.3, or LHC >=0.6 && <0.8.

Modules and preprocessors

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

When building, Cabal will automatically run the appropriate preprocessor and compile the Haskell module it produces.

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.

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.

version: numbers (required)

The package version number, usually consisting of a sequence of natural numbers separated by dots.

cabal-version: >= x.y

The version of the Cabal specification that this package description uses. The Cabal specification does slowly evolve, intoducing new features and occasionally changing the meaning of existing features. By specifying which version of the spec you are using it enables programs which process the package description to know what syntax to expect and what each part means.

For historical reasons this is always expressed using >= version range syntax. No other kinds of version range make sense, in particular upper bounds do not make sense. In future this field will specify just a version number, rather than a version range.

The version number you specify will affect both compatability and behaviour. Most tools (including the Cabal libray and 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. Most of the time, tools that are too old will recognise this fact and produce a suitable error message.

As for behaviour, new versions of the Cabal spec 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 compatability 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.

build-type: identifier

The type of build used by this package. Build types are the constructors of the BuildType type, defaulting to Custom. If this field is given a value other than Custom, some tools such as cabal-install will be able to build the package without using the setup script. So if you are just using the default Setup.hs then set the build type as Simple.

license: identifier (default: AllRightsReserved)

The type of license under which this package is distributed. License names are the constants of the License type.

license-file: filename

The name of a file containing the precise license for this package. It will be installed with the package.

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:

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

bug-reports: http://hackage.haskell.org/trac/hackage/
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 setup 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).

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.

The limitation is that * wildcards are only allowed in place of the file name, not in the directory name or file extension. In particular, wildcards do not include directories contents recursively. Furthermore, if a wildcard is used it must be used with an extension, so data-files: data/* is not allowed. When matching a wildcard plus extension, a file’s full extension must match exactly, so *.gz matches foo.gz but not foo.tar.gz. 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.

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 setup sdist. As with data-files it can use a limited form of * wildcards in file names.

extra-html-files: filename list

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 setup clean. These would typically be additional files created by additional hooks, such as the scheme described in the section on system-dependent parameters.

Library

The library section should contain the following fields:

exposed-modules: identifier list (required if this package contains a library)

A list of modules added by this package.

exposed: boolean (default: True)

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

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

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

Opening an interpreter session

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

cabal repl

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

cabal repl foo
cabal repl exe:foo
cabal repl test:bar
cabal repl bench:baz

Executables

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)
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.

Running executables

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

$ cabal run EXECUTABLE [-- EXECUTABLE_FLAGS]

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

Test suites

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

Test suites using the exitcode-stdio-1.0 interface are executables that indicate test failure with a non-zero exit code when run; they may provide human-readable log information through the standard output and error channels. This interface is provided primarily for compatibility with existing test suites; it is preferred that new test suites be written for the detailed-1.0 interface. The exitcode-stdio-1.0 type requires the main-is field.

main-is: filename (required: exitcode-stdio-1.0, disallowed: detailed-1.0)
The name of the .hs or .lhs file containing the Main module. Note that it is the .hs filename that must be listed, even if that file is generated using a preprocessor. The source file must be relative to one of the directories listed in hs-source-dirs. This field is analogous to the main-is field of an executable section.

Test suites using the detailed-1.0 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-1.0 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-1.0 interface requires the test-module field.

test-module: identifier (required: detailed-1.0, disallowed: exitcode-stdio-1.0)
The module exporting the tests symbol.

Example: Package using exitcode-stdio-1.0 interface

The example package description and executable source file below demonstrate the use of the exitcode-stdio-1.0 interface. For brevity, the example package does not include a library or any normal executables, but a real package would be required to have at least one library or executable.

foo.cabal:

Name:           foo
Version:        1.0
License:        BSD3
Cabal-Version:  >= 1.9.2
Build-Type:     Simple

Test-Suite test-foo
    type:       exitcode-stdio-1.0
    main-is:    test-foo.hs
    build-depends: base

test-foo.hs:

module Main where

import System.Exit (exitFailure)

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

Example: Package using detailed-1.0 interface

The example package description and test module source file below demonstrate the use of the detailed-1.0 interface. For brevity, the example package does note include a library or any normal executables, but a real package would be required to have at least one library or executable. The test module below 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:

Name:           bar
Version:        1.0
License:        BSD3
Cabal-Version:  >= 1.9.2
Build-Type:     Simple

Test-Suite test-bar
    type:       detailed-1.0
    test-module: Bar
    build-depends: base, Cabal >= 1.9.2

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
        }

Running test suites

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

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

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

Benchmarks

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

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

main-is: filename (required: exitcode-stdio-1.0)
The name of the .hs or .lhs file containing the Main module. Note that it is the .hs filename that must be listed, even if that file is generated using a preprocessor. The source file must be relative to one of the directories listed in hs-source-dirs. This field is analogous to the main-is field of an executable section.

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. For brevity, the example package does not include a library or any normal executables, but a real package would be required to have at least one library or executable.

foo.cabal:

Name:           foo
Version:        1.0
License:        BSD3
Cabal-Version:  >= 1.9.2
Build-Type:     Simple

Benchmark bench-foo
    type:       exitcode-stdio-1.0
    main-is:    bench-foo.hs
    build-depends: base, time

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)

Running benchmarks

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

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

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

Build information

The following fields may be optionally present in a library or executable 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: package list

A list of packages needed to build this one. Each package can be annotated with a version constraint.

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

library
  build-depends:
    base >= 2,
    foo >= 1.2 && < 1.3,
    bar

Dependencies like foo >= 1.2 && < 1.3 turn out to be very common because it is recommended practise for package versions to correspond to API versions. As of Cabal 1.6, there is a special syntax to support this use:

build-depends: foo ==1.2.*

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

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.

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, exposed-modules or main-is fields.

hs-source-dirs: directory list (default: “.”)

Root directories for the module hierarchy.

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

extensions: identifier list

A list of Haskell extensions used by every module. Extension names are the constructors of the Extension type. These determine corresponding compiler options. In particular, CPP specifies that Haskell source files are to be preprocessed with a C preprocessor.

Extensions used only by one module may be specified by placing a LANGUAGE pragma in the source file affected, e.g.:

{-# LANGUAGE CPP, MultiParamTypeClasses #-}

Note: GHC versions prior to 6.6 do not support the LANGUAGE pragma.

build-tools: program list

A list of programs, possibly annotated with versions, needed to build this package, e.g. c2hs >= 0.15, cpphs.If no version constraint is specified, any version is assumed to be acceptable.

buildable: boolean (default: True)

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

ghc-options: token list

Additional options for GHC. You can often achieve the same effect using the 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.

ghc-prof-options: token list

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

ghc-shared-options: token list

Additional options for GHC when the package is built as shared library.

hugs-options: token list

Additional options for Hugs. You can often achieve the same effect using the extensions field, which is preferred.

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

nhc98-options: token list

Additional options for nhc98. You can often achieve the same effect using the extensions field, which is preferred.

Options required only by one module may be specified by placing an OPTIONS_NHC98 pragma in the source file affected.

includes: filename list

A list of header files to be included in any compilations via C. This field applies to both header files that are already installed on the system and to those coming with the package to be installed. These files typically contain function prototypes for foreign imports used by the package.

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. Note that to include them 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, ffihugs, cpphs or the C preprocessor, and also when compiling via C.

c-sources: filename list

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

If you use this field, you should also name the C files in CFILES pragmas in the Haskell source files that use them, e.g.: {-# CFILES dir/file1.c dir/file2.c #-} These are ignored by the compilers, but needed by Hugs.

extra-libraries: token list

A list of extra libraries to link with.

extra-lib-dirs: directory list

A list of directories to search for libraries.

cc-options: token list

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

ld-options: token list

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

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 (use pkg-config --list-all to find out if it is supported) then it is much preferable to use this field rather than hard code options into the other fields.

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.

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

Example: A package containing a library and executable programs

Name: Test1
Version: 0.0.1
Cabal-Version: >= 1.2
License: BSD3
Author:  Jane Doe
Synopsis: Test package to test configurations
Category: Example

Flag Debug
  Description: Enable debug support
  Default:     False

Flag WebFrontend
  Description: Include API for web frontend.
  -- Cabal checks if the configuration is possible, first
  -- with this flag set to True and if not it tries with False

Library
  Build-Depends:   base
  Exposed-Modules: Testing.Test1
  Extensions:      CPP

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

  if flag(webfrontend)
    Build-Depends: cgi > 0.42
    Other-Modules: Testing.WebStuff

Executable test1
  Main-is: T1.hs
  Other-Modules: Testing.Test1
  Build-Depends: base

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

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:

Example: Using explicit braces rather than indentation for layout

Name: Test1
Version: 0.0.1
Cabal-Version: >= 1.2
License: BSD3
Author:  Jane Doe
Synopsis: Test package to test configurations
Category: Example

Flag Debug {
  Description: Enable debug support
  Default:     False
}

Library {
  Build-Depends:   base
  Exposed-Modules: Testing.Test1
  Extensions:      CPP
  if flag(debug) {
    GHC-Options: -DDEBUG
    if !os(windows) {
      CC-Options: "-DDEBUG"
    } else {
      CC-Options: "-DNDEBUG"
    }
  }
}

Configuration Flags

A flag section takes the flag name as an argument and may contain the following fields.

description: freeform

The description of this flag.

default: boolean (default: True)

The default value of this flag.

Note that 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: False)

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

Conditional Blocks

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

  `if `_condition_
       _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.

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. The argument is matched against System.Info.arch on the target system. If the arch names match, this test evaluates to true, otherwise false. The match is case-insensitive.

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.

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.

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.

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.

Meaning of field values when using conditionals

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

Source Repositories

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

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

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

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

source-repository head
  type:     darcs
  location: http://darcs.haskell.org/cabal/

source-repository this
  type:     darcs
  location: http://darcs.haskell.org/cabal-branches/cabal-1.6/
  tag:      1.6.1

The exact fields are as follows:

type: token

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

This field is required.

location: URL

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

This field is required.

module: token

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

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

branch: token

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

This field is optional.

tag: token

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

This field is required for the this repo kind.

subdir: directory

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

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

Downloading a package’s source

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

$ cabal get [FLAGS] PACKAGES

The get command supports the following options:

-d --destdir PATH

Where to place the package source, defaults to (a subdirectory of) the current directory.

-s --source-repository [head|this|…]

Fork the package’s source repository using the appropriate version control system. The optional argument allows to choose a specific repo kind.

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 listed in the data-files field, the result is the name of the corresponding file on the system on which the program is running.

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.

The Paths_pkgname module is not platform independent so it does not get included in the source tarballs generated by sdist.

Accessing the package version

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

System-dependent parameters

For some packages, especially those interfacing with C libraries, implementation details and the build procedure depend on the build environment. A variant of the simple build infrastructure (the build-type Configure) handles many such situations using a slightly longer Setup.hs:

import Distribution.Simple
main = defaultMainWithHooks autoconfUserHooks

Most packages, however, would probably do better with configurations.

This program differs from defaultMain in two ways:

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.

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, and extra documentation resources in the extra-html-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.

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 depenency 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).

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.

More complex packages

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