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.. _using-ghc:

Using GHC
=========

.. index::
   single: GHC, using
   single: using GHC

Getting started: compiling programs
-----------------------------------

In this chapter you'll find a complete reference to the GHC command-line
syntax, including all 400+ flags. It's a large and complex system, and
there are lots of details, so it can be quite hard to figure out how to
get started. With that in mind, this introductory section provides a
quick introduction to the basic usage of GHC for compiling a Haskell
program, before the following sections dive into the full syntax.

Let's create a Hello World program, and compile and run it. First,
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create a file :file:`hello.hs` containing the Haskell code: ::
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    main = putStrLn "Hello, World!"

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To compile the program, use GHC like this:

.. code-block:: sh
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    $ ghc hello.hs

(where ``$`` represents the prompt: don't type it). GHC will compile the
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source file :file:`hello.hs`, producing an object file :file:`hello.o` and an
interface file :file:`hello.hi`, and then it will link the object file to
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the libraries that come with GHC to produce an executable called
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:file:`hello` on Unix/Linux/Mac, or :file:`hello.exe` on Windows.
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By default GHC will be very quiet about what it is doing, only printing
error messages. If you want to see in more detail what's going on behind
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the scenes, add :ghc-flag:`-v` to the command line.

Then we can run the program like this:
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.. code-block:: sh
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    $ ./hello
    Hello World!

If your program contains multiple modules, then you only need to tell
GHC the name of the source file containing the ``Main`` module, and GHC
will examine the ``import`` declarations to find the other modules that
make up the program and find their source files. This means that, with
the exception of the ``Main`` module, every source file should be named
after the module name that it contains (with dots replaced by directory
separators). For example, the module ``Data.Person`` would be in the
file ``Data/Person.hs`` on Unix/Linux/Mac, or ``Data\Person.hs`` on
Windows.

Options overview
----------------

GHC's behaviour is controlled by options, which for historical reasons
are also sometimes referred to as command-line flags or arguments.
Options can be specified in three ways:

Command-line arguments
~~~~~~~~~~~~~~~~~~~~~~

.. index::
   single: structure, command-line
   single: command-line; arguments
   single: arguments; command-line

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An invocation of GHC takes the following form:

.. code-block:: none
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    ghc [argument...]

Command-line arguments are either options or file names.

Command-line options begin with ``-``. They may *not* be grouped:
``-vO`` is different from ``-v -O``. Options need not precede filenames:
e.g., ``ghc *.o -o foo``. All options are processed and then applied to
all files; you cannot, for example, invoke
``ghc -c -O1 Foo.hs -O2 Bar.hs`` to apply different optimisation levels
to the files ``Foo.hs`` and ``Bar.hs``.

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.. note::

    .. index::
       single: command-line; order of arguments

    Note that command-line options are *order-dependent*, with arguments being
    evaluated from left-to-right. This can have seemingly strange effects in the
    presence of flag implication. For instance, consider
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    :ghc-flag:`-fno-specialise <-fspecialise>` and :ghc-flag:`-O1` (which implies
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    :ghc-flag:`-fspecialise`). These two command lines mean very different
    things:

    ``-fno-specialise -O1``

        ``-fspecialise`` will be enabled as the ``-fno-specialise`` is overriden
        by the ``-O1``.

    ``-O1 -fno-specialise``

        ``-fspecialise`` will not be enabled, since the ``-fno-specialise``
        overrides the ``-fspecialise`` implied by ``-O1``.

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.. _source-file-options:

Command line options in source files
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. index::
   single: source-file options

Sometimes it is useful to make the connection between a source file and
the command-line options it requires quite tight. For instance, if a
Haskell source file deliberately uses name shadowing, it should be
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compiled with the ``-Wno-name-shadowing`` option. Rather than
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maintaining the list of per-file options in a ``Makefile``, it is
possible to do this directly in the source file using the
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``OPTIONS_GHC`` :ref:`pragma <options-pragma>` ::
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    {-# OPTIONS_GHC -Wno-name-shadowing #-}
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    module X where
    ...

``OPTIONS_GHC`` is a *file-header pragma* (see :ref:`options-pragma`).

Only *dynamic* flags can be used in an ``OPTIONS_GHC`` pragma (see
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:ref:`mode-dynamic-flags`).
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Note that your command shell does not get to the source file options,
they are just included literally in the array of command-line arguments
the compiler maintains internally, so you'll be desperately disappointed
if you try to glob etc. inside ``OPTIONS_GHC``.

.. note::
   The contents of ``OPTIONS_GHC`` are appended to the command-line
   options, so options given in the source file override those given on the
   command-line.

It is not recommended to move all the contents of your Makefiles into
your source files, but in some circumstances, the ``OPTIONS_GHC`` pragma
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is the Right Thing. (If you use :ghc-flag:`-keep-hc-file` and have ``OPTION`` flags in
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your module, the ``OPTIONS_GHC`` will get put into the generated ``.hc`` file).

Setting options in GHCi
~~~~~~~~~~~~~~~~~~~~~~~

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Options may also be modified from within GHCi, using the :ghci-cmd:`:set`
command.
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.. _mode-dynamic-flags:
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Dynamic and Mode options
------------------------
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.. index::
   single: dynamic; options
   single: mode; options

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Each of GHC's command line options is classified as dynamic or mode:
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    Mode: A mode may be used on the command line only.
    You can pass only one mode flag.
    For example, :ghc-flag:`--make` or :ghc-flag:`-E`.
    The available modes are listed in :ref:`modes`.
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    Dynamic: A dynamic flag may be used on the command line,
    in a ``OPTIONS_GHC`` pragma in a source
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    file, or set using :ghci-cmd:`:set` in GHCi.
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The flag reference tables (:ref:`flag-reference`) lists the status of
each flag.

.. _file-suffixes:

Meaningful file suffixes
------------------------

.. index::
   single: suffixes, file
   single: file suffixes for GHC

File names with "meaningful" suffixes (e.g., ``.lhs`` or ``.o``) cause
the "right thing" to happen to those files.

``.hs``
    A Haskell module.

``.lhs``
    .. index::
       single: lhs file extension

    A literate Haskell module.

``.hspp``
    A file created by the preprocessor.

``.hi``
    A Haskell interface file, probably compiler-generated.

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``.hie``
    An extended Haskell interface file, produced by the Haskell compiler.

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``.hc``
    Intermediate C file produced by the Haskell compiler.

``.c``
    A C file not produced by the Haskell compiler.

``.ll``
    An llvm-intermediate-language source file, usually produced by the
    compiler.

``.bc``
    An llvm-intermediate-language bitcode file, usually produced by the
    compiler.

``.s``
    An assembly-language source file, usually produced by the compiler.

``.o``
    An object file, produced by an assembler.

Files with other suffixes (or without suffixes) are passed straight to
the linker.

.. _modes:

Modes of operation
------------------

.. index::
   single: help options

GHC's behaviour is firstly controlled by a mode flag. Only one of these
flags may be given, but it does not necessarily need to be the first
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option on the command-line. For instance,

.. code-block:: none
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    $ ghc Main.hs --make -o my-application

If no mode flag is present, then GHC will enter :ghc-flag:`--make` mode
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(:ref:`make-mode`) if there are any Haskell source files given on the
command line, or else it will link the objects named on the command line
to produce an executable.

The available mode flags are:

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.. ghc-flag:: --interactive
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    :shortdesc: Interactive mode - normally used by just running ``ghci``;
        see :ref:`ghci` for details.
    :type: mode
    :category: modes
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    .. index::
       single: interactive mode
       single: GHCi

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    Interactive mode, which is also available as :program:`ghci`. Interactive
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    mode is described in more detail in :ref:`ghci`.

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.. ghc-flag:: --make
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    :shortdesc: Build a multi-module Haskell program, automatically figuring out
        dependencies. Likely to be much easier, and faster, than using
        ``make``; see :ref:`make-mode` for details.
    :type: mode
    :category: modes
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    .. index::
       single: make mode; of GHC

    In this mode, GHC will build a multi-module Haskell program
    automatically, figuring out dependencies for itself. If you have a
    straightforward Haskell program, this is likely to be much easier,
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    and faster, than using :command:`make`. Make mode is described in
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    :ref:`make-mode`.

    This mode is the default if there are any Haskell source files
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    mentioned on the command line, and in this case the :ghc-flag:`--make`
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    option can be omitted.

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.. ghc-flag:: -e ⟨expr⟩
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    :shortdesc: Evaluate ``expr``; see :ref:`eval-mode` for details.
    :type: mode
    :category: modes
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    .. index::
       single: eval mode; of GHC

    Expression-evaluation mode. This is very similar to interactive
    mode, except that there is a single expression to evaluate (⟨expr⟩)
    which is given on the command line. See :ref:`eval-mode` for more
    details.

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.. ghc-flag:: -E
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    :shortdesc: Stop after preprocessing (``.hspp`` file)
    :type: mode
    :category: phases

    Stop after preprocessing (``.hspp`` file)

.. ghc-flag:: -C
    :shortdesc: Stop after generating C (``.hc`` file)
    :type: mode
    :category: phases
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    Stop after generating C (``.hc`` file)

.. ghc-flag:: -S
    :shortdesc: Stop after generating assembly (``.s`` file)
    :type: mode
    :category: phases

    Stop after generating assembly (``.s`` file)

.. ghc-flag:: -c
    :shortdesc: Stop after generating object (``.o``) file
    :type: mode
    :category: phases
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    Stop after generating object (``.o``) file
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    This is the traditional batch-compiler mode, in which GHC can
    compile source files one at a time, or link objects together into an
    executable. See :ref:`options-order`.

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.. ghc-flag:: -M
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    :shortdesc: generate dependency information suitable for use in a
        ``Makefile``; see :ref:`makefile-dependencies` for details.
    :type: mode
    :category: modes
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    .. index::
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        single: dependency-generation mode; of GHC
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    Dependency-generation mode. In this mode, GHC can be used to
    generate dependency information suitable for use in a ``Makefile``.
    See :ref:`makefile-dependencies`.

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.. ghc-flag:: --frontend ⟨module⟩
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    :shortdesc: run GHC with the given frontend plugin; see
        :ref:`frontend_plugins` for details.
    :type: mode
    :category: modes
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    .. index::
        single: frontend plugins; using

    Run GHC using the given frontend plugin. See :ref:`frontend_plugins` for
    details.

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.. ghc-flag:: --mk-dll
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    :shortdesc: DLL-creation mode (Windows only)
    :type: mode
    :category: modes
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    .. index::
       single: DLL-creation mode

    DLL-creation mode (Windows only). See :ref:`win32-dlls-create`.

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.. ghc-flag:: --help
              -?
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    :shortdesc: Display help
    :type: mode
    :category: modes
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    Cause GHC to spew a long usage message to standard output and then
    exit.

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.. ghc-flag:: --show-iface ⟨file⟩
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    :shortdesc: display the contents of an interface file.
    :type: mode
    :category: modes
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    Read the interface in ⟨file⟩ and dump it as text to ``stdout``. For
    example ``ghc --show-iface M.hi``.

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.. ghc-flag:: --supported-extensions
              --supported-languages
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    :shortdesc: display the supported language extensions
    :type: mode
    :category: modes
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    Print the supported language extensions.

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.. ghc-flag:: --show-options
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    :shortdesc: display the supported command line options
    :type: mode
    :category: modes
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    Print the supported command line options. This flag can be used for
    autocompletion in a shell.

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.. ghc-flag:: --info
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    :shortdesc: display information about the compiler
    :type: mode
    :category: modes
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    Print information about the compiler.

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.. ghc-flag:: --version
              -V
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    :shortdesc: display GHC version
    :type: mode
    :category: modes
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    Print a one-line string including GHC's version number.

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.. ghc-flag:: --numeric-version
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    :shortdesc: display GHC version (numeric only)
    :type: mode
    :category: modes
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    Print GHC's numeric version number only.

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.. ghc-flag:: --print-libdir
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    :shortdesc: display GHC library directory
    :type: mode
    :category: modes
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    .. index::
       single: libdir

    Print the path to GHC's library directory. This is the top of the
    directory tree containing GHC's libraries, interfaces, and include
    files (usually something like ``/usr/local/lib/ghc-5.04`` on Unix).
    This is the value of ``$libdir`` in the package
    configuration file (see :ref:`packages`).

.. _make-mode:

Using ``ghc`` ``--make``
~~~~~~~~~~~~~~~~~~~~~~~~

.. index::
   single: --make; mode of GHC
   single: separate compilation

In this mode, GHC will build a multi-module Haskell program by following
dependencies from one or more root modules (usually just ``Main``). For
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example, if your ``Main`` module is in a file called :file:`Main.hs`, you
could compile and link the program like this:

.. code-block:: none
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    ghc --make Main.hs

In fact, GHC enters make mode automatically if there are any Haskell
source files on the command line and no other mode is specified, so in
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this case we could just type

.. code-block:: none
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    ghc Main.hs

Any number of source file names or module names may be specified; GHC
will figure out all the modules in the program by following the imports
from these initial modules. It will then attempt to compile each module
which is out of date, and finally, if there is a ``Main`` module, the
program will also be linked into an executable.

The main advantages to using ``ghc --make`` over traditional
``Makefile``\s are:

-  GHC doesn't have to be restarted for each compilation, which means it
   can cache information between compilations. Compiling a multi-module
   program with ``ghc --make`` can be up to twice as fast as running
   ``ghc`` individually on each source file.

-  You don't have to write a ``Makefile``.

   .. index::
      single: Makefiles; avoiding

-  GHC re-calculates the dependencies each time it is invoked, so the
   dependencies never get out of sync with the source.

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-  Using the :ghc-flag:`-j[⟨n⟩]` flag, you can compile modules in parallel.
   Specify ``-j ⟨n⟩`` to compile ⟨n⟩ jobs in parallel. If ⟨n⟩ is omitted, then
   it defaults to the number of processors.
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Any of the command-line options described in the rest of this chapter
can be used with ``--make``, but note that any options you give on the
command line will apply to all the source files compiled, so if you want
any options to apply to a single source file only, you'll need to use an
``OPTIONS_GHC`` pragma (see :ref:`source-file-options`).

If the program needs to be linked with additional objects (say, some
auxiliary C code), then the object files can be given on the command
line and GHC will include them when linking the executable.

For backward compatibility with existing make scripts, when used in
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combination with :ghc-flag:`-c`, the linking phase is omitted (same as
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``--make -no-link``).

Note that GHC can only follow dependencies if it has the source file
available, so if your program includes a module for which there is no
source file, even if you have an object and an interface file for the
module, then GHC will complain. The exception to this rule is for
package modules, which may or may not have source files.

The source files for the program don't all need to be in the same
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directory; the :ghc-flag:`-i` option can be used to add directories to the
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search path (see :ref:`search-path`).

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.. ghc-flag:: -j[⟨n⟩]
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    :shortdesc: When compiling with :ghc-flag:`--make`, compile ⟨n⟩ modules
        in parallel.
    :type: dynamic
    :category: misc
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    Perform compilation in parallel when possible. GHC will use up to ⟨N⟩
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    threads during compilation. If N is omitted, then it defaults to the
    number of processors. Note that compilation of a module may not begin
    until its dependencies have been built.
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.. _eval-mode:

Expression evaluation mode
~~~~~~~~~~~~~~~~~~~~~~~~~~

This mode is very similar to interactive mode, except that there is a
single expression to evaluate which is specified on the command line as
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an argument to the ``-e`` option:

.. code-block:: none
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    ghc -e expr

Haskell source files may be named on the command line, and they will be
loaded exactly as in interactive mode. The expression is evaluated in
the context of the loaded modules.

For example, to load and run a Haskell program containing a module
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``Main``, we might say:

.. code-block:: none
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    ghc -e Main.main Main.hs

or we can just use this mode to evaluate expressions in the context of
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the ``Prelude``:

.. code-block:: none
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    $ ghc -e "interact (unlines.map reverse.lines)"
    hello
    olleh

.. _options-order:

Batch compiler mode
~~~~~~~~~~~~~~~~~~~

In *batch mode*, GHC will compile one or more source files given on the
command line.

The first phase to run is determined by each input-file suffix, and the
last phase is determined by a flag. If no relevant flag is present, then
go all the way through to linking. This table summarises:

+-----------------------------------+------------------------------+----------------------------+---------------------------+
| Phase of the compilation system   | Suffix saying “start here”   | Flag saying “stop after”   | (suffix of) output file   |
+===================================+==============================+============================+===========================+
| literate pre-processor            | ``.lhs``                     |                            | ``.hs``                   |
+-----------------------------------+------------------------------+----------------------------+---------------------------+
| C pre-processor (opt.)            | ``.hs`` (with ``-cpp``)      | ``-E``                     | ``.hspp``                 |
+-----------------------------------+------------------------------+----------------------------+---------------------------+
| Haskell compiler                  | ``.hs``                      | ``-C``, ``-S``             | ``.hc``, ``.s``           |
+-----------------------------------+------------------------------+----------------------------+---------------------------+
| C compiler (opt.)                 | ``.hc`` or ``.c``            | ``-S``                     | ``.s``                    |
+-----------------------------------+------------------------------+----------------------------+---------------------------+
| assembler                         | ``.s``                       | ``-c``                     | ``.o``                    |
+-----------------------------------+------------------------------+----------------------------+---------------------------+
| linker                            | ⟨other⟩                      |                            | ``a.out``                 |
+-----------------------------------+------------------------------+----------------------------+---------------------------+

.. index::
   single: -C
   single: -E
   single: -S
   single: -c

Thus, a common invocation would be:

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.. code-block:: none
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    ghc -c Foo.hs

to compile the Haskell source file ``Foo.hs`` to an object file
``Foo.o``.

.. note::
   What the Haskell compiler proper produces depends on what backend
   code generator is used. See :ref:`code-generators` for more details.

.. note::
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   Pre-processing is optional, the :ghc-flag:`-cpp` flag turns it
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   on. See :ref:`c-pre-processor` for more details.

.. note::
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   The option :ghc-flag:`-E` runs just the pre-processing passes of
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   the compiler, dumping the result in a file.

.. note::
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   The option :ghc-flag:`-C` is only available when GHC is built in
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   unregisterised mode. See :ref:`unreg` for more details.

.. _overriding-suffixes:

Overriding the default behaviour for a file
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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As described above, the way in which a file is processed by GHC depends on its
suffix. This behaviour can be overridden using the :ghc-flag:`-x ⟨suffix⟩`
option:
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.. ghc-flag:: -x ⟨suffix⟩
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    :shortdesc: Override default behaviour for source files
    :type: dynamic
    :category: phases
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    Causes all files following this option on the command line to be
    processed as if they had the suffix ⟨suffix⟩. For example, to
    compile a Haskell module in the file ``M.my-hs``, use
    ``ghc -c -x hs M.my-hs``.

.. _options-help:

Verbosity options
-----------------

.. index::
   single: verbosity options

See also the ``--help``, ``--version``, ``--numeric-version``, and
``--print-libdir`` modes in :ref:`modes`.

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.. ghc-flag:: -v
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    :shortdesc: verbose mode (equivalent to ``-v3``)
    :type: dynamic
    :category: verbosity
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    The :ghc-flag:`-v` option makes GHC *verbose*: it reports its version number
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    and shows (on stderr) exactly how it invokes each phase of the
    compilation system. Moreover, it passes the ``-v`` flag to most
    phases; each reports its version number (and possibly some other
    information).

    Please, oh please, use the ``-v`` option when reporting bugs!
    Knowing that you ran the right bits in the right order is always the
    first thing we want to verify.

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.. ghc-flag:: -v⟨n⟩
    :shortdesc: set verbosity level
    :type: dynamic
    :category: verbosity
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    To provide more control over the compiler's verbosity, the ``-v``
    flag takes an optional numeric argument. Specifying ``-v`` on its
    own is equivalent to ``-v3``, and the other levels have the
    following meanings:

    ``-v0``
        Disable all non-essential messages (this is the default).

    ``-v1``
        Minimal verbosity: print one line per compilation (this is the
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        default when :ghc-flag:`--make` or :ghc-flag:`--interactive` is on).
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    ``-v2``
        Print the name of each compilation phase as it is executed.
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        (equivalent to :ghc-flag:`-dshow-passes`).
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    ``-v3``
        The same as ``-v2``, except that in addition the full command
        line (if appropriate) for each compilation phase is also
        printed.

    ``-v4``
        The same as ``-v3`` except that the intermediate program
        representation after each compilation phase is also printed
        (excluding preprocessed and C/assembly files).

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.. ghc-flag:: -fprint-potential-instances
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    :shortdesc: display all available instances in type error messages
    :type: dynamic
    :reverse: -fno-print-potential-instances
    :category: verbosity
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    When GHC can't find an instance for a class, it displays a short
    list of some in the instances it knows about. With this flag it
    prints *all* the instances it knows about.

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.. ghc-flag:: -fhide-source-paths
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    :shortdesc: hide module source and object paths
    :type: dynamic
    :category: verbosity
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    Starting with minimal verbosity (``-v1``, see :ghc-flag:`-v`), GHC
    displays the name, the source path and the target path of each compiled
    module. This flag can be used to reduce GHC's output by hiding source paths
    and target paths.
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The following flags control the way in which GHC displays types in error
messages and in GHCi:

.. ghc-flag:: -fprint-unicode-syntax
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    :shortdesc: Use unicode syntax when printing expressions, types and kinds.
        See also :ghc-flag:`-XUnicodeSyntax`
    :type: dynamic
    :reverse: -fno-print-unicode-syntax
    :category: verbosity
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    When enabled GHC prints type signatures using the unicode symbols from the
    :ghc-flag:`-XUnicodeSyntax` extension. For instance,

    .. code-block:: none

        ghci> :set -fprint-unicode-syntax
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        ghci> :t +v (>>)
        (>>)  Monad m   a b. m a  m b  m b
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.. _pretty-printing-types:
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.. ghc-flag:: -fprint-explicit-foralls
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    :shortdesc: Print explicit ``forall`` quantification in types.
        See also :ghc-flag:`-XExplicitForAll`
    :type: dynamic
    :reverse: -fno-print-explicit-foralls
    :category: verbosity
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    Using :ghc-flag:`-fprint-explicit-foralls` makes
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    GHC print explicit ``forall`` quantification at the top level of a
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    type; normally this is suppressed. For example, in GHCi:

    .. code-block:: none
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        ghci> let f x = x
        ghci> :t f
        f :: a -> a
        ghci> :set -fprint-explicit-foralls
        ghci> :t f
        f :: forall a. a -> a

    However, regardless of the flag setting, the quantifiers are printed
    under these circumstances:

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    -  For nested ``foralls``, e.g.

       .. code-block:: none
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           ghci> :t GHC.ST.runST
           GHC.ST.runST :: (forall s. GHC.ST.ST s a) -> a

    -  If any of the quantified type variables has a kind that mentions
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       a kind variable, e.g.

       .. code-block:: none
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           ghci> :i Data.Type.Equality.sym
           Data.Type.Equality.sym ::
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             forall k (a :: k) (b :: k).
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             (a Data.Type.Equality.:~: b) -> b Data.Type.Equality.:~: a
                   -- Defined in Data.Type.Equality

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    This flag also enables the printing of *inferred* type variables
    inside braces. See :ref:`inferred-vs-specified`.
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.. ghc-flag:: -fprint-explicit-kinds
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    :shortdesc: Print explicit kind foralls and kind arguments in types.
        See also :ghc-flag:`-XKindSignatures`
    :type: dynamic
    :reverse: -fno-print-explicit-kinds
    :category: verbosity
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    Using :ghc-flag:`-fprint-explicit-kinds` makes GHC print kind arguments in
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    types, which are normally suppressed. This can be important when you
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    are using kind polymorphism. For example:

    .. code-block:: none
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           ghci> :set -XPolyKinds
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           ghci> data T a (b :: l) = MkT
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           ghci> :t MkT
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           MkT :: forall k l (a :: k) (b :: l). T a b
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           ghci> :set -fprint-explicit-kinds
           ghci> :t MkT
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           MkT :: forall k l (a :: k) (b :: l). T @{k} @l a b
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           ghci> :set -XNoPolyKinds
           ghci> :t MkT
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           MkT :: T @{*} @* a b

    In the output above, observe that ``T`` has two kind variables
    (``k`` and ``l``) and two type variables (``a`` and ``b``). Note that
    ``k`` is an *inferred* variable and ``l`` is a *specified* variable
    (see :ref:`inferred-vs-specified`), so as a result, they are displayed
    using slightly different syntax in the type ``T @{k} @l a b``. The
    application of ``l`` (with ``@l``) is the standard syntax for visible
    type application (see :ref:`visible-type-application`). The application
    of ``k`` (with ``@{k}``), however, uses a hypothetical syntax for visible
    type application of inferred type variables. This syntax is not currently
    exposed to the programmer, but it is nevertheless displayed when
    :ghc-flag:`-fprint-explicit-kinds` is enabled.
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.. ghc-flag:: -fprint-explicit-coercions
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    :shortdesc: Print coercions in types
    :type: dynamic
    :reverse: -fno-print-explicit-coercions
    :category: verbosity
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    Using :ghc-flag:`-fprint-explicit-coercions` makes GHC print coercions in
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    types. When trying to prove the equality between types of different
    kinds, GHC uses type-level coercions. Users will rarely need to
    see these, as they are meant to be internal.

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.. ghc-flag:: -fprint-equality-relations
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    :shortdesc: Distinguish between equality relations when printing
    :type: dynamic
    :reverse: -fno-print-equality-relations
    :category: verbosity
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    Using :ghc-flag:`-fprint-equality-relations` tells GHC to distinguish between
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    its equality relations when printing. For example, ``~`` is homogeneous
    lifted equality (the kinds of its arguments are the same) while
    ``~~`` is heterogeneous lifted equality (the kinds of its arguments
    might be different) and ``~#`` is heterogeneous unlifted equality,
    the internal equality relation used in GHC's solver. Generally,
    users should not need to worry about the subtleties here; ``~`` is
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    probably what you want. Without :ghc-flag:`-fprint-equality-relations`, GHC
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    prints all of these as ``~``. See also :ref:`equality-constraints`.
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.. ghc-flag:: -fprint-expanded-synonyms
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    :shortdesc: In type errors, also print type-synonym-expanded types.
    :type: dynamic
    :reverse: -fno-print-expanded-synonyms
    :category: verbosity
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    When enabled, GHC also prints type-synonym-expanded types in type
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    errors. For example, with this type synonyms: ::
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        type Foo = Int
        type Bar = Bool
        type MyBarST s = ST s Bar

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    This error message:

    .. code-block:: none
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        Couldn't match type 'Int' with 'Bool'
        Expected type: ST s Foo
          Actual type: MyBarST s

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    Becomes this:

    .. code-block:: none
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        Couldn't match type 'Int' with 'Bool'
        Expected type: ST s Foo
          Actual type: MyBarST s
        Type synonyms expanded:
        Expected type: ST s Int
          Actual type: ST s Bool

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.. ghc-flag:: -fprint-typechecker-elaboration
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    :shortdesc: Print extra information from typechecker.
    :type: dynamic
    :reverse: -fno-print-typechecker-elaboration
    :category: verbosity
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    When enabled, GHC also prints extra information from the typechecker in
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    warnings. For example: ::
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        main :: IO ()
        main = do
          return $ let a = "hello" in a
          return ()

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    This warning message:

    .. code-block:: none
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        A do-notation statement discarded a result of type ‘[Char]’
        Suppress this warning by saying
          ‘_ <- ($) return let a = "hello" in a’
        or by using the flag -fno-warn-unused-do-bind

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    Becomes this:

    .. code-block:: none
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        A do-notation statement discarded a result of type ‘[Char]’
        Suppress this warning by saying
          ‘_ <- ($)
                  return
                  let
                    AbsBinds [] []
                      {Exports: [a <= a
                                   <>]
                       Exported types: a :: [Char]
                                       [LclId, Str=DmdType]
                       Binds: a = "hello"}
                  in a’
        or by using the flag -fno-warn-unused-do-bind

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.. ghc-flag:: -fdefer-diagnostics
    :shortdesc: Defer and group diagnostic messages by severity
    :type: dynamic
    :category: verbosity

    Causes GHC to group diagnostic messages by severity and output them after
    other messages when building a multi-module Haskell program. This flag can
    make diagnostic messages more visible when used in conjunction with
    :ghc-flag:`--make` and :ghc-flag:`-j[⟨n⟩]`. Otherwise, it can be hard to
    find the relevant errors or likely to ignore the warnings when they are
    mixed with many other messages.

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.. ghc-flag:: -fdiagnostics-color=⟨always|auto|never⟩
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    :shortdesc: Use colors in error messages
    :type: dynamic
    :category: verbosity
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    Causes GHC to display error messages with colors.  To do this, the
    terminal must have support for ANSI color codes, or else garbled text will
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    appear.  The default value is ``auto``, which means GHC will make an
    attempt to detect whether terminal supports colors and choose accordingly.

    The precise color scheme is controlled by the environment variable
    ``GHC_COLORS`` (or ``GHC_COLOURS``).  This can be set to colon-separated
    list of ``key=value`` pairs.  These are the default settings:

    .. code-block:: none

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        header=:message=1:warning=1;35:error=1;31:fatal=1;31:margin=1;34
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    Each value is expected to be a `Select Graphic Rendition (SGR) substring
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    <https://en.wikipedia.org/wiki/ANSI_escape_code#graphics>`_.  The
    formatting of each element can inherit from parent elements.  For example,
    if ``header`` is left empty, it will inherit the formatting of
    ``message``.  Alternatively if ``header`` is set to ``1`` (bold), it will
    be bolded but still inherits the color of ``message``.

    Currently, in the primary message, the following inheritance tree is in
    place:

    - ``message``
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      - ``header``
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        - ``warning``
        - ``error``
        - ``fatal``

    In the caret diagnostics, there is currently no inheritance at all between
    ``margin``, ``warning``, ``error``, and ``fatal``.
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    The environment variable can also be set to the magical values ``never``
    or ``always``, which is equivalent to setting the corresponding
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    ``-fdiagnostics-color`` flag but with lower precedence.
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.. ghc-flag:: -fdiagnostics-show-caret
    :shortdesc: Whether to show snippets of original source code
    :type: dynamic
    :reverse: -fno-diagnostics-show-caret
    :category: verbosity
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    Controls whether GHC displays a line of the original source code where the
    error was detected.  This also affects the associated caret symbol that
    points at the region of code at fault.  The flag is on by default.

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.. ghc-flag:: -ferror-spans
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    :shortdesc: Output full span in error messages
    :type: dynamic
    :category: verbosity
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    Causes GHC to emit the full source span of the syntactic entity
    relating to an error message. Normally, GHC emits the source
    location of the start of the syntactic entity only.

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    For example:

    .. code-block:: none
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        test.hs:3:6: parse error on input `where'

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    becomes:

    .. code-block:: none
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        test296.hs:3:6-10: parse error on input `where'

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    And multi-line spans are possible too:

    .. code-block:: none
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        test.hs:(5,4)-(6,7):
            Conflicting definitions for `a'
            Bound at: test.hs:5:4
                      test.hs:6:7
            In the binding group for: a, b, a

    Note that line numbers start counting at one, but column numbers
    start at zero. This choice was made to follow existing convention
    (i.e. this is how Emacs does it).

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.. ghc-flag:: -freverse-errors
    :shortdesc: Output errors in reverse order
    :type: dynamic
    :reverse: -fno-reverse-errors
    :category: verbosity

    Causes GHC to output errors in reverse line-number order, so that
    the errors and warnings that originate later in the file are
    displayed first.

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.. ghc-flag:: -H size
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    :shortdesc: Set the minimum size of the heap to size
    :type: dynamic
    :category: misc
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    Set the minimum size of the heap to size. This option is
    equivalent to ``+RTS -Hsize``, see :ref:`rts-options-gc`.

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.. ghc-flag:: -Rghc-timing
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    :shortdesc: Summarise timing stats for GHC (same as ``+RTS -tstderr``).
    :type: dynamic
    :category: verbosity
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    Prints a one-line summary of timing statistics for the GHC run. This
    option is equivalent to ``+RTS -tstderr``, see
    :ref:`rts-options-gc`.

.. _options-platform:

Platform-specific Flags
-----------------------

.. index::
   single: -m\* options
   single: platform-specific options
   single: machine-specific options

Some flags only make sense for particular target platforms.

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.. ghc-flag:: -msse2
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    :shortdesc: (x86 only) Use SSE2 for floating-point operations
    :type: dynamic
    :category: platform-options
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    (x86 only, added in GHC 7.0.1) Use the SSE2 registers and
    instruction set to implement floating point operations when using
    the :ref:`native code generator <native-code-gen>`. This gives a
    substantial performance improvement for floating point, but the
    resulting compiled code will only run on processors that support
    SSE2 (Intel Pentium 4 and later, or AMD Athlon 64 and later). The
    :ref:`LLVM backend <llvm-code-gen>` will also use SSE2 if your
    processor supports it but detects this automatically so no flag is
    required.

    SSE2 is unconditionally used on x86-64 platforms.

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.. ghc-flag:: -msse4.2
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    :shortdesc: (x86 only) Use SSE4.2 for floating-point operations
    :type: dynamic
    :category: platform-options
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    (x86 only, added in GHC 7.4.1) Use the SSE4.2 instruction set to
    implement some floating point and bit operations when using the
    :ref:`native code generator <native-code-gen>`. The resulting compiled
    code will only run on processors that support SSE4.2 (Intel Core i7
    and later). The :ref:`LLVM backend <llvm-code-gen>` will also use
    SSE4.2 if your processor supports it but detects this automatically
    so no flag is required.
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.. ghc-flag:: -mbmi2
    :shortdesc: (x86 only) Use BMI2 for bit manipulation operations
    :type: dynamic
    :category: platform-options

    (x86 only, added in GHC 7.4.1) Use the BMI2 instruction set to
    implement some bit operations when using the
    :ref:`native code generator <native-code-gen>`. The resulting compiled
    code will only run on processors that support BMI2 (Intel Haswell and newer, AMD Excavator, Zen and newer).

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Miscellaneous flags
-------------------

.. index::
   single: miscellaneous flags

Some flags only make sense for a particular use case.

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.. ghc-flag:: -ghcversion-file path to ghcversion.h
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    :shortdesc: (GHC as a C compiler only) Use this ``ghcversion.h`` file
    :type: dynamic
    :category: misc

    When GHC is used to compile C files, GHC adds package include paths and
    includes ``ghcversion.h`` directly. The compiler will lookup the path for
    the ``ghcversion.h`` file from the ``rts`` package in the package database.
    In some cases, the compiler's package database does not contain the ``rts``
    package, or one wants to specify a specific ``ghcversions.h`` to be
    included. This option can be used to specify the path to the
    ``ghcversions.h`` file to be included. This is primarily intended to be
    used by GHC's build system.