@@ -890,7 +890,7 @@ The library section should contain the following fields:
Supported only in GHC 8.2 and later. A list of `module signatures <https://downloads.haskell.org/~ghc/master/users-guide/separate_compilation.html#module-signatures>`__ required by this package.
Module signatures are part of the Backpack_ extension to
Module signatures are part of the :ref:`Backpack` extension to
the Haskell module system.
Packages that do not export any modules and only export required signatures
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@@ -2211,7 +2211,7 @@ system-dependent values for these fields.
See the :pkg-field:`library:signatures` field for more details.
Mixin packages are part of the Backpack_ extension to the
Mixin packages are part of the :ref:`Backpack` extension to the
Haskell module system.
The matching of the module signatures required by a
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@@ -2224,7 +2224,7 @@ system-dependent values for these fields.
.. Warning::
Backpack_ has the limitation that implementation modules that instantiate
:ref:`Backpack` has the limitation that implementation modules that instantiate
signatures required by a :pkg-field:`build-depends` dependency can't
reside in the same component that has the dependency. They must reside
in a different package dependency, or at least in a separate internal
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@@ -3305,123 +3305,6 @@ a few options:
library for all or part of the work. One option is to copy the source
of ``Distribution.Simple``, and alter it for your needs. Good luck.
.. _Backpack:
Backpack
--------
Cabal and GHC jointly support Backpack, an extension to Haskell's module
system which makes it possible to parametrize a package over some
modules, which can be instantiated later arbitrarily by a user. This
means you can write a library to be agnostic over some data
representation, and then instantiate it several times with different
data representations. Like C++ templates, instantiated packages are
recompiled for each instantiation, which means you do not pay any
runtime cost for parametrizing packages in this way. Backpack modules
are somewhat experimental; while fully supported by cabal-install, they are currently
`not supported by Stack <https://github.com/commercialhaskell/stack/issues/2540>`__.
A Backpack package is defined by use of the
:pkg-field:`library:signatures` field, or by (transitive) dependency on
a package that defines some requirements. To define a parametrized
package, define a signature file (file extension ``hsig``) that
specifies the signature of the module you want to parametrize over, and
add it to your Cabal file in the :pkg-field:`library:signatures` field.
.. code-block:: haskell
:caption: .hsig
signature Str where
data Str
concat :: [Str] -> Str
.. code-block:: cabal
:caption: parametrized.cabal
cabal-version: 2.2
name: parametrized
library
build-depends: base
signatures: Str
exposed-modules: MyModule
You can define any number of regular modules (e.g., ``MyModule``) that
import signatures and use them as regular modules.
If you are familiar with ML modules, you might now expect there to be
some way to apply the parametrized package with an implementation of
the ``Str`` module to get a concrete instantiation of the package.
Backpack operates slightly differently with a concept of *mix-in
linking*, where you provide an implementation of ``Str`` simply by
bringing another module into scope with the same name as the
requirement. For example, if you had a package ``str-impl`` that provided a
module named ``Str``, instantiating ``parametrized`` is as simple as
just depending on both ``str-impl`` and ``parametrized``:
.. code-block:: cabal
:caption: combined.cabal
cabal-version: 2.2
name: combined
library
build-depends: base, str-impl, parametrized
Note that due to technical limitations, you cannot directly define
``Str`` in the ``combined`` library; it must be placed in its own
library (you can use :ref:`Sublibraries <sublibs>` to conveniently
define a sub-library).
However, a more common situation is that your names don't match up
exactly. The :pkg-field:`library:mixins` field can be used to rename
signatures and modules to line up names as necessary. If you have
a requirement ``Str`` and an implementation ``Data.Text``, you can
line up the names in one of two ways:
* Rename the requirement to match the implementation:
``mixins: parametrized requires (Str as Data.Text)``
* Rename the implementation to match the requirement:
``mixins: text (Data.Text as Str)``
The :pkg-field:`library:mixins` field can also be used to disambiguate
between multiple instantiations of the same package; for each
instantiation of the package, give it a separate entry in mixins with
the requirements and provided modules renamed to be distinct.
