path: root/docs/users_guide/separate_compilation.rst

.. _separate-compilation:

Filenames and separate compilation
==================================

.. index::
   single: separate compilation
   single: recompilation checker
   single: make and recompilation

This section describes what files GHC expects to find, what files it
creates, where these files are stored, and what options affect this
behaviour.

Pathname conventions vary from system to system. In particular, the
directory separator is "``/``" on Unix systems and "``\``" on
Windows systems. In the sections that follow, we shall consistently use
"``/``" as the directory separator; substitute this for the
appropriate character for your system.

.. _source-files:

Haskell source files
--------------------

.. index::
   single: file names; of source files

Each Haskell source module should be placed in a file on its own.

Usually, the file should be named after the module name, replacing dots
in the module name by directory separators. For example, on a Unix
system, the module ``A.B.C`` should be placed in the file ``A/B/C.hs``,
relative to some base directory. If the module is not going to be
imported by another module (``Main``, for example), then you are free to
use any filename for it.

.. index::
   single: Unicode
   single: UTF-8
   single: ASCII
   single: Latin-1
   single: encodings; of source files

GHC assumes that source files are ASCII or UTF-8 only, other
encoding are not recognised. However, invalid UTF-8 sequences
will be ignored in comments, so it is possible to use other encodings
such as Latin-1, as long as the non-comment source code is ASCII
only.

.. _output-files:

Output files
------------

.. index::
   single: interface files
   single: .hi files
   single: object files
   single: .o files

When asked to compile a source file, GHC normally generates two files:
an object file, and an interface file.

The object file, which normally ends in a ``.o`` suffix, contains the
compiled code for the module.

The interface file, which normally ends in a ``.hi`` suffix, contains the
information that GHC needs in order to compile further modules that depend on
this module. It contains things like the types of exported functions,
definitions of data types, and so on. It is stored in a binary format, so don't
try to read one; use the :ghc-flag:`--show-iface ⟨file⟩` option instead (see
:ref:`hi-options`).

You should think of the object file and the interface file as a pair,
since the interface file is in a sense a compiler-readable description
of the contents of the object file. If the interface file and object
file get out of sync for any reason, then the compiler may end up making
assumptions about the object file that aren't true; trouble will almost
certainly follow. For this reason, we recommend keeping object files and
interface files in the same place (GHC does this by default, but it is
possible to override the defaults as we'll explain shortly).

Every module has a *module name* defined in its source code
(``module A.B.C where ...``).

The name of the object file generated by GHC is derived according to the
following rules, where ⟨osuf⟩ is the object-file suffix (this can be
changed with the ``-osuf`` option).

-  If there is no ``-odir`` option (the default), then the object
   filename is derived from the source filename (ignoring the module
   name) by replacing the suffix with ⟨osuf⟩.

-  If ``-odir ⟨dir⟩`` has been specified, then the object filename is
   ⟨dir⟩/⟨mod⟩.⟨osuf⟩, where ⟨mod⟩ is the module name with dots replaced
   by slashes. GHC will silently create the necessary directory
   structure underneath ⟨dir⟩, if it does not already exist.

The name of the interface file is derived using the same rules, except that the
suffix is ⟨hisuf⟩ (``.hi`` by default) instead of ⟨osuf⟩, and the relevant
options are :ghc-flag:`-hidir ⟨dir⟩` and :ghc-flag:`-hisuf ⟨suffix⟩` instead of
:ghc-flag:`-odir ⟨dir⟩` and :ghc-flag:`-osuf ⟨suffix⟩` respectively.

For example, if GHC compiles the module ``A.B.C`` in the file
``src/A/B/C.hs``, with no ``-odir`` or ``-hidir`` flags, the interface
file will be put in ``src/A/B/C.hi`` and the object file in
``src/A/B/C.o``.

For any module that is imported, GHC requires that the name of the
module in the import statement exactly matches the name of the module in
the interface file (or source file) found using the strategy specified
in :ref:`search-path`. This means that for most modules, the source file
name should match the module name.

However, note that it is reasonable to have a module ``Main`` in a file
named ``foo.hs``, but this only works because GHC never needs to search
for the interface for module ``Main`` (because it is never imported). It
is therefore possible to have several ``Main`` modules in separate
source files in the same directory, and GHC will not get confused.

In batch compilation mode, the name of the object file can also be overridden
using the :ghc-flag:`-o ⟨file⟩` option, and the name of the interface file can
be specified directly using the :ghc-flag:`-ohi ⟨file⟩` option.

.. _search-path:

The search path
---------------

.. index::
   single: search path
   single: interface files, finding them
   single: finding interface files

In your program, you import a module ``Foo`` by saying ``import Foo``.
In :ghc-flag:`--make` mode or GHCi, GHC will look for a source file for ``Foo``
and arrange to compile it first. Without :ghc-flag:`--make`, GHC will look for
the interface file for ``Foo``, which should have been created by an
earlier compilation of ``Foo``. GHC uses the same strategy in each of
these cases for finding the appropriate file.

This strategy is as follows: GHC keeps a list of directories called the
search path. For each of these directories, it tries appending
``⟨basename⟩.⟨extension⟩`` to the directory, and checks whether the
file exists. The value of ⟨basename⟩ is the module name with dots
replaced by the directory separator ("``/``" or "``\\"``, depending on the
system), and ⟨extension⟩ is a source extension (``hs``, ``lhs``) if we
are in :ghc-flag:`--make` mode or GHCi, or ⟨hisuf⟩ otherwise.

For example, suppose the search path contains directories ``d1``,
``d2``, and ``d3``, and we are in :ghc-flag:`--make` mode looking for the source
file for a module ``A.B.C``. GHC will look in ``d1/A/B/C.hs``,
``d1/A/B/C.lhs``, ``d2/A/B/C.hs``, and so on.

The search path by default contains a single directory: "``.``" (i.e. the
current directory). The following options can be used to add to or change the
contents of the search path:

.. ghc-flag:: -i⟨dir⟩[:⟨dir⟩]*
    :shortdesc: add ⟨dir⟩, ⟨dir2⟩, etc. to import path
    :type: dynamic
    :category: search-path

    .. index::
       single: search path; source code

    This flag appends a colon-separated list of ``dirs`` to
    the search path.

.. ghc-flag:: -i
    :shortdesc: Empty the import directory list
    :type: dynamic
    :category: search-path

    resets the search path back to nothing.

This isn't the whole story: GHC also looks for modules in pre-compiled
libraries, known as packages. See the section on packages
(:ref:`packages`) for details.

.. _options-output:

Redirecting the compilation output(s)
-------------------------------------

.. index::
   single: output-directing options
   single: redirecting compilation output

.. ghc-flag:: -o ⟨file⟩
    :shortdesc: set output filename
    :type: dynamic
    :category:

    GHC's compiled output normally goes into a ``.hc``, ``.o``, etc.,
    file, depending on the last-run compilation phase. The option
    ``-o file`` re-directs the output of that last-run phase to ⟨file⟩.

    .. note::
       This “feature” can be counterintuitive: ``ghc -C -o foo.o foo.hs``
       will put the intermediate C code in the file ``foo.o``, name
       notwithstanding!

    This option is most often used when creating an executable file, to
    set the filename of the executable. For example:

    .. code-block:: none

        ghc -o prog --make Main

    will compile the program starting with module ``Main`` and put the
    executable in the file ``prog``.

    Note: on Windows, if the result is an executable file, the extension
    "``.exe``" is added if the specified filename does not already have
    an extension. Thus

    .. code-block:: none

        ghc -o foo Main.hs

    will compile and link the module ``Main.hs``, and put the resulting
    executable in ``foo.exe`` (not ``foo``).

    If you use ``ghc --make`` and you don't use the ``-o``, the name GHC
    will choose for the executable will be based on the name of the file
    containing the module ``Main``. Note that with GHC the ``Main``
    module doesn't have to be put in file ``Main.hs``. Thus both

    .. code-block:: none

        ghc --make Prog

    and

    .. code-block:: none

        ghc --make Prog.hs

    will produce ``Prog`` (or ``Prog.exe`` if you are on Windows).

.. ghc-flag:: -odir ⟨dir⟩
    :shortdesc: set directory for object files
    :type: dynamic
    :category:

    Redirects object files to directory ⟨dir⟩. For example:

    .. code-block:: none

        $ ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `uname -m`

    The object files, ``Foo.o``, ``Bar.o``, and ``Bumble.o`` would be
    put into a subdirectory named after the architecture of the
    executing machine (``x86``, ``mips``, etc).

    Note that the ``-odir`` option does *not* affect where the interface
    files are put; use the ``-hidir`` option for that. In the above
    example, they would still be put in ``parse/Foo.hi``,
    ``parse/Bar.hi``, and ``gurgle/Bumble.hi``.

    Please also note that when doing incremental compilation, this directory is
    where GHC looks into to find object files from previous builds.

.. ghc-flag:: -ohi ⟨file⟩
    :shortdesc: set the filename in which to put the interface
    :type: dynamic
    :category:

    The interface output may be directed to another file
    ``bar2/Wurble.iface`` with the option ``-ohi bar2/Wurble.iface``
    (not recommended).

    .. warning::
       If you redirect the interface file somewhere that GHC can't
       find it, then the recompilation checker may get confused (at the
       least, you won't get any recompilation avoidance). We recommend
       using a combination of ``-hidir`` and ``-hisuf`` options instead, if
       possible.

    To avoid generating an interface at all, you could use this option
    to redirect the interface into the bit bucket: ``-ohi /dev/null``,
    for example.

.. ghc-flag:: -hidir ⟨dir⟩
    :shortdesc: set directory for interface files
    :type: dynamic
    :category:

    Redirects all generated interface files into ⟨dir⟩, instead of the
    default.

    Please also note that when doing incremental compilation (by ``ghc --make``
    or ``ghc -c``), this directory is where GHC looks into to find interface
    files.

.. ghc-flag:: -hiedir ⟨dir⟩
    :shortdesc: set directory for extended interface files
    :type: dynamic
    :category:

    Redirects all generated extended interface files into ⟨dir⟩, instead of
    the default.

    Please also note that when doing incremental compilation (by ``ghc --make``
    or ``ghc -c``), this directory is where GHC looks into to find extended
    interface files.

.. ghc-flag:: -stubdir ⟨dir⟩
    :shortdesc: redirect FFI stub files
    :type: dynamic
    :category:

    Redirects all generated FFI stub files into ⟨dir⟩. Stub files are
    generated when the Haskell source contains a ``foreign export`` or
    ``foreign import "&wrapper"`` declaration (see
    :ref:`foreign-export-ghc`). The ``-stubdir`` option behaves in
    exactly the same way as ``-odir`` and ``-hidir`` with respect to
    hierarchical modules.

.. ghc-flag:: -dumpdir ⟨dir⟩
    :shortdesc: redirect dump files
    :type: dynamic
    :category:

    Redirects all dump files into ⟨dir⟩. Dump files are generated when
    ``-ddump-to-file`` is used with other ``-ddump-*`` flags.

.. ghc-flag:: -outputdir ⟨dir⟩
    :shortdesc: set output directory
    :type: dynamic
    :category:

    The ``-outputdir`` option is shorthand for the combination of
    :ghc-flag:`-odir ⟨dir⟩`, :ghc-flag:`-hidir ⟨dir⟩`, :ghc-flag:`-stubdir
    ⟨dir⟩` and :ghc-flag:`-dumpdir ⟨dir⟩`.

.. ghc-flag:: -osuf ⟨suffix⟩
    :shortdesc: set the output file suffix
    :type: dynamic
    :category:

    The ``-osuf`` ⟨suffix⟩ will change the ``.o`` file suffix for object
    files to whatever you specify. We use this when compiling libraries,
    so that objects for the profiling versions of the libraries don't
    clobber the normal ones.

.. ghc-flag:: -hisuf ⟨suffix⟩
    :shortdesc: set the suffix to use for interface files
    :type: dynamic
    :category:

    Similarly, the ``-hisuf`` ⟨suffix⟩ will change the ``.hi`` file
    suffix for non-system interface files (see :ref:`hi-options`).

    The ``-hisuf``/``-osuf`` game is particularly useful if you want to
    compile a program both with and without profiling, in the same
    directory. You can say:

    .. code-block:: none

        ghc ...

    to get the ordinary version, and

    .. code-block:: none

        ghc ... -osuf prof.o -hisuf prof.hi -prof -fprof-auto

    to get the profiled version.

.. ghc-flag:: -hiesuf ⟨suffix⟩
    :shortdesc: set the suffix to use for extended interface files
    :type: dynamic

    The ``-hiesuf`` ⟨suffix⟩ will change the ``.hie`` file suffix for
    extended interface files to whatever you specify.

.. ghc-flag:: -hcsuf ⟨suffix⟩
    :shortdesc: set the suffix to use for intermediate C files
    :type: dynamic
    :category:

    Finally, the option ``-hcsuf`` ⟨suffix⟩ will change the ``.hc`` file
    suffix for compiler-generated intermediate C files.

.. _keeping-intermediates:

Keeping Intermediate Files
--------------------------

.. index::
   single: intermediate files, saving
   single: .hc files, saving
   single: .ll files, saving
   single: .s files, saving

The following options are useful for keeping (or not keeping) certain
intermediate files around, when normally GHC would throw these away after
compilation:

.. ghc-flag:: -keep-hc-file
              -keep-hc-files
    :shortdesc: Retain intermediate ``.hc`` files.
    :type: dynamic
    :category: keep-intermediates

    Keep intermediate ``.hc`` files when doing ``.hs``-to-``.o``
    compilations via :ref:`C <c-code-gen>` (Note: ``.hc`` files are only
    generated by :ref:`unregisterised <unreg>` compilers).

.. ghc-flag:: -keep-hi-files
    :shortdesc: Retain intermediate ``.hi`` files (the default).
    :type: dynamic
    :reverse: -no-keep-hi-files
    :category: keep-intermediates

    .. index::
       single: temporary files; keeping

    Keep intermediate ``.hi`` files. This is the default. You may use
    ``-no-keep-hi-files`` if you are not interested in the ``.hi`` files.

.. ghc-flag:: -keep-hscpp-file
              -keep-hscpp-files
    :shortdesc: Retain intermediate ``.hscpp`` files.
    :type: dynamic
    :category: keep-intermediates

    .. index::
       single: temporary files; keeping

    Keep the output of the ``CPP`` pre-processor phase as ``.hscpp`` files.
    A ``.hscpp`` file is only created, if a module gets compiled and uses the
    C pre-processor.

.. ghc-flag:: -keep-llvm-file
              -keep-llvm-files
    :shortdesc: Retain intermediate LLVM ``.ll`` files.
        Implies :ghc-flag:`-fllvm`.
    :type: dynamic
    :category: keep-intermediates

    :implies: :ghc-flag:`-fllvm`

    Keep intermediate ``.ll`` files when doing ``.hs``-to-``.o``
    compilations via :ref:`LLVM <llvm-code-gen>` (Note: ``.ll`` files
    aren't generated when using the native code generator, you may need
    to use :ghc-flag:`-fllvm` to force them to be produced).

.. ghc-flag:: -keep-o-files
    :shortdesc: Retain intermediate ``.o`` files (the default).
    :type: dynamic
    :reverse: -no-keep-o-files
    :category: keep-intermediates

    .. index::
       single: temporary files; keeping

    Keep intermediate ``.o`` files. This is the default. You may use
    ``-no-keep-o-files`` if you are not interested in the ``.o`` files.

.. ghc-flag:: -keep-s-file
              -keep-s-files
    :shortdesc: Retain intermediate ``.s`` files.
    :type: dynamic
    :category: keep-intermediates

    Keep intermediate ``.s`` files.

.. ghc-flag:: -keep-tmp-files
    :shortdesc: Retain all intermediate temporary files.
    :type: dynamic
    :category: keep-intermediates

    .. index::
       single: temporary files; keeping

    Instructs the GHC driver not to delete any of its temporary files,
    which it normally keeps in ``/tmp`` (or possibly elsewhere; see
    :ref:`temp-files`). Running GHC with ``-v`` will show you what
    temporary files were generated along the way.

.. _temp-files:

Redirecting temporary files
---------------------------

.. index::
   single: temporary files; redirecting

.. ghc-flag:: -tmpdir ⟨dir⟩
    :shortdesc: set the directory for temporary files
    :type: dynamic
    :category: temp-files

    If you have trouble because of running out of space in ``/tmp`` (or
    wherever your installation thinks temporary files should go), you
    may use the :ghc-flag:`-tmpdir ⟨dir⟩` option option to specify an
    alternate directory. For example, ``-tmpdir .`` says to put temporary files
    in the current working directory.

    .. index::
        single: TMPDIR environment variable

    Alternatively, use your :envvar:`TMPDIR` environment variable. Set it to the
    name of the directory where temporary files should be put. GCC and other
    programs will honour the :envvar:`TMPDIR` variable as well.

.. _hi-options:

Other options related to interface files
----------------------------------------

.. index::
   single: interface files, options

.. ghc-flag:: -ddump-hi
    :shortdesc: Dump the new interface to stdout
    :type: dynamic
    :category: interface-files

    Dumps the new interface to standard output.

.. ghc-flag:: -ddump-hi-diffs
    :shortdesc: Show the differences vs. the old interface
    :type: dynamic
    :category: interface-files

    The compiler does not overwrite an existing ``.hi`` interface file
    if the new one is the same as the old one; this is friendly to
    :command:`make`. When an interface does change, it is often enlightening to
    be informed. The :ghc-flag:`-ddump-hi-diffs` option will make GHC report the
    differences between the old and new ``.hi`` files.

.. ghc-flag:: -ddump-minimal-imports
    :shortdesc: Dump a minimal set of imports
    :type: dynamic
    :category: interface-files

    Dump to the file :file:`{M}.imports` (where ⟨M⟩ is the name of the module
    being compiled) a "minimal" set of import declarations. The
    directory where the ``.imports`` files are created can be controlled
    via the :ghc-flag:`-dumpdir ⟨dir⟩` option.

    You can safely replace all the import declarations in :file:`{M}.hs` with
    those found in its respective ``.imports`` file. Why would you want
    to do that? Because the "minimal" imports (a) import everything
    explicitly, by name, and (b) import nothing that is not required. It
    can be quite painful to maintain this property by hand, so this flag
    is intended to reduce the labour.

.. ghc-flag:: --show-iface ⟨file⟩
    :shortdesc: See :ref:`modes`.
    :type: mode
    :category: interface-files

    where ⟨file⟩ is the name of an interface file, dumps the contents of
    that interface in a human-readable format. See :ref:`modes`.

.. _hie-options:

Options related to extended interface files
-------------------------------------------

.. index::
   single: extended interface files, options

GHC builds up a wealth of information about a Haskell source file as it compiles
it. Extended interface files are a way of persisting some of this information to
disk so that external tools, such as IDE's, can avoid parsing, typechecking, and
renaming all over again. These files contain

  * a simplified AST

       * nodes are annotated with source positions and types
       * identifiers are annotated with scope information

  * the raw bytes of the initial Haskell source

The GHC API exposes functions for reading and writing these files.

.. ghc-flag:: -fwrite-ide-info
    :shortdesc: Write out extended interface files
    :type: dynamic
    :category: extended-interface-files

    Writes out extended interface files alongside regular interface files.
    Just like regular interface files, GHC has a recompilation check to detect
    out of date or missing extended interface files.

.. ghc-flag:: -fvalidate-ide-info
    :shortdesc: Perform some sanity checks on the extended interface files
    :type: dynamic
    :category: extended-interface-files

    Runs a series of sanity checks and lints on the extended interface files
    that are being written out. These include testing things properties such as
    variables not occurring outside of their expected scopes.

The format in which GHC currently stores its typechecked AST, makes it costly
to collect the types for some expressions nodes. For the sake of performance,
GHC currently chooses to skip over these, so not all expression nodes should be
expected to have type information on them. See :ghc-ticket:`16233` for more.

.. _recomp:

The recompilation checker
-------------------------

.. index::
   single: recompilation checker

.. ghc-flag:: -fforce-recomp
    :shortdesc: Turn off recompilation checking. This is implied by any
        ``-ddump-X`` option when compiling a single file
        (i.e. when using :ghc-flag:`-c`).
    :type: dynamic
    :reverse: -fno-force-recomp
    :category: recompilation

    Turn off recompilation checking (which is on by default).
    Recompilation checking normally stops compilation early, leaving an
    existing ``.o`` file in place, if it can be determined that the
    module does not need to be recompiled.

.. ghc-flag:: -fignore-optim-changes
    :shortdesc: Do not recompile modules just to match changes to
        optimisation flags. This is especially useful for avoiding
        recompilation when using GHCi, and is enabled by default for
        GHCi.
    :type: dynamic
    :reverse: -fno-ignore-optim-changes
    :category: recompilation

.. ghc-flag:: -fignore-hpc-changes
    :shortdesc: Do not recompile modules just to match changes to
        HPC flags. This is especially useful for avoiding recompilation
        when using GHCi, and is enabled by default for GHCi.
    :type: dynamic
    :reverse: -fno-ignore-hpc-changes
    :category: recompilation

In the olden days, GHC compared the newly-generated ``.hi`` file with
the previous version; if they were identical, it left the old one alone
and didn't change its modification date. In consequence, importers of a
module with an unchanged output ``.hi`` file were not recompiled.

This doesn't work any more. Suppose module ``C`` imports module ``B``,
and ``B`` imports module ``A``. So changes to module ``A`` might require
module ``C`` to be recompiled, and hence when ``A.hi`` changes we should
check whether ``C`` should be recompiled. However, the dependencies of
``C`` will only list ``B.hi``, not ``A.hi``, and some changes to ``A``
(changing the definition of a function that appears in an inlining of a
function exported by ``B``, say) may conceivably not change ``B.hi`` one
jot. So now…

GHC calculates a fingerprint (in fact an MD5 hash) of each interface
file, and of each declaration within the interface file. It also keeps
in every interface file a list of the fingerprints of everything it used
when it last compiled the file. If the source file's modification date
is earlier than the ``.o`` file's date (i.e. the source hasn't changed
since the file was last compiled), and the recompilation checking is on,
GHC will be clever. It compares the fingerprints on the things it needs
this time with the fingerprints on the things it needed last time
(gleaned from the interface file of the module being compiled); if they
are all the same it stops compiling early in the process saying
“Compilation IS NOT required”. What a beautiful sight!

You can read about :ghc-wiki:`how all this works <commentary/compiler/recompilation-avoidance>` in the GHC commentary.

.. _mutual-recursion:

How to compile mutually recursive modules
-----------------------------------------

.. index::
   single: module system, recursion
   single: recursion, between modules

GHC supports the compilation of mutually recursive modules. This section
explains how.

Every cycle in the module import graph must be broken by a ``hs-boot``
file. Suppose that modules ``A.hs`` and ``B.hs`` are Haskell source
files, thus: ::

    module A where
        import B( TB(..) )

        newtype TA = MkTA Int

        f :: TB -> TA
        f (MkTB x) = MkTA x

    module B where
        import {-# SOURCE #-} A( TA(..) )

        data TB = MkTB !Int

        g :: TA -> TB
        g (MkTA x) = MkTB x

.. index::
   single: ``hs-boot`` files
   single: importing, ``hi-boot`` files

Here ``A`` imports ``B``, but ``B`` imports ``A`` with a
``{-# SOURCE #-}`` pragma, which breaks the circular dependency. Every
loop in the module import graph must be broken by a ``{-# SOURCE #-}``
import; or, equivalently, the module import graph must be acyclic if
``{-# SOURCE #-}`` imports are ignored.

For every module ``A.hs`` that is ``{-# SOURCE #-}``-imported in this
way there must exist a source file ``A.hs-boot``. This file contains an
abbreviated version of ``A.hs``, thus: ::

    module A where
        newtype TA = MkTA Int

To compile these three files, issue the following commands:

.. code-block:: none

      ghc -c A.hs-boot    -- Produces A.hi-boot, A.o-boot
      ghc -c B.hs         -- Consumes A.hi-boot, produces B.hi, B.o
      ghc -c A.hs         -- Consumes B.hi, produces A.hi, A.o
      ghc -o foo A.o B.o  -- Linking the program

There are several points to note here:

-  The file ``A.hs-boot`` is a programmer-written source file. It must
   live in the same directory as its parent source file ``A.hs``.
   Currently, if you use a literate source file ``A.lhs`` you must also
   use a literate boot file, ``A.lhs-boot``; and vice versa.

-  A ``hs-boot`` file is compiled by GHC, just like a ``hs`` file:

   .. code-block:: none

         ghc -c A.hs-boot

   When a hs-boot file ``A.hs-boot`` is compiled, it is checked for
   scope and type errors. When its parent module ``A.hs`` is compiled,
   the two are compared, and an error is reported if the two are
   inconsistent.

-  Just as compiling ``A.hs`` produces an interface file ``A.hi``, and
   an object file ``A.o``, so compiling ``A.hs-boot`` produces an
   interface file ``A.hi-boot``, and a pseudo-object file ``A.o-boot``:

   -  The pseudo-object file ``A.o-boot`` is empty (don't link it!), but
      it is very useful when using a Makefile, to record when the
      ``A.hi-boot`` was last brought up to date (see :ref:`using-make`).

   -  The ``hi-boot`` generated by compiling a ``hs-boot`` file is in
      the same machine-generated binary format as any other
      GHC-generated interface file (e.g. ``B.hi``). You can display its
      contents with ``ghc --show-iface``. If you specify a directory for
      interface files, the ``-ohidir`` flag, then that affects ``hi-boot`` files
      too.

-  If hs-boot files are considered distinct from their parent source
   files, and if a ``{-# SOURCE #-}`` import is considered to refer to
   the hs-boot file, then the module import graph must have no cycles.
   The command ``ghc -M`` will report an error if a cycle is found.

-  A module ``M`` that is ``{-# SOURCE #-}``\-imported in a program will
   usually also be ordinarily imported elsewhere. If not, ``ghc --make``
   automatically adds ``M`` to the set of modules it tries to compile
   and link, to ensure that ``M``\'s implementation is included in the
   final program.

A hs-boot file need only contain the bare minimum of information needed
to get the bootstrapping process started. For example, it doesn't need
to contain declarations for *everything* that module ``A`` exports, only
the things required by the module(s) that import ``A`` recursively.

A hs-boot file is written in a subset of Haskell:

-  The module header (including the export list), and import statements,
   are exactly as in Haskell, and so are the scoping rules. Hence, to
   mention a non-Prelude type or class, you must import it.

-  There must be no value declarations, but there can be type signatures
   for values. For example: ::

        double :: Int -> Int

-  Fixity declarations are exactly as in Haskell.

-  Vanilla type synonym declarations are exactly as in Haskell.

-  Open type and data family declarations are exactly as in Haskell.

-  A closed type family may optionally omit its equations, as in the
   following example: ::

        type family ClosedFam a where ..

   The ``..`` is meant literally -- you should write two dots in your
   file. Note that the ``where`` clause is still necessary to
   distinguish closed families from open ones. If you give any equations
   of a closed family, you must give all of them, in the same order as
   they appear in the accompanying Haskell file.

-  A data type declaration can either be given in full, exactly as in
   Haskell, or it can be given abstractly, by omitting the '=' sign and
   everything that follows. For example: ::

        data T a b

   In a *source* program this would declare TA to have no constructors
   (a GHC extension: see :ref:`nullary-types`), but in an hi-boot file
   it means "I don't know or care what the constructors are". This is
   the most common form of data type declaration, because it's easy to
   get right. You *can* also write out the constructors but, if you do
   so, you must write it out precisely as in its real definition.

   If you do not write out the constructors, you may need to give a kind
   annotation (:ref:`kinding`), to tell GHC the kind of the type
   variable, if it is not "\*". (In source files, this is worked out
   from the way the type variable is used in the constructors.) For
   example: ::

        data R (x :: * -> *) y

   You cannot use ``deriving`` on a data type declaration; write an
   ``instance`` declaration instead.

-  Class declarations is exactly as in Haskell, except that you may not
   put default method declarations. You can also omit all the
   superclasses and class methods entirely; but you must either omit
   them all or put them all in.

-  You can include instance declarations just as in Haskell; but omit
   the "where" part.

-  The default role for abstract datatype parameters is now
   representational. (An abstract datatype is one with no constructors
   listed.) To get another role, use a role annotation. (See
   :ref:`roles`.)

.. _module-signatures:

Module signatures
-----------------

.. index::
     single: signature files; Backpack; hsig files

GHC 8.2 supports module signatures (``hsig`` files), which allow you to
write a signature in place of a module implementation, deferring the
choice of implementation until a later point in time.  This feature is
not intended to be used without `Cabal
<http://www.haskell.org/cabal/>`__; this manual entry will focus
on the syntax and semantics of signatures.

To start with an example, suppose you had a module ``A`` which made use of some
string operations.  Using normal module imports, you would only
be able to pick a particular implementation of strings::

    module Str where
        type Str = String

        empty :: Str
        empty = ""

        toString :: Str -> String
        toString s = s

    module A where
        import Str
        z = toString empty

By replacing ``Str.hs`` with a signature ``Str.hsig``, ``A`` (and
any other modules in this package) are now parametrized by
a string implementation::

    signature Str where
        data Str
        empty :: Str
        toString :: Str -> String

We can typecheck ``A`` against this signature, or we can instantiate
``Str`` with a module that provides the following declarations.  Refer
to Cabal's documentation for a more in-depth discussion on how to
instantiate signatures.

Module signatures actually consist of two closely related features:

- The ability to define an ``hsig`` file, containing type definitions
  and type signature for values which can be used by modules that
  import the signature, and must be provided by the eventual
  implementing module, and

- The ability to *inherit* required signatures from packages we
  depend upon, combining the signatures into a single merged
  signature which reflects the requirements of any locally defined
  signature, as well as the requirements of our dependencies.

A signature file is denoted by an ``hsig`` file; every required
signature must have an ``hsig`` file (even if it is an empty one),
including required signatures inherited from dependencies.  Signatures
can be imported using an ordinary ``import Sig`` declaration.

``hsig`` files are written in a variant of Haskell similar
to ``hs-boot`` files, but with some slight changes:

- The header of a signature is ``signature A where ...`` (instead
  of the usual ``module A where ...``).

- Import statements and scoping rules are exactly as in Haskell.
  To mention a non-Prelude type or class, you must import it.

- Unlike regular modules, the defined entities of
  a signature include not only those written in the local
  ``hsig`` file, but also those from inherited signatures
  (as inferred from the :ghc-flag:`-package-id ⟨unit-id⟩` flags).
  These entities are not considered in scope when typechecking
  the local ``hsig`` file, but are available for import by
  any module or signature which imports the signature.  The
  one exception to this rule is the export list, described
  below.

  If a declaration occurs in multiple inherited signatures,
  they will be *merged* together.  For values, we require
  that the types from both signatures match exactly; however,
  other declarations may merge in more interesting ways.
  The merging operation in these cases has the effect of
  textually replacing all occurrences of the old name with
  a reference to the new, merged declaration.  For example,
  if we have the following two signatures::

    signature A where
        data T
        f :: T -> T

    signature A where
        data T = MkT
        g :: T

  the resulting merged signature would be::

    signature A where
        data T = MkT
        f :: T -> T
        g :: T

- If no export list is provided for a signature, the exports of
  a signature are all of its defined entities merged with the
  exports of all inherited signatures.

  If you want to reexport an entity from a signature, you must
  also include a ``module SigName`` export, so that all of the
  entities defined in the signature are exported.  For example,
  the following module exports both ``f`` and ``Int`` from
  ``Prelude``::

    signature A(module A, Int) where
        import Prelude (Int)
        f :: Int

  Reexports merge with local declarations; thus, the signature above
  would successfully merge with::

    signature A where
        data Int

  The only permissible implementation of such a signature is a module
  which reexports precisely the same entity::

    module A (f, Int) where
        import Prelude (Int)
        f = 2 :: Int

  Conversely, any entity requested by a signature can be provided
  by a reexport from the implementing module.  This is different from
  ``hs-boot`` files, which require every entity to be defined
  locally in the implementing module.

- GHC has experimental support for *signature thinning*, which is used
  when a signature has an explicit export list without a module export of the
  signature itself.  In this case, the export list applies to the final export
  list *after* merging, in particular, you may refer to entities which are not
  declared in the body of the local ``hsig`` file.

  The semantics in this case is that the set of required entities is defined
  exclusively by its exports; if an entity is not mentioned in the export list,
  it is not required.  The motivation behind this feature is to allow a library
  author to provide an omnibus signature containing the type of every function
  someone might want to use, while a client thins down the exports to the ones
  they actually require.  For example, supposing that you have inherited a
  signature for strings, you might write a local signature of this form, listing
  only the entities that you need::

    signature Str (Str, empty, append, concat) where
        -- empty

  A few caveats apply here.  First, it is illegal to export an entity
  which refers to a locally defined type which itself is not exported
  (GHC will report an error in this case).  Second, signatures which
  come from dependencies which expose modules cannot be thinned in this
  way (after all, the dependency itself may need the entity); these
  requirements are unconditionally exported.  Finally, any module
  reexports must refer to modules imported by the local signature
  (even if an inherited signature exported the module).

  We may change the syntax and semantics of this feature in the future.

- The declarations and types from signatures of dependencies
  that will be merged in are not in scope when type checking
  an ``hsig`` file.  To refer to any such type, you must
  declare it yourself::

    -- OK, assuming we inherited an A that defines T
    signature A (T) where
        -- empty

    -- Not OK
    signature A (T, f) where
        f :: T -> T

    -- OK
    signature A (T, f) where
        data T
        f :: T -> T

- There must be no value declarations, but there can be type signatures
  for values.  For example, we might define the signature::

        signature A where
            double :: Int -> Int

  A module implementing ``A`` would have to export the function
  ``double`` with a type definitionally equal to the signature.
  Note that this means you can't implement ``double`` using
  a polymorphic function ``double :: Num a => a -> a``.

  Note that signature matching does check if *fixity* matches, so be
  sure specify fixity of ordinary identifiers if you intend to use them
  with backticks.

- Fixity, type synonym, open type/data family declarations
  are permitted as in normal Haskell.

- Closed type family declarations are permitted as in normal
  Haskell.  They can also be given abstractly, as in the
  following example::

    type family ClosedFam a where ..

  The ``..`` is meant literally -- you should write two dots in
  your file.  The ``where`` clause distinguishes closed families
  from open ones.

- A data type declaration can either be given in full, exactly
  as in Haskell, or it can be given abstractly, by omitting the '='
  sign and everything that follows.  For example: ::

        signature A where
            data T a b

  Abstract data types can be implemented not only with data
  declarations, but also newtypes and type synonyms (with the
  restriction that a type synonym must be fully eta-reduced,
  e.g., ``type T = ...`` to be accepted.)  For example,
  the following are all valid implementations of the T above::

        -- Algebraic data type
        data T a b = MkT a b

        -- Newtype
        newtype T a b = MkT (a, b)

        -- Type synonym
        data T2 a b = MkT2 a a b b
        type T = T2

  Data type declarations merge only with other data type
  declarations which match exactly, except abstract data,
  which can merge with ``data``, ``newtype`` or ``type``
  declarations.  Merges with type synonyms are especially useful:
  suppose you are using a package of strings which has left the type of
  characters in the string unspecified::

        signature Str where
            data Str
            data Elem
            head :: Str -> Elem

  If you locally define a signature which specifies
  ``type Elem = Char``, you can now use ``head`` from the
  inherited signature as if it returned a ``Char``.

  If you do not write out the constructors, you may need to give a kind to tell
  GHC what the kinds of the type variables are, if they are not the default
  ``*``.  Unlike regular data type declarations, the return kind of an
  abstract data declaration can be anything (in which case it probably
  will be implemented using a type synonym.)  This can be used
  to allow compile-time representation polymorphism (as opposed to
  `run-time representation polymorphism <#runtime-rep>`__),
  as in this example::

        signature Number where
            import GHC.Types
            data Rep :: RuntimeRep
            data Number :: TYPE Rep
            plus :: Number -> Number -> Number

  Roles of type parameters are subject to the subtyping
  relation ``phantom < representational < nominal``: for example,
  an abstract type with a nominal type parameter can be implemented
  using a concrete type with a representational type parameter.
  Merging respects this subtyping relation (e.g., ``nominal``
  merged with ``representational`` is ``representational``.)
  Roles in signatures default to ``nominal``, which gives maximum
  flexibility on the implementor's side.  You should only need to
  give an explicit role annotation if a client of the signature
  would like to coerce the abstract type in a type parameter (in which case you
  should specify ``representational`` explicitly.)  Unlike
  regular data types, we do *not* assume that abstract
  data types are representationally injective: if we have
  ``Coercible (T a) (T b)``, and ``T`` has role ``nominal``,
  this does not imply that ``a ~ b``.

- A class declarations can either be abstract or concrete.  An
  abstract class is one with no superclasses or class methods::

    signature A where
        class Key k

  It can be implemented in any way, with any set of superclasses
  and methods; however, modules depending on an abstract class
  are not permitted to define instances (as of GHC 8.2, this
  restriction is not checked, see :ghc-ticket:`13086`.)
  These declarations can be implemented by type synonyms
  of kind ``Constraint``; this can be useful if you want to parametrize
  over a constraint in functions.  For example, with the
  ``ConstraintKinds`` extension, this type synonym is a valid
  implementation of the signature above::

    module A where
        type Key = Eq

  A concrete class specifies its superclasses, methods,
  default method signatures (but not their implementations)
  and a ``MINIMAL`` pragma.  Unlike regular Haskell classes,
  you don't have to explicitly declare a default for a method
  to make it optional vis-a-vis the ``MINIMAL`` pragma.

  When merging class declarations, we require that the superclasses
  and methods match exactly; however, ``MINIMAL`` pragmas are logically
  ORed together, and a method with a default signature will merge
  successfully against one that does not.

- You can include instance declarations as in Haskell; just omit the
  "where" part.  An instance declaration need not be implemented directly;
  if an instance can be derived based on instances in the environment,
  it is considered implemented.  For example, the following signature::

    signature A where
        data Str
        instance Eq Str

  is considered implemented by the following module, since there
  are instances of ``Eq`` for ``[]`` and ``Char`` which can be combined
  to form an instance ``Eq [Char]``::

    module A where
        type Str = [Char]

  Unlike other declarations, for which only the entities declared
  in a signature file are brought into scope, instances from the
  implementation are always brought into scope, even if they were
  not declared in the signature file.  This means that a module may
  typecheck against a signature, but not against a matching
  implementation.  You can avoid situations like this by never
  defining orphan instances inside a package that has signatures.

  Instance declarations are only merged if their heads are exactly
  the same, so it is possible to get into a situation where GHC
  thinks that instances in a signature are overlapping, even if
  they are implemented in a non-overlapping way.  If this is
  giving you problems give us a shout.

- Any orphan instances which are brought into scope by an import
  from a signature are unconditionally considered in scope, even
  if the eventual implementing module doesn't actually import the
  same orphans.

Known limitations:

- Pattern synonyms are not supported.

- Algebraic data types specified in a signature cannot be implemented using
  pattern synonyms.  See :ghc-ticket:`12717`

.. _using-make:

Using ``make``
--------------

.. index::
   single: make; building programs with

It is reasonably straightforward to set up a ``Makefile`` to use with
GHC, assuming you name your source files the same as your modules. Thus:

.. code-block:: makefile

    HC      = ghc
    HC_OPTS = -cpp $(EXTRA_HC_OPTS)

    SRCS = Main.lhs Foo.lhs Bar.lhs
    OBJS = Main.o   Foo.o   Bar.o

    .SUFFIXES : .o .hs .hi .lhs .hc .s

    cool_pgm : $(OBJS)
            rm -f $@
            $(HC) -o $@ $(HC_OPTS) $(OBJS)

    # Standard suffix rules
    .o.hi:
            @:

    .lhs.o:
            $(HC) -c $< $(HC_OPTS)

    .hs.o:
            $(HC) -c $< $(HC_OPTS)

    .o-boot.hi-boot:
            @:

    .lhs-boot.o-boot:
            $(HC) -c $< $(HC_OPTS)

    .hs-boot.o-boot:
            $(HC) -c $< $(HC_OPTS)

    # Inter-module dependencies
    Foo.o Foo.hc Foo.s    : Baz.hi          # Foo imports Baz
    Main.o Main.hc Main.s : Foo.hi Baz.hi   # Main imports Foo and Baz

.. note::
    Sophisticated :command:`make` variants may achieve some of the above more
    elegantly. Notably, :command:`gmake`\'s pattern rules let you write the more
    comprehensible:

    .. code-block:: make

        %.o : %.lhs
                $(HC) -c $< $(HC_OPTS)

    What we've shown should work with any ``make``.

Note the cheesy ``.o.hi`` rule: It records the dependency of the
interface (``.hi``) file on the source. The rule says a ``.hi`` file can
be made from a ``.o`` file by doing…nothing. Which is true.

Note that the suffix rules are all repeated twice, once for normal
Haskell source files, and once for ``hs-boot`` files (see
:ref:`mutual-recursion`).

Note also the inter-module dependencies at the end of the Makefile,
which take the form

.. code-block:: make

    Foo.o Foo.hc Foo.s    : Baz.hi          # Foo imports Baz

They tell ``make`` that if any of ``Foo.o``, ``Foo.hc`` or ``Foo.s``
have an earlier modification date than ``Baz.hi``, then the out-of-date
file must be brought up to date. To bring it up to date, ``make`` looks
for a rule to do so; one of the preceding suffix rules does the job
nicely. These dependencies can be generated automatically by ``ghc``;
see :ref:`makefile-dependencies`

.. _makefile-dependencies:

Dependency generation
---------------------

.. index::
   single: dependencies in Makefiles
   single: Makefile dependencies

Putting inter-dependencies of the form ``Foo.o : Bar.hi`` into your
``Makefile`` by hand is rather error-prone. Don't worry, GHC has support
for automatically generating the required dependencies. Add the
following to your ``Makefile``:

.. code-block:: make

    depend :
            ghc -dep-suffix '' -M $(HC_OPTS) $(SRCS)

Now, before you start compiling, and any time you change the ``imports``
in your program, do ``make depend`` before you do ``make cool_pgm``. The command
``ghc -M`` will append the needed dependencies to your ``Makefile``.

In general, ``ghc -M Foo`` does the following. For each module ``M`` in
the set ``Foo`` plus all its imports (transitively), it adds to the
Makefile:

-  A line recording the dependence of the object file on the source
   file.

   .. code-block:: make

       M.o : M.hs

   (or ``M.lhs`` if that is the filename you used).

-  For each import declaration ``import X`` in ``M``, a line recording
   the dependence of ``M`` on ``X``:

   .. code-block:: make

       M.o : X.hi

-  For each import declaration ``import {-# SOURCE #-} X`` in ``M``, a
   line recording the dependence of ``M`` on ``X``:

   .. code-block:: make

       M.o : X.hi-boot

   (See :ref:`mutual-recursion` for details of ``hi-boot`` style
   interface files.)

If ``M`` imports multiple modules, then there will be multiple lines
with ``M.o`` as the target.

There is no need to list all of the source files as arguments to the
``ghc -M`` command; ``ghc`` traces the dependencies, just like
``ghc --make`` (a new feature in GHC 6.4).

Note that ``ghc -M`` needs to find a *source file* for each module in
the dependency graph, so that it can parse the import declarations and
follow dependencies. Any pre-compiled modules without source files must
therefore belong to a package [1]_.

By default, ``ghc -M`` generates all the dependencies, and then
concatenates them onto the end of ``makefile`` (or ``Makefile`` if
``makefile`` doesn't exist) bracketed by the lines
"``# DO NOT DELETE: Beginning of Haskell dependencies``" and
"``# DO NOT DELETE: End of Haskell dependencies``". If these lines
already exist in the ``makefile``, then the old dependencies are deleted
first.

Don't forget to use the same ``-package`` options on the ``ghc -M``
command line as you would when compiling; this enables the dependency
generator to locate any imported modules that come from packages. The
package modules won't be included in the dependencies generated, though
(but see the ``-include-pkg-deps`` option below).

The dependency generation phase of GHC can take some additional options,
which you may find useful. The options which affect dependency
generation are:

.. ghc-flag:: -ddump-mod-cycles
    :shortdesc: Dump module cycles
    :type: dynamic
    :category:

    Display a list of the cycles in the module graph. This is useful
    when trying to eliminate such cycles.

.. ghc-flag:: -v2
    :noindex:

    Print a full list of the module dependencies to stdout. (This is the
    standard verbosity flag, so the list will also be displayed with
    ``-v3`` and ``-v4``; see :ref:`options-help`.)

.. ghc-flag:: -dep-makefile ⟨file⟩
    :shortdesc: Use ⟨file⟩ as the makefile
    :type: dynamic
    :category:

    Use ⟨file⟩ as the makefile, rather than ``makefile`` or
    ``Makefile``. If ⟨file⟩ doesn't exist, ``mkdependHS`` creates it. We
    often use ``-dep-makefile .depend`` to put the dependencies in
    ``.depend`` and then ``include`` the file ``.depend`` into
    ``Makefile``.

.. ghc-flag:: -dep-suffix ⟨suffix⟩
    :shortdesc: Make dependencies that declare that files with suffix
        ``.⟨suf⟩⟨osuf⟩`` depend on interface files with suffix ``.⟨suf⟩hi``
    :type: dynamic
    :category:

    Make dependencies that declare that files with suffix
    ``.⟨suf⟩⟨osuf⟩`` depend on interface files with suffix
    ``.⟨suf⟩hi``, or (for ``{-# SOURCE #-}`` imports) on ``.hi-boot``.
    Multiple ``-dep-suffix`` flags are permitted. For example,
    ``-dep-suffix a_ -dep-suffix b_`` will make dependencies for ``.hs``
    on ``.hi``, ``.a_hs`` on ``.a_hi``, and ``.b_hs`` on ``.b_hi``.
    Note that you must provide at least one suffix; if you do not want a suffix
    then pass ``-dep-suffix ''``.

.. ghc-flag:: --exclude-module=⟨file⟩
    :shortdesc: Regard ``⟨file⟩`` as "stable"; i.e., exclude it from having
        dependencies on it.
    :type: dynamic
    :category:

    Regard ``⟨file⟩`` as "stable"; i.e., exclude it from having
    dependencies on it.

.. ghc-flag:: -include-pkg-deps
    :shortdesc: Regard modules imported from packages as unstable
    :type: dynamic
    :category:

    Regard modules imported from packages as unstable, i.e., generate
    dependencies on any imported package modules (including ``Prelude``,
    and all other standard Haskell libraries). Dependencies are not
    traced recursively into packages; dependencies are only generated
    for home-package modules on external-package modules directly
    imported by the home package module. This option is normally only
    used by the various system libraries.

.. ghc-flag:: -include-cpp-deps
    :shortdesc: Include preprocessor dependencies
    :type: dynamic
    :category:

    Output preprocessor dependencies. This only has an effect when the CPP
    language extension is enabled. These dependencies are files included with
    the ``#include`` preprocessor directive (as well as transitive includes) and
    implicitly included files such as standard c preprocessor headers and a GHC
    version header. One exception to this is that GHC generates a temporary
    header file (during compilation) containing package version macros. As this
    is only a temporary file that GHC will always generate, it is not output as
    a dependency.

.. _orphan-modules:

Orphan modules and instance declarations
----------------------------------------

Haskell specifies that when compiling module ``M``, any instance declaration
in any module "below" ``M`` is visible. (Module ``A`` is "below" ``M`` if ``A`` is
imported directly by ``M``, or if ``A`` is below a module that ``M`` imports
directly.) In principle, GHC must therefore read the interface files of
every module below ``M``, just in case they contain an instance declaration
that matters to ``M``. This would be a disaster in practice, so GHC tries to
be clever.

In particular, if an instance declaration is in the same module as the
definition of any type or class mentioned in the *head* of the instance
declaration (the part after the "``=>``"; see :ref:`instance-rules`), then GHC
has to visit that interface file anyway. Example: ::

      module A where
        instance C a => D (T a) where ...
        data T a = ...

The instance declaration is only relevant if the type ``T`` is in use, and
if so, GHC will have visited ``A``\'s interface file to find ``T``\'s definition.

The only problem comes when a module contains an instance declaration
and GHC has no other reason for visiting the module. Example: ::

      module Orphan where
        instance C a => D (T a) where ...
        class C a where ...

Here, neither ``D`` nor ``T`` is declared in module ``Orphan``. We call such modules
"orphan modules". GHC identifies orphan modules, and visits the
interface file of every orphan module below the module being compiled.
This is usually wasted work, but there is no avoiding it. You should
therefore do your best to have as few orphan modules as possible.

Functional dependencies complicate matters. Suppose we have: ::

      module B where
        instance E T Int where ...
        data T = ...

Is this an orphan module? Apparently not, because ``T`` is declared in
the same module. But suppose class ``E`` had a functional dependency: ::

      module Lib where
        class E x y | y -> x where ...

Then in some importing module ``M``, the constraint ``(E a Int)`` should be
"improved" by setting ``a = T``, *even though there is no explicit
mention* of ``T`` in ``M``.

These considerations lead to the following definition of an orphan
module:

-  An *orphan module* orphan module contains at least one *orphan
   instance* or at least one *orphan rule*.

-  An instance declaration in a module ``M`` is an *orphan instance* if

   -  The class of the instance declaration is not declared in ``M``, and

   -  *Either* the class has no functional dependencies, and none of the
      type constructors in the instance head is declared in ``M``; *or*
      there is a functional dependency for which none of the type
      constructors mentioned in the *non-determined* part of the
      instance head is defined in ``M``.

   Only the instance head counts. In the example above, it is not good
   enough for ``C``\'s declaration to be in module ``A``; it must be the
   declaration of ``D`` or ``T``.

-  A rewrite rule in a module ``M`` is an *orphan rule* orphan rule if none
   of the variables, type constructors, or classes that are free in the
   left hand side of the rule are declared in ``M``.

If you use the flag :ghc-flag:`-Worphans`, GHC will warn you if you are
creating an orphan module. Like any warning, you can switch the warning
off with :ghc-flag:`-Wno-orphans <-Worphans>`, and :ghc-flag:`-Werror` will make
the compilation fail if the warning is issued.

You can identify an orphan module by looking in its interface file, ``M.hi``,
using the :ghc-flag:`--show-iface ⟨file⟩` :ref:`mode <modes>`. If there is a
``[orphan module]`` on the first line, GHC considers it an orphan module.

.. [1]
   This is a change in behaviour relative to 6.2 and earlier.