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+Hadrian
+=======
+
+[![Linux & OS X status](https://img.shields.io/travis/snowleopard/hadrian/master.svg?label=Linux%20%26%20OS%20X)](https://travis-ci.org/snowleopard/hadrian) [![Windows status](https://img.shields.io/appveyor/ci/snowleopard/hadrian/master.svg?label=Windows)](https://ci.appveyor.com/project/snowleopard/hadrian) [![OS X status](https://img.shields.io/circleci/project/github/snowleopard/hadrian.svg?label=OS%20X)](https://circleci.com/gh/snowleopard/hadrian)
+
+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
+simply as `build`. Note that Hadrian runs the `boot` and `configure` scripts automatically when
+needed. Use `--skip-configure` to suppress this behaviour (see the overview of command line
+flags below).
+
+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].
+
+* Hadrian is written in Haskell and depends on the following packages:
+`ansi-terminal extra mtl quickcheck shake`.
+
+* 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:
+* `--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).
+
+* `--skip-configure`: use this flag to suppress the default behaviour of Hadrian that
+runs the `boot` and `configure` scripts automatically when needed, so that you don't have
+to remember to run them manually. With `--skip-configure` you will need to manually run:
+    ```bash
+    ./boot
+    ./configure # On Windows run ./configure --enable-tarballs-autodownload
+    ```
+    as you normally do when using `make`. Beware, by default Hadrian may do network I/O on
+Windows to download necessary tarballs, which may sometimes be undesirable; `--skip-configure`
+is your friend in such cases.
+
+* `--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