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Hadrian
=======

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Hadrian is a new build system for the [Glasgow Haskell Compiler][ghc]. It is based
on [Shake][shake] and we hope that it will soon replace the current
[Make-based build system][make]. If you are curious about the rationale behind the
project and the architecture of the build system you can find more details in
this [Haskell Symposium 2016 paper][paper] and this [Haskell eXchange 2016 talk][talk].

The new build system can work side-by-side with the existing build system. Note, there is
some interaction between them: they put (some) build results in the same directories,
e.g. the resulting GHC is `inplace/bin/ghc-stage2`.

Your first build
----------------

Beware, the build system is in the alpha development phase. Things are shaky and sometimes
break; there are numerous [known issues][issues]. Not afraid? Then put on the helmet and
run the following command from root of the GHC tree:

```
hadrian/build.sh -j
```

or on Windows:

```
hadrian/build.bat -j
```

Here flag `-j` enables parallelism and is optional. We will further refer to the build script
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simply as `build`. Note that Hadrian can also run the `boot` and `configure` scripts
automatically if you pass the flag `--configure`, or simply `-c`. See the overview of
command line flags below.
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Notes:

* If the default build script doesn't work, you might want to give a try to another one, e.g. based
on Cabal sandboxes (`build.cabal.*`), Stack (`build.stack.*`) or the global package database
(`build.global-db.*`). Also see [instructions for building GHC on Windows using Stack][windows-build].

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* Hadrian is written in Haskell and depends on `shake` (plus a few packages that `shake` depends on),
`ansi-terminal`, `mtl`, `quickcheck`, and GHC core libraries.
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* If you have never built GHC before, start with the [preparation guide][ghc-preparation].

Using the build system
----------------------
Once your first build is successful, simply run `build` to rebuild. Build results
are placed into `_build` and `inplace` directories.

#### Command line flags

In addition to standard Shake flags (try `--help`), the build system
currently supports several others:
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* `--configure` or `-c`: use this flag to run the `boot` and `configure` scripts
automatically, so that you don't have to remember to run them manually as you normally
do when using Make (typically only in the first build):
    ```bash
    ./boot
    ./configure # On Windows run ./configure --enable-tarballs-autodownload
    ```
    Beware that with this flag Hadrian may do network I/O on Windows to download necessary
    tarballs, which may sometimes be undesirable.

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* `--flavour=FLAVOUR`: choose a build flavour. The following settings are currently supported:
`default`, `quick`, `quickest`, `perf`, `prof`, `devel1` and `devel2`. As an example, the
`quickest` flavour adds `-O0` flag to all GHC invocations and builds libraries only in the
`vanilla` way, which speeds up builds by 3-4x. Build flavours are documented
[here](https://github.com/snowleopard/hadrian/blob/master/doc/flavours.md).

* `--freeze1`: freeze Stage1 GHC, i.e. do not rebuild it even if some of its source files
are out-of-date. This allows to significantly reduce the rebuild time when you are working
on a feature that affects both Stage1 and Stage2 compilers, but may lead to incorrect
build results. To unfreeze Stage1 GHC simply drop the `--freeze1` flag and Hadrian will
rebuild all out-of-date files.

* `--integer-simple`: build GHC using the `integer-simple` integer library (instead
of `integer-gmp`).

* `--progress-colour=MODE`: choose whether to use colours when printing build progress
info. There are three settings: `never` (do not use colours), `auto` (attempt to detect
whether the console supports colours; this is the default setting), and `always` (use
colours).

* `--progress-info=STYLE`: choose how build progress info is printed. There are four
settings: `none`, `brief` (one line per build command; this is the default setting),
`normal` (typically a box per build command), and `unicorn` (when `normal` just won't do).

* `--split-objects`: generate split objects, which are switched off by default. Due to
a GHC [bug][ghc-split-objs-bug], you need a full clean rebuild when using this flag.

* `--verbose`: run Hadrian in verbose mode. In particular this prints diagnostic messages
by Shake oracles.

#### User settings

The Make-based build system uses `mk/build.mk` to specify user build settings. We
use `hadrian/UserSettings.hs` for the same purpose, see [documentation](doc/user-settings.md).

#### Clean and full rebuild

* `build clean` removes all build artefacts.

* `build -B` forces Shake to rerun all rules, even if the previous build results are
are still up-to-date.

#### Documentation

To build GHC documentation, run `build docs`. Note that finer-grain documentation
targets (e.g. building only HTML documentation or only the GHC User's Guide)
are currently not supported.

#### Source distribution

To build a GHC source distribution tarball, run `build sdist-ghc`.

#### Installation

To build and install GHC artifacts, run `build install`.

By default, GHC will be installed to the specified _prefix_ path on your system,
relative to the root of the file system. For example on UNIX, GHC will be installed
to `/usr/local/bin`. By setting the command line flag `--install-destdir=[DESTDIR]`,
you can install GHC to path `DESTDIR/<prefix>` instead. Make sure you use correct
absolute path as `DESTDIR` on Windows, e.g. `C:/path`, which installs GHC
into `C:/path/usr/local`.

#### Testing

* `build validate` runs GHC tests by simply executing `make fast` in `testsuite/tests`
directory. This can be used instead of `sh validate --fast --no-clean` in the existing
build system. Note: this will rebuild Stage2 GHC, `ghc-pkg` and `hpc` if they are out of date.

* `build test` runs GHC tests by calling the `testsuite/driver/runtests.py` python
script with appropriate flags. The current implementation is limited and cannot
replace the `validate` script (see [#187][validation-issue]).

* `build selftest` runs tests of the build system. Current test coverage is close to
zero (see [#197][test-issue]).

Current limitations
-------------------
The new build system still lacks many important features:
* Validation is not implemented: [#187][validation-issue].
* Dynamic linking on Windows is not supported [#343][dynamic-windows-issue].
* There is no support for binary distribution: [#219][bin-dist-issue].

Check out [milestones] to see when we hope to resolve the above limitations.

How to contribute
-----------------

The best way to contribute is to try the new build system, report the issues
you found, and attempt to fix them. Please note: the codebase is very unstable
at present and we expect a lot of further refactoring. If you would like to
work on a particular issue, please let everyone know by adding a comment about
this. The issues that are currently on the critical path and therefore require
particular attention are listed in [#239](https://github.com/snowleopard/hadrian/issues/239).
Also have a look at [projects](https://github.com/snowleopard/hadrian/projects)
where open issues and pull requests are grouped into categories.

Acknowledgements
----------------

I started this project as part of my 6-month research visit to Microsoft
Research Cambridge, which was funded by Newcastle University, EPSRC, and
Microsoft Research. I would like to thank Simon Peyton Jones, Neil Mitchell
and Simon Marlow for kick-starting the project and for their guidance.
Zhen Zhang has done fantastic work on Hadrian as part of his Summer of
Haskell 2017 [project](https://summer.haskell.org/ideas.html#hadrian-ghc),
solving a few heavy and long-overdue issues. Last but not least, big thanks
to all other project [contributors][contributors], who helped me endure and
enjoy the project.

[ghc]: https://en.wikipedia.org/wiki/Glasgow_Haskell_Compiler
[shake]: https://github.com/ndmitchell/shake
[make]: https://ghc.haskell.org/trac/ghc/wiki/Building/Architecture
[paper]: https://www.staff.ncl.ac.uk/andrey.mokhov/Hadrian.pdf
[talk]: https://skillsmatter.com/skillscasts/8722-meet-hadrian-a-new-build-system-for-ghc
[issues]: https://github.com/snowleopard/hadrian/issues
[ghc-preparation]: https://ghc.haskell.org/trac/ghc/wiki/Building/Preparation
[ghc-windows-quick-build]: https://ghc.haskell.org/trac/ghc/wiki/Building/Preparation/Windows#AQuickBuild
[windows-build]: https://github.com/snowleopard/hadrian/blob/master/doc/windows.md
[ghc-split-objs-bug]: https://ghc.haskell.org/trac/ghc/ticket/11315
[test-issue]: https://github.com/snowleopard/hadrian/issues/197
[validation-issue]: https://github.com/snowleopard/hadrian/issues/187
[dynamic-windows-issue]: https://github.com/snowleopard/hadrian/issues/343
[bin-dist-issue]: https://github.com/snowleopard/hadrian/issues/219
[milestones]: https://github.com/snowleopard/hadrian/milestones
[contributors]: https://github.com/snowleopard/hadrian/graphs/contributors