4.9. Packages
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## 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 7.27, “Safe Haskell”) with the `-fpackage-trust` flag enabled.

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 `base` and `rts` packages. 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 7.27, “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 7.27, “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 7.27, “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, and they are arranged in a stack structure. Packages closer to the top of the stack will override (shadow) those below them. By default, the stack contains just the global and the user's package databases, in that order.

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. Packages in additional databases read this way will override those in the initial stack and those in previously specified databases.

`-no-global-package-db`

  Remove the global package database from the package database stack.

`-no-user-package-db`

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

`-clear-package-db`

  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.

`-global-package-db`

  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.

`-user-package-db`

  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.

#### 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 package database stack (i.e. the user and global package databases, in that order) is appended. 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:

(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-db` 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.

`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`:

`-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.

`--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.

- 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-objs` then building the `HSfoo.o` is worthwhile because it saves time when loading the package into GHCi. Without `-split-objs`, 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 Section 4.12.6, “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 4.12.5, “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: libraries@haskell.org
    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

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