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 the accompanying library documentation), 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:

A package may contain several Haskell modules. A package may span many directories, or many packages may exist in a single directory. Packages may not be mutually recursive.
A package has a name (e.g. base)
The Haskell code in a package may be built into one or more archive libraries (e.g. libHSfoo.a), or a single DLL on Windows (e.g. HSfoo.dll). The restriction to a single DLL on Windows is because the package system is used to tell the compiler when it should make an inter-DLL call rather than an intra-DLL call (inter-DLL calls require an extra indirection). Building packages as DLLs doesn't work at the moment; see Section 11.3 for the gory details.
Building a static library is done by using the ar tool, like so:
ar cqs libHSfoo.a A.o B.o C.o ...
where A.o, B.o and so on are the compiled Haskell modules, and libHSfoo.a is the library you wish to create. The syntax may differ slightly on your system, so check the documentation if you run into difficulties.
Versions of the Haskell libraries for use with GHCi may also be included: GHCi cannot load .a files directly, instead it will look for an object file called HSfoo.o and load that. On some systems, the ghc-pkg tool can automatically build the GHCi version of each library, see Section 4.10.4. To build these libraries by hand from the .a archive, it is possible to use GNU ld as follows:
ld -r ––whole-archive -o HSfoo.o libHSfoo.a
GHC does not maintain detailed cross-package dependency information. It does remember which modules in other packages the current module depends on, but not which things within those imported things.

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, updated 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 package specification from the input (see below), 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.

––auto-ghci-libs, -g

Automatically generate the GHCi .o version of each .a Haskell library, using GNU ld (if that is available). Without this option, ghc-pkg will warn if GHCi versions of any Haskell libraries in the package don't exist.

GHCi .o libraries don't necessarily have to live in the same directory as the corresponding .a library. However, this option will cause the GHCi library to be created in the same directory as the .a library.

––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.

––update-package, -u

Reads package specification from the input, and adds it to the database of installed packages. If a package with the same name is already installed, its configuration data is replaced with the new information. If the package doesn't already exist, it's added.

––force

Causes ghc-pkg to ignore missing directories and libraries when adding a package, and just go ahead and add it anyway. This might be useful if your package installation system needs to add the package to GHC before building and installing the files.

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     = ["${installdir}/imports/mypkg"],
     source_dirs     = [],
     library_dirs    = ["${installdir}"],
     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:

name

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

import_dirs

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

If the package contains profiling libraries, then the interface files for those library modules should have the suffix .p_hi. So the package can contain both normal and profiling versions of the same library without conflict (see also library_dirs below).

source_dirs

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.

library_dirs

A list of directories containing libraries for this package.

hs_libraries

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:

libHSfoo.a: The name of the library on Unix and Windows (mingw) systems. Note that we don't support building dynamic libraries of Haskell code on Unix systems.
HSfoo.dll: The name of the dynamic library on Windows systems (optional).
HSfoo.o, HSfoo.obj: The object version of the library used by GHCi.

extra_libraries

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.

The libraries listed in extra_libraries may be any libraries supported by your system's linker, including dynamic libraries (.so on Unix, .DLL on Windows).

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.

include_dirs

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

c_includes

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.

package_deps

A list of packages which this package depends on.

extra_ghc_opts

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

extra_cc_opts

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

extra_ld_opts

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

framework_dirs

On Darwin/MacOS X, a list of directories containing frameworks for this package. This corresponds to the -framework-path option. It is ignored on all other platforms.

extra_frameworks

On Darwin/MacOS X, a list of frameworks to link to. This corresponds to the -framework option. Take a look at Apple's developer documentation to find out what frameworks actually are. This entry is ignored on all other platforms.

The ghc-pkg tool performs expansion of environment variables occurring in input package specifications. So, if the mypkg was added to the package database as follows:

  $ installdir=/usr/local/lib ghc-pkg -a < mypkg.pkg

The occurrence of ${installdir} is replaced with /usr/local/lib in the package data that is added for mypkg.

This feature enables the distribution of package specification files that can be easily configured when installing.

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