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Quickstart
==========

Suppose that you are in a directory containing a single Cabal package
which you wish to build. You can configure and build it using Nix-style
local builds with this command (configuring is not necessary):

::

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    $ cabal new-build
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To open a GHCi shell with this package, use this command:

::

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    $ cabal new-repl
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Developing multiple packages
----------------------------

Many Cabal projects involve multiple packages which need to be built
together. To build multiple Cabal packages, you need to first create a
``cabal.project`` file which declares where all the local package
directories live. For example, in the Cabal repository, there is a root
directory with a folder per package, e.g., the folders ``Cabal`` and
``cabal-install``. The ``cabal.project`` file specifies each folder as
part of the project:

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.. code-block:: cabal
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    packages: Cabal/
              cabal-install/

The expectation is that a ``cabal.project`` is checked into your source
control, to be used by all developers of a project. If you need to make
local changes, they can be placed in ``cabal.project.local`` (which
should not be checked in.)

Then, to build every component of every package, from the top-level
directory, run the command: (Warning: cabal-install-1.24 does NOT have
this behavior; you will need to upgrade to HEAD.)

::

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    $ cabal new-build
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To build a specific package, you can either run ``new-build`` from the
directory of the package in question:

::

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    $ cd cabal-install
    $ cabal new-build
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or you can pass the name of the package as an argument to
``cabal new-build`` (this works in any subdirectory of the project):

::

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    $ cabal new-build cabal-install
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You can also specify a specific component of the package to build. For
example, to build a test suite named ``package-tests``, use the command:

::

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    $ cabal new-build package-tests
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Targets can be qualified with package names. So to request
``package-tests`` *from* the ``Cabal`` package, use
``Cabal:package-tests``.

Unlike sandboxes, there is no need to setup a sandbox or ``add-source``
projects; just check in ``cabal.project`` to your repository and
``new-build`` will just work.

How it works
============

Local versus external packages
------------------------------

One of the primary innovations of Nix-style local builds is the
distinction between local packages, which users edit and recompile and
must be built per-project, versus external packages, which can be cached
across packages. To be more precise:

1. A **local package** is one that is listed explicitly in the
   ``packages``, ``optional-packages`` or ``extra-packages`` field of a
   project. Usually, these refer to packages whose source code lives
   directly in a folder in your project (although, you can list an
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   arbitrary Hackage package in ``extra-packages`` to force it to be
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   treated as local).

Local packages, as well as the external packages (below) which depend on
them, are built **inplace**, meaning that they are always built
specifically for the project and are not installed globally. Inplace
packages are not cached and not given unique hashes, which makes them
suitable for packages which you want to edit and recompile.

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2. An **external package** is any package which is not listed in the
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   ``packages`` field. The source code for external packages is usually
   retrieved from Hackage.

When an external package does not depend on an inplace package, it can
be built and installed to a **global** store, which can be shared across
projects. These build products are identified by a hash that over all of
the inputs which would influence the compilation of a package (flags,
dependency selection, etc.). Just as in Nix, these hashes uniquely
identify the result of a build; if we compute this identifier and we
find that we already have this ID built, we can just use the already
built version.

The global package store is ``~/.cabal/store``; if you need to clear
your store for whatever reason (e.g., to reclaim disk space or because
the global store is corrupted), deleting this directory is safe
(``new-build`` will just rebuild everything it needs on its next
invocation).

This split motivates some of the UI choices for Nix-style local build
commands. For example, flags passed to ``cabal new-build`` are only
applied to *local* packages, so that adding a flag to
``cabal new-build`` doesn't necessitate a rebuild of *every* transitive
dependency in the global package store.

In cabal-install HEAD, Nix-style local builds also take advantage of a
new Cabal library feature, `per-component
builds <https://github.com/ezyang/ghc-proposals/blob/master/proposals/0000-componentized-cabal.rst>`__,
where each component of a package is configured and built separately.
This can massively speed up rebuilds of packages with lots of components
(e.g., a package that defines multiple executables), as only one
executable needs to be rebuilt. Packages that use Custom setup scripts
are not currently built on a per-component basis.

Where are my build products?
----------------------------

A major deficiency in the current implementation of new-build is that
there is no programmatic way to access the location of build products.
The location of the build products is intended to be an internal
implementation detail of new-build, but we also understand that many
unimplemented features (e.g., ``new-test``) can only be reasonably
worked around by accessing build products directly.

The location where build products can be found varies depending on the
version of cabal-install:

-  In cabal-install-1.24, the dist directory for a package ``p-0.1`` is
   stored in ``dist-newstyle/build/p-0.1``. For example, if you built an
   executable or test suite named ``pexe``, it would be located at
   ``dist-newstyle/build/p-0.1/build/pexe/pexe``.

-  In cabal-install HEAD, the dist directory for a package ``p-0.1``
   defining a library built with GHC 8.0.1 on 64-bit Linux is
   ``dist-newstyle/build/x86_64-linux/ghc-8.0.1/p-0.1``. When
   per-component builds are enabled (any non-Custom package), a
   subcomponent like an executable or test suite named ``pexe`` will be
   stored at
   ``dist-newstyle/build/x86_64-linux/ghc-8.0.1/p-0.1/c/pexe``; thus,
   the full path of the executable is
   ``dist-newstyle/build/x86_64-linux/ghc-8.0.1/p-0.1/c/pexe/build/pexe/pexe``
   (you can see why we want this to be an implementation detail!)

The paths are a bit longer in HEAD but the benefit is that you can
transparently have multiple builds with different versions of GHC. We
plan to add the ability to create aliases for certain build
configurations, and more convenient paths to access particularly useful
build products like executables.

Caching
-------

Nix-style local builds sport a robust caching system which help reduce
the time it takes to execute a rebuild cycle. While the details of how
``cabal-install`` does caching are an implementation detail and may
change in the future, knowing what gets cached is helpful for
understanding the performance characteristics of invocations to
``new-build``. The cached intermediate results are stored in
``dist-newstyle/cache``; this folder can be safely deleted to clear the
cache.

The following intermediate results are cached in the following files in
this folder (the most important two are first):

``solver-plan`` (binary)
    The result of calling the dependency solver, assuming that the
    Hackage index, local ``cabal.project`` file, and local ``cabal``
    files are unmodified. (Notably, we do NOT have to dependency solve
    again if new build products are stored in the global store; the
    invocation of the dependency solver is independent of what is
    already available in the store.)
``source-hashes`` (binary)
    The hashes of all local source files. When all local source files of
    a local package are unchanged, ``cabal new-build`` will skip
    invoking ``setup build`` entirely (saving us from a possibly
    expensive call to ``ghc --make``). The full list of source files
    participating in compilation are determined using
    ``setup sdist --list-sources`` (thus, if you do not list all your
    source files in a Cabal file, you may fail to recompile when you
    edit them.)
``config`` (same format as ``cabal.project``)
    The full project configuration, merged from ``cabal.project`` (and
    friends) as well as the command line arguments.
``compiler`` (binary)
    The configuration of the compiler being used to build the project.
``improved-plan`` (binary)
    Like ``solver-plan``, but with all non-inplace packages improved
    into pre-existing copies from the store.

Note that every package also has a local cache managed by the Cabal
build system, e.g., in ``$distdir/cache``.

There is another useful file in ``dist-newstyle/cache``, ``plan.json``,
which is a JSON serialization of the computed install plan. (TODO: docs)

Commands
========

We now give an in-depth description of all the commands, describing the
arguments and flags they accept.

cabal new-configure
-------------------

``cabal new-configure`` takes a set of arguments and writes a
``cabal.project.local`` file based on the flags passed to this command.
``cabal new-configure FLAGS; cabal new-build`` is roughly equivalent to
``cabal new-build FLAGS``, except that with ``new-configure`` the flags
are persisted to all subsequent calls to ``new-build``.

``cabal new-configure`` is intended to be a convenient way to write out
a ``cabal.project.local`` for simple configurations; e.g.,
``cabal new-configure -w ghc-7.8`` would ensure that all subsequent
builds with ``cabal new-build`` are performed with the compiler
``ghc-7.8``. For more complex configuration, we recommend writing the
``cabal.project.local`` file directly (or placing it in
``cabal.project``!)

``cabal new-configure`` inherits options from ``Cabal``. semantics:

-  Any flag accepted by ``./Setup configure``.

-  Any flag accepted by ``cabal configure`` beyond
   ``./Setup configure``, namely ``--cabal-lib-version``,
   ``--constraint``, ``--preference`` and ``--solver.``

-  Any flag accepted by ``cabal install`` beyond ``./Setup configure``.

-  Any flag accepted by ``./Setup haddock``.

The options of all of these flags apply only to *local* packages in a
project; this behavior is different than that of ``cabal install``,
which applies flags to every package that would be built. The motivation
for this is to avoid an innocuous addition to the flags of a package
resulting in a rebuild of every package in the store (which might need
to happen if a flag actually applied to every transitive dependency). To
apply options to an external package, use a ``package`` stanza in a
``cabal.project`` file.

cabal new-build
---------------

``cabal new-build`` takes a set of targets and builds them. It
automatically handles building and installing any dependencies of these
targets.

A target can take any of the following forms:

-  A package target: ``package``, which specifies that all enabled
   components of a package to be built. By default, test suites and
   benchmarks are *not* enabled, unless they are explicitly requested
   (e.g., via ``--enable-tests``.)

-  A component target: ``[package:][ctype:]component``, which specifies
   a specific component (e.g., a library, executable, test suite or
   benchmark) to be built.

In component targets, ``package:`` and ``ctype:`` (valid component types
are ``lib``, ``exe``, ``test`` and ``bench``) can be used to
disambiguate when multiple packages define the same component, or the
same component name is used in a package (e.g., a package ``foo``
defines both an executable and library named ``foo``). We always prefer
interpreting a target as a package name rather than as a component name.

Some example targets:

::

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    $ cabal new-build lib:foo-pkg       # build the library named foo-pkg
    $ cabal new-build foo-pkg:foo-tests # build foo-tests in foo-pkg
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(There is also syntax for specifying module and file targets, but it
doesn't currently do anything.)

Beyond a list of targets, ``cabal new-build`` accepts all the flags that
``cabal new-configure`` takes. Most of these flags are only taken into
consideration when building local packages; however, some flags may
cause extra store packages to be built (for example,
``--enable-profiling`` will automatically make sure profiling libraries
for all transitive dependencies are built and installed.)

cabal new-repl
--------------

``cabal new-repl TARGET`` loads all of the modules of the target into
GHCi as interpreted bytecode. It takes the same flags as
``cabal new-build``.

Currently, it is not supported to pass multiple targets to ``new-repl``
(``new-repl`` will just successively open a separate GHCi session for
each target.)

cabal new-freeze
----------------

``cabal new-freeze`` writes out a ``cabal.project.freeze`` file which
records all of the versions and flags which that are picked by the
solver under the current index and flags. A ``cabal.project.freeze``
file has the same syntax as ``cabal.project`` and looks something like
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this:
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::
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    constraints: HTTP ==4000.3.3,
                 HTTP +warp-tests -warn-as-error -network23 +network-uri -mtl1 -conduit10,
                 QuickCheck ==2.9.1,
                 QuickCheck +templatehaskell,
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                 -- etc...

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For end-user executables, it is recommended that you distribute the
``cabal.project.freeze`` file in your source repository so that all
users see a consistent set of dependencies. For libraries, this is not
recommended: users often need to build against different versions of
libraries than what you developed against.

Unsupported commands
--------------------

The following commands are not currently supported:

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``cabal new-test`` (:issue:`3638`)
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    Workaround: run the test executable directly (see `Where are my
    build products <#where-are-my-build-products>`__?)
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``cabal new-bench`` (:issue:`3638`)
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    Workaround: run the benchmark executable directly (see `Where are my
    build products <#where-are-my-build-products>`__?)
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``cabal new-run`` (:issue:`3638`)
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    Workaround: run the executable directly (see `Where are my build
    products <#where-are-my-build-products>`__?)
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``cabal new-exec``
    Workaround: if you wanted to execute GHCi, consider using
    ``cabal new-repl`` instead. Otherwise, use ``-v`` to find the list
    of flags GHC is being invoked with and pass it manually.
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``cabal new-haddock`` (:issue:`3535`)
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    Workaround: run
    ``cabal act-as-setup -- haddock --builddir=dist-newstyle/build/pkg-0.1``
    (or execute the Custom setup script directly).
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``cabal new-install`` (:issue:`3737`)
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    Workaround: no good workaround at the moment. (But note that you no
    longer need to install libraries before building!)

Configuring builds with cabal.project
=====================================

``cabal.project`` files support a variety of options which configure the
details of your build. The general syntax of a ``cabal.project`` file is
similar to that of a Cabal file: there are a number of fields, some of
which live inside stanzas:

::

    packages: */*.cabal
    with-compiler: /opt/ghc/8.0.1/bin/ghc

    package cryptohash
      optimization: False

In general, the accepted field names coincide with the accepted command
line flags that ``cabal install`` and other commands take. For example,
``cabal new-configure --library-profiling`` will write out a project
file with ``library-profiling: True``.

The full configuration of a project is determined by combining the
following sources (later entries override earlier ones):

1. ``~/.cabal/config`` (the user-wide global configuration)

2. ``cabal.project`` (the project configuratoin)

3. ``cabal.project.freeze`` (the output of ``cabal new-freeze``)

4. ``cabal.project.local`` (the output of ``cabal new-configure``)

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Specifying the local packages
-----------------------------

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.. cfg-section:: packages

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The following top-level options specify what the local packages of a
project are:

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.. cfg-field:: packages: package location list (space or comma separated)
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    :synopsis: Project packages.
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    :default: ``./*.cabal``
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    Specifies the list of package locations which contain the local
    packages to be built by this project. Package locations can take the
    following forms:

    1. They can specify a Cabal file, or a directory containing a Cabal
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       file, e.g., ``packages: Cabal cabal-install/cabal-install.cabal``.
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    2. They can specify a glob-style wildcards, which must match one or
       more (a) directories containing a (single) Cabal file, (b) Cabal
       files (extension ``.cabal``), or (c) [STRIKEOUT:tarballs which
       contain Cabal packages (extension ``.tar.gz``)] (not implemented
       yet). For example, to match all Cabal files in all
       subdirectories, as well as the Cabal projects in the parent
       directories ``foo`` and ``bar``, use
       ``packages: */*.cabal ../{foo,bar}/``

    3. [STRIKEOUT:They can specify an ``http``, ``https`` or ``file``
       URL, representing the path to a remote tarball to be downloaded
       and built.] (not implemented yet)

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    There is no command line variant of this field; see :issue:`3585`.
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.. cfg-field:: optional-packages: package location list (space or comma-separated)
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    :synopsis: Optional project packages.
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    :default: ``./*/*.cabal``

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    Like :cfg-field:`packages`, specifies a list of package locations
    containing local packages to be built. Unlike :cfg-field:`packages`,
    if we glob for a package, it is permissible for the glob to match against
    zero packages. The intended use-case for :cfg-field:`optional-packages`
    is to make it so that vendored packages can be automatically picked up if
    they are placed in a subdirectory, but not error if there aren't any.
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    There is no command line variant of this field.

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.. cfg-field:: extra-packages: package list with version bounds (comma separated)
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    :synopsis: Adds external pacakges as local
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    [STRIKEOUT:Specifies a list of external packages from Hackage which
    should be considered local packages.] (Not implemented)

    There is no command line variant of this field.

[STRIKEOUT:There is also a stanza ``source-repository-package`` for
specifying packages from an external version control.] (Not
implemented.)

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All local packages are *vendored*, in the sense that if other packages
(including external ones from Hackage) depend on a package with the name
of a local package, the local package is preferentially used.  This
motivates the default settings::

    packages: ./*.cabal
    optional-packages: ./*/*.cabal

...any package can be vendored simply by making a checkout in the
top-level project directory, as might be seen in this hypothetical
directory layout::

    foo.cabal
    foo-helper/     # local package
    unix/           # vendored external package

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All of these options support globs. ``cabal new-build`` has its own glob
format:

-  Anywhere in a path, as many times as you like, you can specify an
   asterisk ``*`` wildcard. E.g., ``*/*.cabal`` matches all ``.cabal``
   files in all immediate subdirectories. Like in glob(7), asterisks do
   not match hidden files unless there is an explicit period, e.g.,
   ``.*/foo.cabal`` will match ``.private/foo.cabal`` (but
   ``*/foo.cabal`` will not).

-  You can use braces to specify specific directories; e.g.,
   ``{vendor,pkgs}/*.cabal`` matches all Cabal files in the ``vendor``
   and ``pkgs`` subdirectories.

Formally, the format described by the following BNF:

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.. code-block:: abnf
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    FilePathGlob    ::= FilePathRoot FilePathGlobRel
    FilePathRoot    ::= {- empty -}        # relative to cabal.project
                      | "/"                # Unix root
                      | [a-zA-Z] ":" [/\\] # Windows root
                      | "~"                # home directory
    FilePathGlobRel ::= Glob "/"  FilePathGlobRel # Unix directory
                      | Glob "\\" FilePathGlobRel # Windows directory
                      | Glob         # file
                      | {- empty -}  # trailing slash
    Glob      ::= GlobPiece *
    GlobPiece ::= "*"            # wildcard
                | [^*{},/\\] *   # literal string
                | "\\" [*{},]    # escaped reserved character
                | "{" Glob "," ... "," Glob "}" # union (match any of these)

Global configuration options
----------------------------

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.. cfg-section:: global

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The following top-level configuration options are not specific to any
package, and thus apply globally:

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.. cfg-field:: verbose: nat
               --verbose=n, -vn
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    :synopsis: Build verbosity level.
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    :default: 1

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    Control the verbosity of ``cabal`` commands, valid values are from 0
    to 3.

    The command line variant of this field is ``--verbose=2``; a short
    form ``-v2`` is also supported.

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.. cfg-field:: jobs: nat or $ncpus
               --jobs=n, -jn, --jobs=$ncpus
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    :synopsis: Number of builds running in parallel.
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    :default: 1

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    Run *nat* jobs simultaneously when building. If ``$ncpus`` is
    specified, run the number of jobs equal to the number of CPUs.
    Package building is often quite parallel, so turning on parallelism
    can speed up build times quite a bit!

    The command line variant of this field is ``--jobs=2``; a short form
    ``-j2`` is also supported; a bare ``--jobs`` or ``-j`` is equivalent
    to ``--jobs=$ncpus``.

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.. cfg-field::  keep-going: boolean
                --keep-going
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    :synopsis: Try to continue building on failure.
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    :default: False

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    If true, after a build failure, continue to build other unaffected
    packages.

    The command line variant of this field is ``--keep-going``.

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Solver configuration options
----------------------------

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.. cfg-section:: solver

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The following settings control the behavior of the dependency solver:

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.. cfg-field:: constraints: constraints list (comma separated)
               --constrant="pkg >= 2.0"
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    :synopsis: Extra dependencies constraints.
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    Add extra constraints to the version bounds, flag settings, and
    other properties a solver can pick for a package. For example, to
    only consider install plans that do not use ``bar`` at all, or use
    ``bar-2.1``, write:

    ::

        constraints: bar == 2.1

    Version bounds have the same syntax as ``build-depends``. You can
    also specify flag assignments:

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    ::
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        -- Require bar to be installed with the foo flag turned on and
        -- the baz flag turned off
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        constraints: bar +foo -baz

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        -- Require that bar NOT be present in the install plan. Note:
        -- this is just syntax sugar for '> 1 && < 1', and is supported
        -- by build-depends.
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        constraints: bar -none

    A package can be specified multiple times in ``constraints``, in
    which case the specified constraints are intersected. This is
    useful, since the syntax does not allow you to specify multiple
    constraints at once. For example, to specify both version bounds and
    flag assignments, you would write:

    ::

        constraints: bar == 2.1,
                     bar +foo -baz,

    There are also some more specialized constraints, which most people
    don't generally need:

    ::

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        -- Require bar to be preinstalled in the global package database
        -- (this does NOT include the Nix-local build global store.)
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        constraints: bar installed

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        -- Require the local source copy of bar to be used
        -- (Note: By default, if we have a local package we will
        -- automatically use it, so it generally not be necessary to
        -- specify this)
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        constraints: bar source

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        -- Require that bar be solved with test suites and benchmarks enabled
        -- (Note: By default, new-build configures the solver to make
        -- a best-effort attempt to enable these stanzas, so this generally
        -- should not be necessary.)
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        constraints: bar test,
                     bar bench

    The command line variant of this field is
    ``--constraint="pkg >= 2.0"``; to specify multiple constraints, pass
    the flag multiple times.

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.. cfg-field:: preferences: preference (comma separated)
               --preference="pkg >= 2.0"
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    :synopsis: Prefered dependency versions.
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    Like :cfg-field:`constraints`, but the solver will attempt to satisfy
    these preferences on a best-effort basis. The resulting install is locally
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    optimal with respect to preferences; specifically, no single package
    could be replaced with a more preferred version that still satisfies
    the hard constraints.

    Operationally, preferences can cause the solver to attempt certain
    version choices of a package before others, which can improve
    dependency solver runtime.

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    One way to use :cfg-field:`preferences` is to take a known working set of
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    constraints (e.g., via ``cabal new-freeze``) and record them as
    preferences. In this case, the solver will first attempt to use this
    configuration, and if this violates hard constraints, it will try to
    find the minimal number of upgrades to satisfy the hard constraints
    again.

    The command line variant of this field is
    ``--preference="pkg >= 2.0"``; to specify multiple preferences, pass
    the flag multiple times.

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.. cfg-field:: allow-newer: none, all or list of scoped package names (space or comma separated)
               --allow-newer, --allow-newer=[none,all,pkg]
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    :synopsis: Lift dependencies upper bound constaints.
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    :default: ``none``

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    Allow the solver to pick an newer version of some packages than
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    would normally be permitted by than the :pkg-field:`build-depends` bounds
    of packages in the install plan. This option may be useful if the
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    dependency solver cannot otherwise find a valid install plan.

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    For example, to relax ``pkg``\ s :pkg-field:`build-depends` upper bound on
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    ``dep-pkg``, write a scoped package name of the form:

    ::

        allow-newer: pkg:dep-pkg

    This syntax is recommended, as it is often only a single package
    whose upper bound is misbehaving. In this case, the upper bounds of
    other packages should still be respected; indeed, relaxing the bound
    can break some packages which test the selected version of packages.

    However, in some situations (e.g., when attempting to build packages
    on a new version of GHC), it is useful to disregard *all*
    upper-bounds, with respect to a package or all packages. This can be
    done by specifying just a package name, or using the keyword ``all``
    to specify all packages:

    ::

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        -- Disregard upper bounds involving the dependencies on
        -- packages bar, baz and quux
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        allow-newer: bar, baz, quux

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        -- Disregard all upper bounds when dependency solving
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        allow-newer: all

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    :cfg-field:`allow-newer` is often used in conjunction with a constraint
    (in the cfg-field:`constraints` field) forcing the usage of a specific,
    newer version of a package.
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    The command line variant of this field is ``--allow-newer=bar``. A
    bare ``--allow-newer`` is equivalent to ``--allow-newer=all``.

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.. cfg-field:: allow-older: none, all, list of scoped package names (space or comma separated)
               --allow-older, --allow-older=[none,all,pkg]
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    :synopsis: Lift dependency lower bound constaints.
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    :default: ``none``

    Like :cfg-field:`allow-newer`, but applied to lower bounds rather than
    upper bounds.
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    The command line variant of this field is ``--allow-older=all``. A
    bare ``--allow-older`` is equivalent to ``--allow-older=all``.

Package configuration options
-----------------------------

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.. cfg-section:: package

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Package options affect the building of specific packages. There are two
ways a package option can be specified:

-  They can be specified at the top-level, in which case they apply only
   to **local package**, or

-  They can be specified inside a ``package`` stanza, in which case they
   apply to the build of the package, whether or not it is local or
   external.

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For example, the following options specify that :cfg-field:`optimization`
should be turned off for all local packages, and that ``bytestring`` (possibly
an external dependency) should be built with ``-fno-state-hack``::
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    optimization: False

    package bytestring
        ghc-options: -fno-state-hack

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``ghc-options`` is not specifically described in this documentation,
but is one of many fields for configuring programs.  They take the form
``progname-options`` and ``progname-location``, and
can only be set inside package stanzas.  (TODO: They are not supported
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at top-level, see :issue:`3579`.
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At the moment, there is no way to specify an option to apply to all
external packages or all inplace packages. Additionally, it is only
possible to specify these options on the command line for all local
packages (there is no per-package command line interface.)

Some flags were added by more recent versions of the Cabal library. This
means that they are NOT supported by packages which use Custom setup
scripts that require a version of the Cabal library older than when the
feature was added.

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.. cfg-field:: flags: list of +flagname or -flagname (space separated)
               --flags="+foo -bar", -ffoo, -f-bar
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    :synopsis: Enable or disable package flags.
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    Force all flags specified as ``+flagname`` to be true, and all flags
    specified as ``-flagname`` to be false. For example, to enable the
    flag ``foo`` and disable ``bar``, set:

    ::

        flags: +foo -bar

    If there is no leading punctuation, it is assumed that the flag
    should be enabled; e.g., this is equivalent:

    ::

        flags: foo -bar

    Flags are *per-package*, so it doesn't make much sense to specify
    flags at the top-level, unless you happen to know that *all* of your
    local packages support the same named flags. If a flag is not
    supported by a package, it is ignored.

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    See also the solver configuration field :cfg-field:`constraints`.
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    The command line variant of this flag is ``--flags``. There is also
    a shortened form ``-ffoo -f-bar``.

    A common mistake is to say ``cabal new-build -fhans``, where
    ``hans`` is a flag for a transitive dependency that is not in the
    local package; in this case, the flag will be silently ignored. If
    ``haskell-tor`` is the package you want this flag to apply to, try
    ``--constraint="haskell-tor +hans"`` instead.

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.. cfg-field:: with-compiler: executable
               --with-compiler=executable
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    :synopsis: Path to compiler executable.
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    Specify the path to a particular compiler to be used. If not an
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    absolute path, it will be resolved according to the :envvar:`PATH`
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    environment. The type of the compiler (GHC, GHCJS, etc) must be
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    consistent with the setting of the :cfg-field:`compiler` field.
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    The most common use of this option is to specify a different version
    of your compiler to be used; e.g., if you have ``ghc-7.8`` in your
    path, you can specify ``with-compiler: ghc-7.8`` to use it.

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    This flag also sets the default value of :cfg-field:`with-hc-pkg`, using
    the heuristic that it is named ``ghc-pkg-7.8`` (if your executable name
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    is suffixed with a version number), or is the executable named
    ``ghc-pkg`` in the same directory as the ``ghc`` directory. If this
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    heuristic does not work, set :cfg-field:`with-hc-pkg` explicitly.
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    For inplace packages, ``cabal new-build`` maintains a separate build
    directory for each version of GHC, so you can maintain multiple
    build trees for different versions of GHC without clobbering each
    other.

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    At the moment, it's not possible to set :cfg-field:`with-compiler` on a
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    per-package basis, but eventually we plan on relaxing this
    restriction. If this is something you need, give us a shout.

    The command line variant of this flag is
    ``--with-compiler=ghc-7.8``; there is also a short version
    ``-w ghc-7.8``.

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.. cfg-field:: with-hc-pkg: executable
               --with-hc-pkg=executable
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    :synopsis: Specifies package tool.
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    Specify the path to the package tool, e.g., ``ghc-pkg``. This
    package tool must be compatible with the compiler specified by
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    :cfg-field:`with-compiler` (generally speaking, it should be precisely
    the tool that was distributed with the compiler). If this option is
    omitted, the default value is determined from :cfg-field:`with-compiler`.
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    The command line variant of this flag is
    ``--with-hc-pkg=ghc-pkg-7.8``.

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.. cfg-field:: optimization: nat
               --enable-optimization
               --disable-optimization
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    :synopsis: Build with optimization.
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    :default: ``1``

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    Build with optimization. This is appropriate for production use,
    taking more time to build faster libraries and programs.

    The optional *nat* value is the optimisation level. Some compilers
    support multiple optimisation levels. The range is 0 to 2. Level 0
    disables optimization, level 1 is the default. Level 2 is higher
    optimisation if the compiler supports it. Level 2 is likely to lead
    to longer compile times and bigger generated code. If you are not
    planning to run code, turning off optimization will lead to better
    build times and less code to be rebuilt when a module changes.

    We also accept ``True`` (equivalent to 1) and ``False`` (equivalent
    to 0).

    Note that as of GHC 8.0, GHC does not recompile when optimization
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    levels change (see :ghc-ticket:`10923`), so if
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    you change the optimization level for a local package you may need
    to blow away your old build products in order to rebuild with the
    new optimization level.

    The command line variant of this flag is ``-O2`` (with ``-O1``
    equivalent to ``-O``). There are also long-form variants
    ``--enable-optimization`` and ``--disable-optimization``.

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.. cfg-field:: configure-options: args (space separated)
               --configure-option=arg
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    :synopsis: Options to pass to configure script.
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    A list of extra arguments to pass to the external ``./configure``
    script, if one is used. This is only useful for packages which have
    the ``Configure`` build type. See also the section on
    `system-dependent
    parameters <developing-packages.html#system-dependent-parameters>`__.

    The command line variant of this flag is ``--configure-option=arg``,
    which can be specified multiple times to pass multiple options.

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.. cfg-field:: compiler: ghc, ghcjs, jhc, lhc, uhc or haskell-suite
               --compiler=compiler
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    :synopsis: Compiler to build with.
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    :default: ``ghc``

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    Specify which compiler toolchain to be used. This is independent of
    ``with-compiler``, because the choice of toolchain affects Cabal's
    build logic.

    The command line variant of this flag is ``--compiler=ghc``.

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.. cfg-field:: tests: boolean
               --enable-tests
               --disable-tests
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    :synopsis: Build tests.
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    :default: ``False``

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    Force test suites to be enabled. For most users this should not be
    needed, as we always attempt to solve for test suite dependencies,
    even when this value is ``False``; furthermore, test suites are
    automatically enabled if they are requested as a built target.

    The command line variant of this flag is ``--enable-tests`` and
    ``--disable-tests``.

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.. cfg-field:: benchmarks: boolean
               --enable-benchmarks
               --disable-benchmarks
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    :synopsis: Build benchmarks.
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    :default: ``False``

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    Force benchmarks to be enabled. For most users this should not be
    needed, as we always attempt to solve for benchmark dependencies,
    even when this value is ``False``; furthermore, benchmarks are
    automatically enabled if they are requested as a built target.

    The command line variant of this flag is ``--enable-benchmarks`` and
    ``--disable-benchmarks``.

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.. cfg-field:: extra-prog-path: paths (newline or comma separated)
               --extra-prog-path=PATH
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    :synopsis: Add directories to program search path.
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    :since: 1.18

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    A list of directories to search for extra required programs. Most
    users should not need this, as programs like ``happy`` and ``alex``
    will automatically be installed and added to the path. This can be
    useful if a ``Custom`` setup script relies on an exotic extra
    program.

    The command line variant of this flag is ``--extra-prog-path=PATH``,
    which can be specified multiple times.

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.. cfg-field:: run-tests: boolean
               --run-tests
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    :synopsis: Run package test suite upon installation.
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    :default: ``False``

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    Run the package test suite upon installation. This is useful for
    saying "When this package is installed, check that the test suite
    passes, terminating the rest of the build if it is broken."

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    .. warning::

      One deficiency: the :cfg-field:`run-tests` setting of a package is NOT
      recorded as part of the hash, so if you install something without
      :cfg-field:`run-tests` and then turn on ``run-tests``, we won't
      subsequently test the package. If this is causing you problems, give
      us a shout.
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    The command line variant of this flag is ``--run-tests``.

Object code options
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^^^^^^^^^^^^^^^^^^^
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.. cfg-section:: object

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.. cfg-field:: debug-info: boolean
               --enable-debug-info
               --disable-debug-info
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    :synopsis: Build with debug info enabled.
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    :since: 1.22

    :default: False

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    If the compiler (e.g., GHC 7.10 and later) supports outputing OS
    native debug info (e.g., DWARF), setting ``debug-info: True`` will
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    instruct it to do so. See the GHC wiki page on :ghc-wiki:`DWARF`
    for more information about this feature.
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    (This field also accepts numeric syntax, but as of GHC 8.0 this
    doesn't do anything.)

    The command line variant of this flag is ``--enable-debug-info`` and
    ``--disable-debug-info``.

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.. cfg-field:: split-objs: boolean
               --enable-split-objs
               --disable-split-objs
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    :synopsis: Use GHC split objects feature.
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    :default: False

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    Use the GHC ``-split-objs`` feature when building the library. This
    reduces the final size of the executables that use the library by
    allowing them to link with only the bits that they use rather than
    the entire library. The downside is that building the library takes
    longer and uses considerably more memory.

    The command line variant of this flag is ``--enable-split-objs`` and
    ``--disable-split-objs``.

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.. cfg-field:: executable-stripping: boolean
               --enable-executable-stripping
               --disable-executable-stripping
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    :synopsis: Strip installed programs.
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    :default: True

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    When installing binary executable programs, run the ``strip``
    program on the binary. This can considerably reduce the size of the
    executable binary file. It does this by removing debugging
    information and symbols.

    Not all Haskell implementations generate native binaries. For such
    implementations this option has no effect.

    (TODO: Check what happens if you combine this with ``debug-info``.)

    The command line variant of this flag is
    ``--enable-executable-stripping`` and
    ``--disable-executable-stripping``.

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.. cfg-field:: library-stripping: boolean
               --enable-library-stripping
               --disable-library-stripping
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    :synopsis: Strip installed libraries.
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    :since: 1.19

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    When installing binary libraries, run the ``strip`` program on the
    binary, saving space on the file system. See also
    ``executable-stripping``.

    The command line variant of this flag is
    ``--enable-library-stripping`` and ``--disable-library-stripping``.

Executable options
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^^^^^^^^^^^^^^^^^^
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.. cfg-section:: exectuable

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.. cfg-field:: program-prefix: prefix
               --program-prefix=prefix
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    :synopsis: Prepend prefix to program names.
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    [STRIKEOUT:Prepend *prefix* to installed program names.] (Currently
    implemented in a silly and not useful way. If you need this to work
    give us a shout.)

    *prefix* may contain the following path variables: ``$pkgid``,
    ``$pkg``, ``$version``, ``$compiler``, ``$os``, ``$arch``, ``$abi``,
    ``$abitag``

    The command line variant of this flag is ``--program-prefix=foo-``.

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.. cfg-field:: program-suffix: suffix
               --program-suffix=suffix
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    :synopsis: Append refix to program names.
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    [STRIKEOUT:Append *suffix* to installed program names.] (Currently
    implemented in a silly and not useful way. If you need this to work
    give us a shout.)

    The most obvious use for this is to append the program's version
    number to make it possible to install several versions of a program
    at once: ``program-suffix: $version``.

    *suffix* may contain the following path variables: ``$pkgid``,
    ``$pkg``, ``$version``, ``$compiler``, ``$os``, ``$arch``, ``$abi``,
    ``$abitag``

    The command line variant of this flag is
    ``--program-suffix='$version'``.

Dynamic linking options
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^^^^^^^^^^^^^^^^^^^^^^^
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.. cfg-section:: linking

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.. cfg-field:: shared: boolean
               --enable-shared
               --disable-shared
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    :synopsis: Build shared library.
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    :default: False

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    Build shared library. This implies a separate compiler run to
    generate position independent code as required on most platforms.

    The command line variant of this flag is ``--enable-shared`` and
    ``--disable-shared``.

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.. cfg-field:: executable-dynamic: boolean
               --enable-executable-dynamic
               --disable-executable-dynamic
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    :synopsis: Link executables dynamically.
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    :default: False

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    Link executables dynamically. The executable's library dependencies
    should be built as shared objects. This implies ``shared: True``
    unless ``shared: False`` is explicitly specified.

    The command line variant of this flag is
    ``--enable-executable-dynamic`` and
    ``--disable-executable-dynamic``.

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.. cfg-field:: library-for-ghci: boolean
               --enable-library-for-ghci
               --disable-library-for-ghci
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    :synopsis: Build libraries suitable for use with GHCi.
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    :default: True

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    Build libraries suitable for use with GHCi. This involves an extra
    linking step after the build.

    Not all platforms support GHCi and indeed on some platforms, trying
    to build GHCi libs fails. In such cases, consider setting
    ``library-for-ghci: False``.

    The command line variant of this flag is
    ``--enable-library-for-ghci`` and ``--disable-library-for-ghci``.

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.. cfg-field:: relocatable:
               --relocatable
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    :synopsis: Build relocatable package.
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    :since: 1.21

    :default: False

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    [STRIKEOUT:Build a package which is relocatable.] (TODO: It is not
    clear what this actually does, or if it works at all.)

    The command line variant of this flag is ``--relocatable``.

Foreign function interface options
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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.. cfg-section:: ffi

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.. cfg-field:: extra-include-dirs: directories (comma or newline separated list)
               --extra-include-dirs=DIR
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    :synopsis: Adds C header search path.
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    An extra directory to search for C header files. You can use this
    flag multiple times to get a list of directories.

    You might need to use this flag if you have standard system header
    files in a non-standard location that is not mentioned in the
    package's ``.cabal`` file. Using this option has the same affect as
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    appending the directory *dir* to the :pkg-field:`include-dirs` field in each
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    library and executable in the package's ``.cabal`` file. The
    advantage of course is that you do not have to modify the package at
    all. These extra directories will be used while building the package
    and for libraries it is also saved in the package registration
    information and used when compiling modules that use the library.

    The command line variant of this flag is
    ``--extra-include-dirs=DIR``, which can be specified multiple times.

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.. cfg-field:: extra-lib-dirs: directories (comma or newline separated list)
               --extra-lib-dirs=DIR
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    :synopsis: Adds library search directory.
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    An extra directory to search for system libraries files.

    The command line variant of this flag is ``--extra-lib-dirs=DIR``,
    which can be specified multiple times.

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.. cfg-field:: extra-framework-dirs: directories (comma or newline separated list)
               --extra-framework-dirs=DIR
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    :synopsis: Adds framework search directory (OS X only).
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    An extra directory to search for frameworks (OS X only).

    You might need to use this flag if you have standard system
    libraries in a non-standard location that is not mentioned in the
    package's ``.cabal`` file. Using this option has the same affect as
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    appending the directory *dir* to the :cfg-field:`extra-lib-dirs` field in
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    each library and executable in the package's ``.cabal`` file. The
    advantage of course is that you do not have to modify the package at
    all. These extra directories will be used while building the package
    and for libraries it is also saved in the package registration
    information and used when compiling modules that use the library.

    The command line variant of this flag is
    ``--extra-framework-dirs=DIR``, which can be specified multiple
    times.

Profiling options
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^^^^^^^^^^^^^^^^^
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.. cfg-section:: profiling

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.. cfg-field:: profiling: boolean
               --enable-profiling
               --disable-profiling
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    :synopsis: Enable profiling builds.
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    :since: 1.21

    :default: False

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    Build libraries and executables with profiling enabled (for
    compilers that support profiling as a separate mode). It is only
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    necessary to specify :cfg-field:`profiling` for the specific package you
    want to profile; ``cabal new-build`` will ensure that all of its
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    transitive dependencies are built with profiling enabled.

    To enable profiling for only libraries or executables, see
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    :cfg-field:`library-profiling` and :cfg-field:`executable-profiling`.
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    For useful profiling, it can be important to control precisely what