5.9. Packages

A package is a library of Haskell modules known to the compiler. GHC comes with several packages: see the accompanying library documentation. More packages to install can be obtained from HackageDB.

Using a package couldn’t be simpler: if you’re using --make or GHCi, then most of the installed packages will be automatically available to your program without any further options. The exceptions to this rule are covered below in Using Packages.

Building your own packages is also quite straightforward: we provide the Cabal infrastructure which automates the process of configuring, building, installing and distributing a package. All you need to do is write a simple configuration file, put a few files in the right places, and you have a package. See the Cabal documentation for details, and also the Cabal libraries (Distribution.Simple, for example).

5.9.1. Using Packages

GHC only knows about packages that are installed. Installed packages live in package databases. For details on package databases and how to control which package databases or specific set of packages are visible to GHC, see Package Databases.

To see which packages are currently available, use the ghc-pkg list command:

$ ghc-pkg list

An installed package is either exposed or hidden by default. Packages hidden by default are listed in parentheses (e.g. (lang-1.0)), or possibly in blue if your terminal supports colour, in the output of ghc-pkg list. Command-line flags, described below, allow you to expose a hidden package or hide an exposed one. Only modules from exposed packages may be imported by your Haskell code; if you try to import a module from a hidden package, GHC will emit an error message. It should be noted that a hidden package might still get linked with your program as a dependency of an exposed package, it is only restricted from direct imports.

If there are multiple exposed versions of a package, GHC will prefer the latest one. Additionally, some packages may be broken: that is, they are missing from the package database, or one of their dependencies are broken; in this case; these packages are excluded from the default set of packages.


If you’re using Cabal, then the exposed or hidden status of a package is irrelevant: the available packages are instead determined by the dependencies listed in your .cabal specification. The exposed/hidden status of packages is only relevant when using ghc or ghci directly.

Similar to a package’s hidden status is a package’s trusted status. A package can be either trusted or not trusted (distrusted). By default packages are distrusted. This property of a package only plays a role when compiling code using GHC’s Safe Haskell feature (see Safe Haskell) with the -fpackage-trust flag enabled.

To see which modules are provided by a package use the ghc-pkg command (see Package management (the ghc-pkg command)):

$ ghc-pkg field network exposed-modules
exposed-modules: Network.BSD,

The GHC command line options that control packages are:

-package ⟨pkg⟩

This option causes the installed package ⟨pkg⟩ to be exposed. The package ⟨pkg⟩ can be specified in full with its version number (e.g. network-1.0) or the version number can be omitted in which case GHC will automatically expose the latest non-broken version from the installed versions of the package.

By default (when -hide-all-packages is not specified), GHC exposes only one version of a package, all other versions become hidden. If -package option is specified multiple times for the same package the last one overrides the previous ones. On the other hand, if -hide-all-packages is used, GHC allows you to expose multiple versions of a package by using the -package option multiple times with different versions of the same package.

-package supports thinning and renaming described in Thinning and renaming modules.

The -package ⟨pkg⟩ option also causes package ⟨pkg⟩ to be linked into the resulting executable or shared object. Whether a packages’ library is linked statically or dynamically is controlled by the flag pair -static/ -dynamic.

In --make mode and --interactive mode (see Modes of operation), the compiler normally determines which packages are required by the current Haskell modules, and links only those. In batch mode however, the dependency information isn’t available, and explicit -package options must be given when linking. The one other time you might need to use -package to force linking a package is when the package does not contain any Haskell modules (it might contain a C library only, for example). In that case, GHC will never discover a dependency on it, so it has to be mentioned explicitly.

For example, to link a program consisting of objects Foo.o and Main.o, where we made use of the network package, we need to give GHC the -package flag thus:

$ ghc -o myprog Foo.o Main.o -package network

The same flag is necessary even if we compiled the modules from source, because GHC still reckons it’s in batch mode:

$ ghc -o myprog Foo.hs Main.hs -package network
-package-id ⟨unit-id⟩

Exposes a package like -package ⟨pkg⟩, but the package is named by its unit ID (i.e. the value of id in its entry in the installed package database, also previously known as an installed package ID) rather than by name. This is a more robust way to name packages, and can be used to select packages that would otherwise be shadowed. Cabal passes -package-id flags to GHC. -package-id supports thinning and renaming described in Thinning and renaming modules.


Ignore the exposed flag on installed packages, and hide them all by default. If you use this flag, then any packages you require (including base) need to be explicitly exposed using -package ⟨pkg⟩ options.

This is a good way to insulate your program from differences in the globally exposed packages, and being explicit about package dependencies is a Good Thing. Cabal always passes the -hide-all-packages flag to GHC, for exactly this reason.

-hide-package ⟨pkg⟩

This option does the opposite of -package ⟨pkg⟩: it causes the specified package to be hidden, which means that none of its modules will be available for import by Haskell import directives.

Note that the package might still end up being linked into the final program, if it is a dependency (direct or indirect) of another exposed package.

-ignore-package ⟨pkg⟩

Causes the compiler to behave as if package ⟨pkg⟩, and any packages that depend on ⟨pkg⟩, are not installed at all.

Saying -ignore-package ⟨pkg⟩ is the same as giving -hide-package ⟨pkg⟩ flags for ⟨pkg⟩ and all the packages that depend on ⟨pkg⟩. Sometimes we don’t know ahead of time which packages will be installed that depend on ⟨pkg⟩, which is when the -ignore-package ⟨pkg⟩ flag can be useful.

By default, GHC will automatically link in the base and rts packages. This flag disables that behaviour.

-this-unit-id ⟨unit-id⟩

Tells GHC that the module being compiled forms part of unit ID ⟨unit-id⟩; internally, these keys are used to determine type equality and linker symbols. As of GHC 8.0, unit IDs must consist solely of alphanumeric characters, dashes, underscores and periods. GHC reserves the right to interpret other characters in a special way in later releases.

-trust ⟨pkg⟩

This option causes the install package ⟨pkg⟩ to be both exposed and trusted by GHC. This command functions in a very similar way to the -package ⟨pkg⟩ command but in addition sets the selected packages to be trusted by GHC, regardless of the contents of the package database. (see Safe Haskell).

-distrust ⟨pkg⟩

This option causes the install package ⟨pkg⟩ to be both exposed and distrusted by GHC. This command functions in a very similar way to the -package ⟨pkg⟩ command but in addition sets the selected packages to be distrusted by GHC, regardless of the contents of the package database. (see Safe Haskell).


Ignore the trusted flag on installed packages, and distrust them by default. If you use this flag and Safe Haskell then any packages you require to be trusted (including base) need to be explicitly trusted using -trust ⟨pkg⟩ options. This option does not change the exposed/hidden status of a package, so it isn’t equivalent to applying -distrust ⟨pkg⟩ to all packages on the system. (see Safe Haskell).

5.9.2. The main package

Every complete Haskell program must define main in module Main in package main. Omitting the -this-unit-id ⟨unit-id⟩ flag compiles code for package main. Failure to do so leads to a somewhat obscure link-time error of the form:

/usr/bin/ld: Undefined symbols:

5.9.3. Consequences of packages for the Haskell language

It is possible that by using packages you might end up with a program that contains two modules with the same name: perhaps you used a package P that has a hidden module M, and there is also a module M in your program. Or perhaps the dependencies of packages that you used contain some overlapping modules. Perhaps the program even contains multiple versions of a certain package, due to dependencies from other packages.

None of these scenarios gives rise to an error on its own [1], but they may have some interesting consequences. For instance, if you have a type M.T from version 1 of package P, then this is not the same as the type M.T from version 2 of package P, and GHC will report an error if you try to use one where the other is expected.

Formally speaking, in Haskell 98, an entity (function, type or class) in a program is uniquely identified by the pair of the module name in which it is defined and its name. In GHC, an entity is uniquely defined by a triple: package, module, and name.

5.9.4. Thinning and renaming modules

When incorporating packages from multiple sources, you may end up in a situation where multiple packages publish modules with the same name. Previously, the only way to distinguish between these modules was to use Package-qualified imports. However, since GHC 7.10, the -package ⟨pkg⟩ flags (and their variants) have been extended to allow a user to explicitly control what modules a package brings into scope, by analogy to the import lists that users can attach to module imports.

The basic syntax is that instead of specifying a package name P to the package flag -package, instead we specify both a package name and a parenthesized, comma-separated list of module names to import. For example, -package "base (Data.List, Data.Bool)" makes only Data.List and Data.Bool visible from package base. We also support renaming of modules, in case you need to refer to both modules simultaneously; this is supporting by writing OldModName as NewModName, e.g. -package "base (Data.Bool as Bool). You can also write -package "base with (Data.Bool as Bool) to include all of the original bindings (e.g. the renaming is strictly additive). It’s important to specify quotes so that your shell passes the package name and thinning/renaming list as a single argument to GHC.

Package imports with thinning/renaming do not hide other versions of the package: e.g. if containers-0.9 is already exposed, -package "containers-0.8 (Data.List as ListV8)" will only add an additional binding to the environment. Similarly, -package "base (Data.Bool as Bool)" -package "base (Data.List as List)" is equivalent to -package "base (Data.Bool as Bool, Data.List as List)". Literal names must refer to modules defined by the original package, so for example -package "base (Data.Bool as Bool, Bool as Baz)" is invalid unless there was a Bool module defined in the original package. Hiding a package also clears all of its renamings.

You can use renaming to provide an alternate prelude, e.g. -hide-all-packages -package "basic-prelude (BasicPrelude as Prelude)", in lieu of the Rebindable syntax and the implicit Prelude import extension.

5.9.5. Package Databases

A package database is where the details about installed packages are stored. It is a directory, usually called package.conf.d, that contains a file for each package, together with a binary cache of the package data in the file package.cache. Normally you won’t need to look at or modify the contents of a package database directly; all management of package databases can be done through the ghc-pkg tool (see Package management (the ghc-pkg command)).

GHC knows about two package databases in particular:

  • The global package database, which comes with your GHC installation, e.g. /usr/lib/ghc-6.12.1/package.conf.d.
  • The user package database private to each user. On Unix systems this will be $HOME/.ghc/arch-os-version/package.conf.d, and on Windows it will be something like C:\Documents And Settings\user\ghc\package.conf.d. The ghc-pkg tool knows where this file should be located, and will create it if it doesn’t exist (see Package management (the ghc-pkg command)).

Package database stack: Package databases are arranged in a stack structure. When GHC starts up it adds the global and the user package databases to the stack, in that order, unless GHC_PACKAGE_PATH is specified. When GHC_PACKAGE_PATH is specified then it will determine the initial database stack. Several command line options described below can further manipulate this initial stack. You can see GHC’s effective package database stack by running GHC with the -v flag.

This stack structure means that the order of -package-db ⟨file⟩ flags or GHC_PACKAGE_PATH is important. Each substack of the stack must be well formed (packages in databases on top of the stack can refer to packages below, but not vice versa).

Package shadowing: When multiple package databases are in use it is possible, though rarely, that the same installed package id is present in more than one database. In that case, packages closer to the top of the stack will override (shadow) those below them. If the conflicting packages are found to be equivalent (by ABI hash comparison) then one of them replaces all references to the other, otherwise the overridden package and all those depending on it will be removed.

Package version selection: When selecting a package, GHC will search for packages in all available databases. If multiple versions of the same package are available the latest non-broken version will be chosen.

Version conflict resolution: If multiple instances of a package version chosen by GHC are available then GHC will choose an unspecified instance.

You can control GHC’s package database stack using the following options:

-package-db ⟨file⟩

Add the package database ⟨file⟩ on top of the current stack.


Remove the global package database from the package database stack.


Prevent loading of the user’s local package database in the initial stack.


Reset the current package database stack. This option removes every previously specified package database (including those read from the GHC_PACKAGE_PATH environment variable) from the package database stack.


Add the global package database on top of the current stack. This option can be used after -no-global-package-db to specify the position in the stack where the global package database should be loaded.


Add the user’s package database on top of the current stack. This option can be used after -no-user-package-db to specify the position in the stack where the user’s package database should be loaded. The GHC_PACKAGE_PATH environment variable


The GHC_PACKAGE_PATH environment variable may be set to a :-separated (;-separated on Windows) list of files containing package databases. This list of package databases, used by GHC and ghc-pkg, specifies a stack of package databases from top to bottom. This order was chosen to match the behaviour of the PATH environment variable where entries earlier in the PATH override ones that come later. See Package Databases for details on how the package database stack is used.

Normally GHC_PACKAGE_PATH replaces the default package stack. For example, all of the following commands are equivalent, creating a stack with db1 at the top followed by db2 (use ; instead of : on Windows):

$ ghc -clear-package-db -package-db db2.conf -package-db db1.conf
$ env GHC_PACKAGE_PATH=db1.conf:db2.conf ghc
$ env GHC_PACKAGE_PATH=db2.conf ghc -package-db db1.conf

However, if GHC_PACKAGE_PATH ends in a separator, the default databases (i.e. the user and global package databases, in that order) are appended to the path. For example, to augment the usual set of packages with a database of your own, you could say (on Unix):

$ export GHC_PACKAGE_PATH=$HOME/.my-ghc-packages.conf:


A package environment file is a file that tells ghc precisely which packages should be visible. It can be used to create environments for ghc or ghci that are local to a shell session or to some file system location. They are intended to be managed by build/package tools, to enable ghc and ghci to automatically use an environment created by the tool.

The file contains package IDs and optionally package databases, one directive per line:

package-db db.d/
package-id id_1
package-id id_2
package-id id_n

If such a package environment is found, it is equivalent to passing these command line arguments to ghc:

-package-db db.d/
-package-id id_1
-package-id id_2
-package-id id_n

Note the implicit -hide-all-packages and the fact that it is -package-id ⟨unit-id⟩, not -package ⟨pkg⟩. This is because the environment specifies precisely which packages should be visible.

Note that for the package-db directive, if a relative path is given it must be relative to the location of the package environment file.

-package-env ⟨file⟩|⟨name⟩

Use the package environment in ⟨file⟩, or in $HOME/.ghc/arch-os-version/environments/⟨name⟩ If set to - no package environment is read.


Specifies the path to the package environment file to be used by GHC. Overridden by the -package-env ⟨file⟩|⟨name⟩ flag if set.

In order, ghc will look for the package environment in the following locations:

  • File ⟨file⟩ if you pass the option -package-env ⟨file⟩|⟨name⟩.
  • File $HOME/.ghc/arch-os-version/environments/name if you pass the option -package-env ⟨name⟩.
  • File ⟨file⟩ if the environment variable GHC_ENVIRONMENT is set to ⟨file⟩.
  • File $HOME/.ghc/arch-os-version/environments/name if the environment variable GHC_ENVIRONMENT is set to ⟨name⟩.

Additionally, unless -hide-all-packages is specified ghc will also look for the package environment in the following locations:

  • File .ghc.environment.arch-os-version if it exists in the current directory or any parent directory (but not the user’s home directory).
  • File $HOME/.ghc/arch-os-version/environments/default if it exists.

Package environments can be modified by further command line arguments; for example, if you specify -package foo on the command line, then package ⟨foo⟩ will be visible even if it’s not listed in the currently active package environment.

5.9.6. Installed package IDs, dependencies, and broken packages

Each installed package has a unique identifier (the “installed package ID”), which distinguishes it from all other installed packages on the system. To see the installed package IDs associated with each installed package, use ghc-pkg list -v:

$ ghc-pkg list -v
using cache: /usr/lib/ghc-6.12.1/package.conf.d/package.cache
   Cabal-1.7.4 (Cabal-1.7.4-48f5247e06853af93593883240e11238)
   array- (array-
   base- (base-
   base- (base-

The string in parentheses after the package name is the installed package ID: it normally begins with the package name and version, and ends in a hash string derived from the compiled package. Dependencies between packages are expressed in terms of installed package IDs, rather than just packages and versions. For example, take a look at the dependencies of the haskell98 package:

$ ghc-pkg field haskell98 depends
depends: array-

The purpose of the installed package ID is to detect problems caused by re-installing a package without also recompiling the packages that depend on it. Recompiling dependencies is necessary, because the newly compiled package may have a different ABI (Application Binary Interface) than the previous version, even if both packages were built from the same source code using the same compiler. With installed package IDs, a recompiled package will have a different installed package ID from the previous version, so packages that depended on the previous version are now orphaned - one of their dependencies is not satisfied. Packages that are broken in this way are shown in the ghc-pkg list output either in red (if possible) or otherwise surrounded by braces. In the following example, we have recompiled and reinstalled the filepath package, and this has caused various dependencies including Cabal to break:

$ ghc-pkg list
WARNING: there are broken packages.  Run 'ghc-pkg check' for more details.
    ... etc ...

Additionally, ghc-pkg list reminds you that there are broken packages and suggests ghc-pkg check, which displays more information about the nature of the failure:

$ ghc-pkg check
There are problems in package ghc-6.12.1:
  dependency "filepath-" doesn't exist
There are problems in package haskeline-0.6.2:
  dependency "filepath-" doesn't exist
There are problems in package Cabal-1.7.4:
  dependency "filepath-" doesn't exist
There are problems in package process-
  dependency "filepath-" doesn't exist
There are problems in package directory-
  dependency "filepath-" doesn't exist

The following packages are broken, either because they have a problem
listed above, or because they depend on a broken package.

To fix the problem, you need to recompile the broken packages against the new dependencies. The easiest way to do this is to use cabal-install, or download the packages from HackageDB and build and install them as normal.

Be careful not to recompile any packages that GHC itself depends on, as this may render the ghc package itself broken, and ghc cannot be simply recompiled. The only way to recover from this would be to re-install GHC.

5.9.7. Package management (the ghc-pkg command)

The ghc-pkg tool is for querying and modifying package databases. To see what package databases are in use, use ghc-pkg list. The stack of databases that ghc-pkg knows about can be modified using the GHC_PACKAGE_PATH environment variable (see The GHC_PACKAGE_PATH environment variable, and using -package-db ⟨file⟩ options on the ghc-pkg command line.

When asked to modify a database, ghc-pkg modifies the global database by default. Specifying --user causes it to act on the user database, or --package-db can be used to act on another database entirely. When multiple of these options are given, the rightmost one is used as the database to act upon.

Commands that query the package database (list, latest, describe, field, dot) operate on the list of databases specified by the flags --user, --global, and --package-db. If none of these flags are given, the default is --global --user.

If the environment variable GHC_PACKAGE_PATH is set, and its value does not end in a separator (: on Unix, ; on Windows), then the last database is considered to be the global database, and will be modified by default by ghc-pkg. The intention here is that GHC_PACKAGE_PATH can be used to create a virtual package environment into which Cabal packages can be installed without setting anything other than GHC_PACKAGE_PATH.

The ghc-pkg program may be run in the ways listed below. Where a package name is required, the package can be named in full including the version number (e.g. network-1.0), or without the version number. Naming a package without the version number matches all versions of the package; the specified action will be applied to all the matching packages. A package specifier that matches all version of the package can also be written ⟨pkg⟩ -*, to make it clearer that multiple packages are being matched. To match against the installed package ID instead of just package name and version, pass the --ipid flag.

ghc-pkg init path
Creates a new, empty, package database at ⟨path⟩, which must not already exist.
ghc-pkg register ⟨file⟩

Reads a package specification from ⟨file⟩ (which may be “-” to indicate standard input), and adds it to the database of installed packages. The syntax of ⟨file⟩ is given in InstalledPackageInfo: a package specification.

The package specification must be a package that isn’t already installed.

ghc-pkg update ⟨file⟩
The same as register, except that if a package of the same name is already installed, it is replaced by the new one.
ghc-pkg unregister ⟨P⟩
Remove the specified package from the database.
ghc-pkg check
Check consistency of dependencies in the package database, and report packages that have missing dependencies.
ghc-pkg expose ⟨P⟩
Sets the exposed flag for package ⟨P⟩ to True.
ghc-pkg hide ⟨P⟩
Sets the exposed flag for package ⟨P⟩ to False.
ghc-pkg trust ⟨P⟩
Sets the trusted flag for package ⟨P⟩ to True.
ghc-pkg distrust ⟨P⟩
Sets the trusted flag for package ⟨P⟩ to False.
ghc-pkg list [⟨P⟩] [--simple-output]

This option displays the currently installed packages, for each of the databases known to ghc-pkg. That includes the global database, the user’s local database, and any further files specified using the -f option on the command line.

Hidden packages (those for which the exposed flag is False) are shown in parentheses in the list of packages.

If an optional package identifier ⟨P⟩ is given, then only packages matching that identifier are shown.

If the option --simple-output is given, then the packages are listed on a single line separated by spaces, and the database names are not included. This is intended to make it easier to parse the output of ghc-pkg list using a script.

ghc-pkg find-module ⟨M⟩ [--simple-output]

This option lists registered packages exposing module ⟨M⟩. Examples:

$ ghc-pkg find-module Var

$ ghc-pkg find-module Data.Sequence

Otherwise, it behaves like ghc-pkg list, including options.

ghc-pkg latest ⟨P⟩
Prints the latest available version of package ⟨P⟩.
ghc-pkg describe ⟨P⟩

Emit the full description of the specified package. The description is in the form of an InstalledPackageInfo, the same as the input file format for ghc-pkg register. See InstalledPackageInfo: a package specification for details.

If the pattern matches multiple packages, the description for each package is emitted, separated by the string --- on a line by itself.

ghc-pkg field ⟨P⟩ ⟨field⟩[,⟨field⟩]*
Show just a single field of the installed package description for P. Multiple fields can be selected by separating them with commas
ghc-pkg dot


ghc-pkg dot | tred | dot -Tpdf >pkgs.pdf
ghc-pkg dump

Emit the full description of every package, in the form of an InstalledPackageInfo. Multiple package descriptions are separated by the string --- on a line by itself.

This is almost the same as ghc-pkg describe '*', except that ghc-pkg dump is intended for use by tools that parse the results, so for example where ghc-pkg describe '*' will emit an error if it can’t find any packages that match the pattern, ghc-pkg dump will simply emit nothing.

ghc-pkg recache

Re-creates the binary cache file package.cache for the selected database. This may be necessary if the cache has somehow become out-of-sync with the contents of the database (ghc-pkg will warn you if this might be the case).

The other time when ghc-pkg recache is useful is for registering packages manually: it is possible to register a package by simply putting the appropriate file in the package database directory and invoking ghc-pkg recache to update the cache. This method of registering packages may be more convenient for automated packaging systems.

Substring matching is supported for ⟨M⟩ in find-module and for ⟨P⟩ in list, describe, and field, where a '*' indicates open substring ends (prefix*, *suffix, *infix*). Examples (output omitted):

-- list all regex-related packages
ghc-pkg list '*regex*' --ignore-case
-- list all string-related packages
ghc-pkg list '*string*' --ignore-case
-- list OpenGL-related packages
ghc-pkg list '*gl*' --ignore-case
-- list packages exporting modules in the Data hierarchy
ghc-pkg find-module 'Data.*'
-- list packages exporting Monad modules
ghc-pkg find-module '*Monad*'
-- list names and maintainers for all packages
ghc-pkg field '*' name,maintainer
-- list location of haddock htmls for all packages
ghc-pkg field '*' haddock-html
-- dump the whole database
ghc-pkg describe '*'

Additionally, the following flags are accepted by ghc-pkg:

-f ⟨file⟩, -package-db ⟨file⟩

Adds ⟨file⟩ to the stack of package databases. Additionally, ⟨file⟩ will also be the database modified by a register, unregister, expose or hide command, unless it is overridden by a later --package-db, --user or --global option.


Causes ghc-pkg to ignore missing dependencies, directories and libraries when registering 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.


Operate on the global package database (this is the default). This flag affects the register, update, unregister, expose, and hide commands.

--help, -?

Outputs the command-line syntax.


Operate on the current user’s local package database. This flag affects the register, update, unregister, expose, and hide commands.

-v [⟨n⟩], --verbose [=⟨n⟩]

Control verbosity. Verbosity levels range from 0-2, where the default is 1, and -v alone selects level 2.

-V; --version

Output the ghc-pkg version number.

--ipid, --unit-id

Causes ghc-pkg to interpret arguments as installed unit IDs (e.g., an identifier like unix- This is useful if providing just the package name and version are ambiguous (in old versions of GHC, this was guaranteed to be unique, but this invariant no longer necessarily holds).

5.9.8. Building a package from Haskell source

We don’t recommend building packages the hard way. Instead, use the Cabal infrastructure if possible. If your package is particularly complicated or requires a lot of configuration, then you might have to fall back to the low-level mechanisms, so a few hints for those brave souls follow.

You need to build an “installed package info” file for passing to ghc-pkg when installing your package. The contents of this file are described in InstalledPackageInfo: a package specification.

The Haskell code in a package may be built into one or more archive libraries (e.g. libHSfoo.a), or a single shared object (e.g. libHSfoo.dll/.so/.dylib). The restriction to a single shared object is because the package system is used to tell the compiler when it should make an inter-shared-object call rather than an intra-shared-object-call call (inter-shared-object calls require an extra indirection).

  • Building a static library is done by using the ar tool, like so:

    ar cqs libHSfoo-1.0.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.

  • To load a package foo, GHCi can load its libHSfoo.a library directly, but it can also load a package in the form of a single HSfoo.o file that has been pre-linked. Loading the .o file is slightly quicker, but at the expense of having another copy of the compiled package. The rule of thumb is that if the modules of the package were compiled with -split-sections then building the HSfoo.o is worthwhile because it saves time when loading the package into GHCi. Without -split-sections, there is not much difference in load time between the .o and .a libraries, so it is better to save the disk space and only keep the .a around. In a GHC distribution we provide .o files for most packages except the GHC package itself.

    The HSfoo.o file is built by Cabal automatically; use --disable-library-for-ghci to disable it. To build one manually, the following GNU ld command can be used:

    ld -r --whole-archive -o HSfoo.o libHSfoo.a

    (replace --whole-archive with -all_load on MacOS X)

  • When building the package as shared library, GHC can be used to perform the link step. This hides some of the details out the underlying linker and provides a common interface to all shared object variants that are supported by GHC (DLLs, ELF DSOs, and Mac OS dylibs). The shared object must be named in specific way for two reasons: (1) the name must contain the GHC compiler version, so that two library variants don’t collide that are compiled by different versions of GHC and that therefore are most likely incompatible with respect to calling conventions, (2) it must be different from the static name otherwise we would not be able to control the linker as precisely as necessary to make the -static/-dynamic flags work, see Options affecting linking.

    ghc -shared -dynamic -o libHSfoo-1.0-ghcGHCVersion.so A.o B.o C.o

    Using GHC’s version number in the shared object name allows different library versions compiled by different GHC versions to be installed in standard system locations, e.g. under *nix /usr/lib. To obtain the version number of GHC invoke ghc --numeric-version and use its output in place of ⟨GHCVersion⟩. See also Options affecting code generation on how object files must be prepared for shared object linking.

  • When building a shared library, care must be taken to ensure that the resulting object is named appropriately. In particular, GHC expects the name of a shared object to have the form libHS<unit id>-ghc<ghc version>.<ext> where unit id is the unit ID given during compilation via the -this-unit-id ⟨unit-id⟩ flag, ghc version is the version of GHC that produced/consumes the object and ext is the host system’s usual file extension for shared objects.

To compile a module which is to be part of a new package, use the -package-name (to identify the name of the package) and -library-name (to identify the version and the version hashes of its identities.) options (Using Packages). Failure to use these options when compiling a package will probably result in disaster, but you will only discover later when you attempt to import modules from the package. At this point GHC will complain that the package name it was expecting the module to come from is not the same as the package name stored in the .hi file.

It is worth noting with shared objects, when each package is built as a single shared object file, since a reference to a shared object costs an extra indirection, intra-package references are cheaper than inter-package references. Of course, this applies to the main package as well.

5.9.9. InstalledPackageInfo: a package specification

A package specification is a Haskell record; in particular, it is the record Distribution.InstalledPackageInfo.InstalledPackageInfo in the module Distribution.InstalledPackageInfo, which is part of the Cabal package distributed with GHC.

An InstalledPackageInfo has a human readable/writable syntax. The functions parseInstalledPackageInfo and showInstalledPackageInfo read and write this syntax respectively. Here’s an example of the InstalledPackageInfo for the unix package:

$ ghc-pkg describe unix
name: unix
id: unix-
license: BSD3
maintainer: libraries@haskell.org
description: This package gives you access to the set of operating system
             services standardised by POSIX 1003.1b (or the IEEE Portable
             Operating System Interface for Computing Environments -
             IEEE Std. 1003.1).
category: System
exposed: True
exposed-modules: System.Posix System.Posix.DynamicLinker.Module
                 System.Posix.DynamicLinker.Prim System.Posix.Directory
                 System.Posix.DynamicLinker System.Posix.Env System.Posix.Error
                 System.Posix.Files System.Posix.IO System.Posix.Process
                 System.Posix.Process.Internals System.Posix.Resource
                 System.Posix.Temp System.Posix.Terminal System.Posix.Time
                 System.Posix.Unistd System.Posix.User System.Posix.Signals
                 System.Posix.Signals.Exts System.Posix.Semaphore
trusted: False
import-dirs: /usr/lib/ghc-6.12.1/unix-
library-dirs: /usr/lib/ghc-6.12.1/unix-
hs-libraries: HSunix-
extra-libraries: rt util dl
include-dirs: /usr/lib/ghc-6.12.1/unix-
includes: HsUnix.h execvpe.h
depends: base-
haddock-interfaces: /usr/share/doc/ghc/html/libraries/unix/unix.haddock
haddock-html: /usr/share/doc/ghc/html/libraries/unix

Here is a brief description of the syntax of this file:

A package description consists of a number of field/value pairs. A field starts with the field name in the left-hand column followed by a “:”, and the value continues until the next line that begins in the left-hand column, or the end of file.

The syntax of the value depends on the field. The various field types are:

Any arbitrary string, no interpretation or parsing is done.
A sequence of non-space characters, or a sequence of arbitrary characters surrounded by quotes "....".
string list
A sequence of strings, separated by commas. The sequence may be empty.

In addition, there are some fields with special syntax (e.g. package names, version, dependencies).

The allowed fields, with their types, are:


(string) The package’s name (without the version).


(string) The installed package ID. It is up to you to choose a suitable one.


(string) The package’s version, usually in the form A.B (any number of components are allowed).


(string) The type of license under which this package is distributed. This field is a value of the Distribution.License.License type.


(optional string) The name of a file giving detailed license information for this package.


(optional freeform) The copyright string.


(optional freeform) The email address of the package’s maintainer.


(optional freeform) A string describing the stability of the package (e.g. stable, provisional or experimental).


(optional freeform) URL of the package’s home page.


(optional freeform) URL of a downloadable distribution for this package. The distribution should be a Cabal package.


(optional freeform) Description of the package.


(optional freeform) Which category the package belongs to. This field is for use in conjunction with a future centralised package distribution framework, tentatively titled Hackage.


(optional freeform) Author of the package.


(bool) Whether the package is exposed or not.


(string list) modules exposed by this package.


(string list) modules provided by this package, but not exposed to the programmer. These modules cannot be imported, but they are still subject to the overlapping constraint: no other package in the same program may provide a module of the same name.


Modules reexported by this package. This list takes the form of pkg:OldName as NewName (A@orig-pkg-0.1-HASH): the first portion of the string is the user-written reexport specification (possibly omitting the package qualifier and the renaming), while the parenthetical is the original package which exposed the module under are particular name. Reexported modules have a relaxed overlap constraint: it’s permissible for two packages to reexport the same module as the same name if the reexported moduleis identical.


(bool) Whether the package is trusted or not.


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



(string list) A list of directories containing libraries for this package.





HSfoo.o; HSfoo.obj
The object version of the library used by GHCi.

(string list) 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 libHSbase_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.


(string list) A list of directories containing C includes for this package.


(string list) A list of files to include for via-C compilations using this package. Typically the include file(s) 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 id list) Packages on which this package depends.


(string list) Options to pass to Hugs for this package.


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






(string list) 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.


(string list) A list of filenames containing Haddock interface files (.haddock files) for this package.


(optional string) The directory containing the Haddock-generated HTML for this package.

[1]it used to in GHC 6.4, but not since 6.6