4.9.  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 Section 4.9.1, “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).

4.9.1. Using Packages

GHC only knows about packages that are installed. To see which packages are installed, use the ghc-pkg list command:

$ ghc-pkg list
/usr/lib/ghc-6.12.1/package.conf.d:
    Cabal-1.7.4
    array-0.2.0.1
    base-3.0.3.0
    base-4.2.0.0
    bin-package-db-0.0.0.0
    binary-0.5.0.1
    bytestring-0.9.1.4
    containers-0.2.0.1
    directory-1.0.0.2
    (dph-base-0.4.0)
    (dph-par-0.4.0)
    (dph-prim-interface-0.4.0)
    (dph-prim-par-0.4.0)
    (dph-prim-seq-0.4.0)
    (dph-seq-0.4.0)
    extensible-exceptions-0.1.1.0
    ffi-1.0
    filepath-1.1.0.1
    (ghc-6.12.1)
    ghc-prim-0.1.0.0
    haskeline-0.6.2
    haskell98-1.0.1.0
    hpc-0.5.0.2
    integer-gmp-0.1.0.0
    mtl-1.1.0.2
    old-locale-1.0.0.1
    old-time-1.0.0.1
    pretty-1.0.1.0
    process-1.0.1.1
    random-1.0.0.1
    rts-1.0
    syb-0.1.0.0
    template-haskell-2.4.0.0
    terminfo-0.3.1
    time-1.1.4
    unix-2.3.1.0
    utf8-string-0.3.4

An installed package is either exposed or hidden by default. Packages hidden by default are listed in parentheses (eg. (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.

Note: 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 Section 7.20, “Safe Haskell”).

To see which modules are provided by a package use the ghc-pkg command (see Section 4.9.6, “Package management (the ghc-pkg command)”):

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

The GHC command line options that control packages are:

-package P

This option causes the installed package P to be exposed. The package P can be specified in full with its version number (e.g. network-1.0) or the version number can be omitted if there is only one version of the package installed. If there are multiple versions of P installed, then all other versions will become hidden.

The -package P option also causes package P 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 Section 4.5, “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 P

Exposes a package like -package, but the package is named by its 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.

-hide-all-packages

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 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 P

This option does the opposite of -package: 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 P

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

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

-no-auto-link-packages

By default, GHC will automatically link in the haskell98 package. This flag disables that behaviour.

-package-name foo

Tells GHC the the module being compiled forms part of package foo. If this flag is omitted (a very common case) then the default package main is assumed.

Note: the argument to -package-name should be the full package name-version for the package. For example: -package mypkg-1.2.

-trust P

This option causes the install package P to be both exposed and trusted by GHC. This command functions in the in a very similar way to the -package command but in addition sets the selected packaged to be trusted by GHC, regardless of the contents of the package database. (see Section 7.20, “Safe Haskell”).

-distrust P

This option causes the install package P to be both exposed and distrusted by GHC. This command functions in the in a very similar way to the -package command but in addition sets the selected packaged to be distrusted by GHC, regardless of the contents of the package database. (see Section 7.20, “Safe Haskell”).

-distrust-all

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 options. This option does not change the exposed/hidden status of a package, so it isn't equivalent to applying -distrust to all packages on the system. (see Section 7.20, “Safe Haskell”).

4.9.2. The main package

Every complete Haskell program must define main in module Main in package main. (Omitting the -package-name 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:
_ZCMain_main_closure

4.9.3. 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[8], 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.

4.9.4. 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 Section 4.9.6, “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.

  • A 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 Section 4.9.6, “Package management (the ghc-pkg command)”).

When GHC starts up, it reads the contents of these two package databases, and builds up a list of the packages it knows about. You can see GHC's package table by running GHC with the -v flag.

Package databases may overlap: for example, packages in the user database will override (shadow) those of the same name and version in the global database.

You can control the loading of package databases using the following GHC options:

-package-conf file

Read in the package configuration file file in addition to the system default file and the user's local file. Packages in additional files read this way will override those in the global and user databases.

-no-user-package-conf

Prevent loading of the user's local package database.

4.9.4.1. 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 is used by GHC and ghc-pkg, with earlier databases in the list overriding later ones. This order was chosen to match the behaviour of the PATH environment variable; think of it as a list of package databases that are searched left-to-right for packages.

If GHC_PACKAGE_PATH ends in a separator, then the default user and system package databases are appended, in that order. e.g. 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:

(use ; instead of : on Windows).

To check whether your GHC_PACKAGE_PATH setting is doing the right thing, ghc-pkg list will list all the databases in use, in the reverse order they are searched.

4.9.5. Package IDs, dependencies, and broken packages

Each installed package has a unique identifier (the “installed package ID”, or just “package ID” for short) , which distinguishes it from all other installed packages on the system. To see the 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
/usr/lib/ghc-6.12.1/package.conf.d
   Cabal-1.7.4 (Cabal-1.7.4-48f5247e06853af93593883240e11238)
   array-0.2.0.1 (array-0.2.0.1-9cbf76a576b6ee9c1f880cf171a0928d)
   base-3.0.3.0 (base-3.0.3.0-6cbb157b9ae852096266e113b8fac4a2)
   base-4.2.0.0 (base-4.2.0.0-247bb20cde37c3ef4093ee124e04bc1c)
   ...

The string in parentheses after the package name is the 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 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-0.2.0.1-9cbf76a576b6ee9c1f880cf171a0928d
         base-4.2.0.0-247bb20cde37c3ef4093ee124e04bc1c
         directory-1.0.0.2-f51711bc872c35ce4a453aa19c799008
         old-locale-1.0.0.1-d17c9777c8ee53a0d459734e27f2b8e9
         old-time-1.0.0.1-1c0d8ea38056e5087ef1e75cb0d139d1
         process-1.0.1.1-d8fc6d3baf44678a29b9d59ca0ad5780
         random-1.0.0.1-423d08c90f004795fd10e60384ce6561

The purpose of the 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 package IDs, a recompiled package will have a different 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.
/usr/lib/ghc-6.12.1/package.conf.d:
    {Cabal-1.7.4}
    array-0.2.0.1
    base-3.0.3.0
    ... 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-1.1.0.1-87511764eb0af2bce4db05e702750e63" doesn't exist
There are problems in package haskeline-0.6.2:
  dependency "filepath-1.1.0.1-87511764eb0af2bce4db05e702750e63" doesn't exist
There are problems in package Cabal-1.7.4:
  dependency "filepath-1.1.0.1-87511764eb0af2bce4db05e702750e63" doesn't exist
There are problems in package process-1.0.1.1:
  dependency "filepath-1.1.0.1-87511764eb0af2bce4db05e702750e63" doesn't exist
There are problems in package directory-1.0.0.2:
  dependency "filepath-1.1.0.1-87511764eb0af2bce4db05e702750e63" doesn't exist

The following packages are broken, either because they have a problem
listed above, or because they depend on a broken package.
ghc-6.12.1
haskeline-0.6.2
Cabal-1.7.4
process-1.0.1.1
directory-1.0.0.2
bin-package-db-0.0.0.0
hpc-0.5.0.2
haskell98-1.0.1.0

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.

4.9.6. 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 Section 4.9.4.1, “The GHC_PACKAGE_PATH environment variable”, and using --package-conf 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-conf 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-conf. 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.

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 Section 4.9.8, “ 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
c:/fptools/validate/ghc/driver/package.conf.inplace:
    (ghc-6.9.20080428)

$ ghc-pkg find-module Data.Sequence
c:/fptools/validate/ghc/driver/package.conf.inplace:
    containers-0.1

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 Section 4.9.8, “ 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

Generate a graph of the package dependencies in a form suitable for input for the graphviz tools. For example, to generate a PDF of the dependency graph:

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:

––auto-ghci-libs

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.

-f file , -package-conf 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-conf, --user or --global option.

––force

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.

––global

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.

––user

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.

4.9.7. 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 Section 4.9.8, “ 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.

  • 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 4.9.6, “Package management (the ghc-pkg command)”. 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

    (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 Section 4.11.6, “Options affecting linking”.

    ghc -shared 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 Section 4.11.5, “Options affecting code generation” on how object files must be prepared for shared object linking.

To compile a module which is to be part of a new package, use the -package-name option (Section 4.9.1, “Using Packages ”). Failure to use the -package-name option 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.

4.9.8.  InstalledPackageInfo: a package specification

A package specification is a Haskell record; in particular, it is the record 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
version: 2.3.1.0
id: unix-2.3.1.0-de7803f1a8cd88d2161b29b083c94240
license: BSD3
copyright:
maintainer: [email protected]
stability:
homepage:
package-url:
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).
             .
             The package is not supported under Windows (except under Cygwin).
category: System
author:
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
                 System.Posix.SharedMem
hidden-modules:
trusted: False
import-dirs: /usr/lib/ghc-6.12.1/unix-2.3.1.0
library-dirs: /usr/lib/ghc-6.12.1/unix-2.3.1.0
hs-libraries: HSunix-2.3.1.0
extra-libraries: rt util dl
extra-ghci-libraries:
include-dirs: /usr/lib/ghc-6.12.1/unix-2.3.1.0/include
includes: HsUnix.h execvpe.h
depends: base-4.2.0.0-247bb20cde37c3ef4093ee124e04bc1c
hugs-options:
cc-options:
ld-options:
framework-dirs:
frameworks:
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:

freeform

Any arbitrary string, no interpretation or parsing is done.

string

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:

name

The package's name (without the version).

id

The package ID. It is up to you to choose a suitable one.

version

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

license

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

license-file

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

copyright

(optional freeform) The copyright string.

maintainer

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

stability

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

homepage

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

package-url

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

description

(optional freeform) Description of the package.

category

(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.

author

(optional freeform) Author of the package.

exposed

(bool) Whether the package is exposed or not.

exposed-modules

(string list) modules exposed by this package.

hidden-modules

(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.

trusted

(bool) Whether the package is trusted or not.

import-dirs

(string list) 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).

library-dirs

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

hs-libraries

(string list) 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

(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.

include-dirs

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

includes

(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.

depends

(package id list) Packages on which this package depends.

hugs-options

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

cc-options

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

ld-options

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

framework-dirs

(string list) 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.

frameworks

(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.

haddock-interfaces

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

haddock-html

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



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