4.10. Packages

Packages are collections of libraries, conveniently grouped together as a single entity. The package system is flexible: a package may consist of Haskell code, foreign language code (eg. C libraries), or a mixture of the two. A package is a good way to group together related Haskell modules, and is essential if you intend to make the modules into a Windows DLL (see below).

Because packages can contain both Haskell and C libraries, they are also a good way to provide convenient access to a Haskell layer over a C library.

GHC comes with several packages (see Haskell Libraries), and packages can be added to or removed from an existing GHC installation, using the supplied ghc-pkg tool, described in Section 4.10.4.

4.10.1. Using a package

To use a package, add the -package flag to the GHC command line:

-package lib

This option brings into scope all the modules from package lib (they still have to be imported in your Haskell source, however). It also causes the relevant libraries to be linked when linking is being done.

Some packages depend on other packages, for example the text package makes use of some of the modules in the lang package. The package system takes care of all these dependencies, so that when you say -package text on the command line, you automatically get -package lang too.

4.10.2. Maintaining a local set of packages

When GHC starts up, it automatically reads the default set of packages from a configuration file, normally named package.conf in your GHC installation directory.

You can load in additional package configuration files using the -package-conf option:

-package-conf file

Read in the package configuration file file in addition to the system default file. This allows the user to have a local set of packages in addition to the system-wide ones.

To create your own package configuration file, just create a new file and put the string "[]" in it. Packages can be added to the new configuration file using the ghc-pkg tool, described in Section 4.10.4.

4.10.3. Building a package from Haskell source

It takes some special considerations to build a new package:

To compile a module which is to be part of a new package, use the -package-name option:

-package-name foo

This option is added to the command line when compiling a module that is destined to be part of package foo. If this flag is omitted then the default package Main is assumed.

Failure to use the -package-name option when compiling a package will result in disaster on Windows, but is relatively harmless on Unix at the moment (it will just cause a few extra dependencies in some interface files). However, bear in mind that we might add support for Unix shared libraries at some point in the future.

It is worth noting that on Windows, when each package is built as a DLL, since a reference to a DLL costs an extra indirection, intra-package references are cheaper than inter-package references. Of course, this applies to the Main package as well.

4.10.4. Package management

The ghc-pkg tool allows packages to be added or removed from a package configuration file. By default, the system-wide configuration file is used, but alternatively packages can be added or removed from a user-specified configuration file using the ––config-file option. An empty package configuration file consists of the string "[]".

The ghc-pkg program accepts the following options:

––add-package, -a

Reads a package specification (see below) on stdin, and adds it to the database of installed packages. The package specification must be a package that isn't already installed.

––input-file=file, -i file

Read new package specifications from file file. If a value of "-" is given, standard input is used. If no -i is present on the command-line, an input file of "-" is assumed.

––config-file file, -f file

Use file instead of the default package configuration file. This, in conjunction with GHC's -package-conf option, allows a user to have a local set of packages in addition to the system-wide installed set.

––list-packages, -l

This option displays the list of currently installed packages.

  $ ghc-pkg ––list-packages
  gmp, rts, std, lang, concurrent, data, net, posix, text, util

Note that your GHC installation might have a slightly different set of packages installed.

The gmp and rts packages are always present, and represent the multi-precision integer and runtime system libraries respectively. The std package contains the Haskell prelude and standard libraries. The rest of the packages are optional libraries.

––remove-package foo, -r foo

Removes the specified package from the installed configuration.

When modifying the configuration file file, a copy of the original file is saved in file.old, so in an emergency you can always restore the old settings by copying the old file back again.

A package specification looks like this:

  Package {
     name            = "mypkg",
     import_dirs     = ["/usr/local/lib/imports/mypkg"],
     source_dirs     = [],
     library_dirs    = ["/usr/local/lib"],
     hs_libraries    = ["HSmypkg" ],
     extra_libraries = ["HSmypkg_cbits"],
     include_dirs    = [],
     c_includes      = ["HsMyPkg.h"],
     package_deps    = ["text", "data"],
     extra_ghc_opts  = [],
     extra_cc_opts   = [],
     extra_ld_opts   = ["-lmy_clib"]

Components of a package specification may be specified in any order, and are:


The package's name, for use with the -package flag and as listed in the ––list-packages list.


A list of directories containing interface files (.hi files) for this package.


A list of directories containing Haskell source files for this package. This field isn't used by GHC, but could potentially be used by an all-interpreted system like Hugs.


A list of directories containing libraries for this package.


A list of libraries containing Haskell code for this package, with the .a or .dll suffix omitted. When packages are built as libraries, the lib prefix is also omitted.

For use with GHCi, each library should have an object file too. The name of the object file does not have a lib prefix, and has the normal object suffix for your platform.

For example, if we specify a Haskell library as HSfoo in the package spec, then the various flavours of library that GHC actually uses will be called:


The name of the library on Unix systems.


The name of the dynamic library on Windows systems.

HSfoo.o, HSfoo.obj

The object version of the library used by GHCi.


A list of extra libraries for this package. The difference between hs_libraries and extra_libraries is that hs_libraries normally have several versions, to support profiling, parallel and other build options. The various versions are given different suffixes to distinguish them, for example the profiling version of the standard prelude library is named libHSstd_p.a, with the _p indicating that this is a profiling version. The suffix is added automatically by GHC for hs_libraries only, no suffix is added for libraries in extra_libraries.

Also, extra_libraries are placed on the linker command line after the hs_libraries for the same package. If your package has dependencies in the other direction (i.e. extra_libraries depends on hs_libraries), and the libraries are static, you might need to make two separate packages.


A list of directories containing C includes for this package (maybe the empty list).


A list of files to include for via-C compilations using this package. Typically this include file will contain function prototypes for any C functions used in the package, in case they end up being called as a result of Haskell functions from the package being inlined.


A list of packages which this package depends on.


Extra arguments to be added to the GHC command line when this package is being used.


Extra arguments to be added to the gcc command line when this package is being used (only for via-C compilations).


Extra arguments to be added to the gcc command line (for linking) when this package is being used.

For examples of more package specifications, take a look at the package.conf in your GHC installation.