Multiple Home Units

Multiple home units allows you to load different packages which may depend on
each other into one GHC session. This will allow both GHCi and HLS to support
multi component projects more naturally.

Public Interface
~~~~~~~~~~~~~~~~

In order to specify multiple units, the -unit @⟨filename⟩ flag
is given multiple times with a response file containing the arguments for each unit.
The response file contains a newline separated list of arguments.

```
ghc -unit @unitLibCore -unit @unitLib
```

where the `unitLibCore` response file contains the normal arguments that cabal would pass to `--make` mode.

```
-this-unit-id lib-core-0.1.0.0
-i
-isrc
LibCore.Utils
LibCore.Types
```

The response file for lib, can specify a dependency on lib-core, so then modules in lib can use modules from lib-core.

```
-this-unit-id lib-0.1.0.0
-package-id lib-core-0.1.0.0
-i
-isrc
Lib.Parse
Lib.Render
```

Then when the compiler starts in --make mode it will compile both units lib and lib-core.

There is also very basic support for multiple home units in GHCi, at the
moment you can start a GHCi session with multiple units but only the
:reload is supported. Most commands in GHCi assume a single home unit,
and so it is additional work to work out how to modify the interface to
support multiple loaded home units.

Options used when working with Multiple Home Units

There are a few extra flags which have been introduced specifically for
working with multiple home units. The flags allow a home unit to pretend
it’s more like an installed package, for example, specifying the package
name, module visibility and reexported modules.

-working-dir ⟨dir⟩

    It is common to assume that a package is compiled in the directory
    where its cabal file resides. Thus, all paths used in the compiler
    are assumed to be relative to this directory. When there are
    multiple home units the compiler is often not operating in the
    standard directory and instead where the cabal.project file is
    located. In this case the -working-dir option can be passed which
    specifies the path from the current directory to the directory the
    unit assumes to be it’s root, normally the directory which contains
    the cabal file.

    When the flag is passed, any relative paths used by the compiler are
    offset by the working directory. Notably this includes -i and
    -I⟨dir⟩ flags.

-this-package-name ⟨name⟩

    This flag papers over the awkward interaction of the PackageImports
    and multiple home units. When using PackageImports you can specify
    the name of the package in an import to disambiguate between modules
    which appear in multiple packages with the same name.

    This flag allows a home unit to be given a package name so that you
    can also disambiguate between multiple home units which provide
    modules with the same name.

-hidden-module ⟨module name⟩

    This flag can be supplied multiple times in order to specify which
    modules in a home unit should not be visible outside of the unit it
    belongs to.

    The main use of this flag is to be able to recreate the difference
    between an exposed and hidden module for installed packages.

-reexported-module ⟨module name⟩

    This flag can be supplied multiple times in order to specify which
    modules are not defined in a unit but should be reexported. The
    effect is that other units will see this module as if it was defined
    in this unit.

    The use of this flag is to be able to replicate the reexported
    modules feature of packages with multiple home units.

Offsetting Paths in Template Haskell splices
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When using Template Haskell to embed files into your program,
traditionally the paths have been interpreted relative to the directory
where the .cabal file resides. This causes problems for multiple home
units as we are compiling many different libraries at once which have
.cabal files in different directories.

For this purpose we have introduced a way to query the value of the
-working-dir flag to the Template Haskell API. By using this function we
can implement a makeRelativeToProject function which offsets a path
which is relative to the original project root by the value of
-working-dir.

```
import Language.Haskell.TH.Syntax ( makeRelativeToProject )

foo = $(makeRelativeToProject "./relative/path" >>= embedFile)
```

> If you write a relative path in a Template Haskell splice you should use the makeRelativeToProject function so that your library works correctly with multiple home units.

A similar function already exists in the file-embed library. The
function in template-haskell implements this function in a more robust
manner by honouring the -working-dir flag rather than searching the file
system.

Closure Property for Home Units
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

For tools or libraries using the API there is one very important closure
property which must be adhered to:

> Any dependency which is not a home unit must not (transitively) depend
  on a home unit.

For example, if you have three packages p, q and r, then if p depends on
q which depends on r then it is illegal to load both p and r as home
units but not q, because q is a dependency of the home unit p which
depends on another home unit r.

If you are using GHC by the command line then this property is checked,
but if you are using the API then you need to check this property
yourself. If you get it wrong you will probably get some very confusing
errors about overlapping instances.

Limitations of Multiple Home Units
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There are a few limitations of the initial implementation which will be smoothed out on user demand.

    * Package thinning/renaming syntax is not supported
    * More complicated reexports/renaming are not yet supported.
    * It’s more common to run into existing linker bugs when loading a
      large number of packages in a session (for example #20674, #20689)
    * Backpack is not yet supported when using multiple home units.
    * Dependency chasing can be quite slow with a large number of
      modules and packages.
    * Loading wired-in packages as home units is currently not supported
      (this only really affects GHC developers attempting to load
      template-haskell).
    * Barely any normal GHCi features are supported, it would be good to
      support enough for ghcid to work correctly.

Despite these limitations, the implementation works already for nearly
all packages. It has been testing on large dependency closures,
including the whole of head.hackage which is a total of 4784 modules
from 452 packages.

Internal Changes
~~~~~~~~~~~~~~~~

* The biggest change is that the HomePackageTable is replaced with the
  HomeUnitGraph. The HomeUnitGraph is a map from UnitId to HomeUnitEnv,
  which contains information specific to each home unit.
* The HomeUnitEnv contains:
    - A unit state, each home unit can have different package db flags
    - A set of dynflags, each home unit can have different flags
    - A HomePackageTable
* LinkNode: A new node type is added to the ModuleGraph, this is used to
  place the linking step into the build plan so linking can proceed in
  parralel with other packages being built.
* New invariant: Dependencies of a ModuleGraphNode can be completely
  determined by looking at the value of the node. In order to achieve
  this, downsweep now performs a more complete job of downsweeping and
  then the dependenices are recorded forever in the node rather than
  being computed again from the ModSummary.
* Some transitive module calculations are rewritten to use the
  ModuleGraph which is more efficient.
* There is always an active home unit, which simplifies modifying a lot
  of the existing API code which is unit agnostic (for example, in the
  driver).

The road may be bumpy for a little while after this change but the
basics are well-tested.

One small metric increase, which we accept and also submodule update to
haddock which removes ExtendedModSummary.

Closes #10827

-------------------------
Metric Increase:
    MultiLayerModules
-------------------------

Co-authored-by: Fendor <power.walross@gmail.com>

1 files changed, 125 insertions, 0 deletions

diff --git a/docs/users_guide/using.rst b/docs/users_guide/using.rst
index 2184946571..994dc66d74 100644
--- a/docs/users_guide/using.rst
+++ b/docs/users_guide/using.rst
@@ -740,6 +740,131 @@ search path (see :ref:`search-path`).
     number of processors. Note that compilation of a module may not begin
     until its dependencies have been built.
 
+.. _multi-home-units:
+
+Multiple Home Units
+~~~~~~~~~~~~~~~~~~~
+
+The compiler also has support for building multiple units in a single compiler
+invocation. In modern projects it is common to work on multiple interdependent
+packages at once, using the support for multiple home units you can load all
+these local packages into one ghc session and quickly get feedback about how
+changes affect other dependent packages.
+
+In order to specify multiple units, the `-unit @⟨filename⟩`:ghc-flag: is given multiple times
+with a response file containing the arguments for each unit. The response file contains
+a newline separated list of arguments.
+
+.. code-block:: none
+
+    ghc -unit @unitA -unit @unitB
+
+where the ``unitA`` response file contains the normal arguments that you would
+pass to ``--make`` mode.
+
+.. code-block:: none
+
+    -this-unit-id a-0.1.0.0
+    -i
+    -isrc
+    A1
+    A2
+    ...
+
+Then when the compiler starts in ``--make`` mode it will compile both units ``a`` and ``b``.
+
+There is also very basic support for multple home units in GHCi, at the moment you can start
+a GHCi session with multiple units but only the `:reload`:ghci-cmd: is supported.
+
+.. ghc-flag:: -unit @⟨filename⟩
+    :shortdesc: Specify the options to build a specific unit.
+    :type: dynamic
+    :category: misc
+
+    This option is passed multiple times to inform the compiler about all the
+    home units which it will compile. The options for each unit are supplied
+    in a response file which contains a newline separated list of normal arguments.
+
+There are a few extra flags which have been introduced to make working with multiple
+units easier.
+
+.. ghc-flag:: -working-dir ⟨dir⟩
+    :shortdesc: Specify the directory a unit is expected to be compiled in.
+    :type: dynamic
+    :category:
+
+    It is common to assume that a package is compiled in the directory where its
+    cabal file resides. Thus, all paths used in the compiler are assumed to be relative
+    to this directory. When there are multiple home units the compiler is often
+    not operating in the standard directory and instead where the cabal.project
+    file is located. In this case the `-working-dir` option can be passed which specifies
+    the path from the current directory to the directory the unit assumes to be it's root,
+    normally the directory which contains the cabal file.
+
+    When the flag is passed, any relative paths used by the compiler are offset
+    by the working directory. Notably this includes `-i`:ghc-flag: and `-I⟨dir⟩`:ghc-flag: flags.
+
+
+    This option can also be queried by the ``getPackageRoot`` Template Haskell
+    function. It is intended to be used with helper functions such as ``makeRelativeToProject``
+    which make relative filepaths relative to the compilation directory rather than
+    the directory which contains the .cabal file.
+
+.. ghc-flag:: -this-package-name ⟨unit-id⟩
+    :shortdesc: The name of the package which this module would be part of when installed.
+    :type: dynamic
+    :category:
+
+    This flag papers over the awkward interaction of the `PackageImports`:extension:
+    and multiple home units. When using ``PackageImports`` you can specify the name
+    of the package in an import to disambiguate between modules which appear in multiple
+    packages with the same name.
+
+    This flag allows a home unit to be given a package name so that you can also
+    disambiguate between multiple home units which provide modules with the same name.
+
+.. ghc-flag:: -hidden-module ⟨module name⟩
+    :shortdesc: A module which should not be visible outside its unit.
+    :type: dynamic
+    :category:
+
+    This flag can be supplied multiple times in order to specify which modules
+    in a home unit should not be visible outside of the unit it belongs to.
+
+    The main use of this flag is to be able to recreate the difference between
+    an exposed and hidden module for installed packages.
+
+.. ghc-flag:: -reexported-module ⟨module name⟩
+    :shortdesc: A module which should be reexported from this unit.
+    :type: dynamic
+    :category:
+
+    This flag can be supplied multiple times in order to specify which modules
+    are not defined in a unit but should be reexported. The effect is that other
+    units will see this module as if it was defined in this unit.
+
+    The use of this flag is to be able to replicate the reexported modules
+    feature of packages with multiple home units.
+
+
+
+The home unit closure requirement
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+There is one very important closure property which you must ensure when using
+multiple home units.
+
+  Any external unit must not depend on any home unit.
+
+This closure property is checked by the compiler but it's up to the tool invoking
+GHC to ensure that the supplied list of home units obey this invariant.
+
+For example, if we have three units, ``p``, ``q`` and ``r``, where ``p`` depends on ``q`` and
+``q`` depends on ``r``, then the closure property states that if we load ``p`` and ``r`` as
+home units then we must also load ``q``, because ``q`` depends on the home unit ``r`` and we need
+``q`` because ``p`` depends on it.
+
+
 .. _eval-mode:
 
 Expression evaluation mode