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196 commits behind the upstream repository.
Simon Peyton Jones's avatar
Simon Peyton Jones authored
This big MR entirely removes the "flattener" that took a type and
replaced each type-family application with a fresh type variable.
The flattener had its origin in the paper
     Injective type families for Haskell

But (a) #25657 showed that flattening doesn't really work.
    (b) since we wrote the paper we have introduced the so-called
        "fine-grained" unifier GHC.Core.Unify, which can return
                 * SurelyApart
                 * Unifiable subst
                 * MaybeApart subst
        where the MaybeApart says that the two types are not unifiable by a
        substitution, but could (perhaps) be unified "later" after some type
        family reductions.  This turns out to subsume flattening.

This MR does a major refactor of GHC.Core.Unify to make it capable of
subsuming flattening.   The main payload is described in
       Note [Apartness and type families]
and its many wrinkles.

The key (non-refactoring) implementation change is to add `um_fam_env`
to the `UMState` in the unification monad.

Careful review with Richard revealed various bugs in the treament of
`kco`, the kind coercion carried around by the unifier, so that is
substantially fixed too: see Note [Kind coercions in Unify].

Compile-time performance is improved by 0.1% with a few improvements over
1% and one worsening by 1.3% namely T9872a.  (I have not investigated the
latter.)

Metric Decrease:
    T9872b
    T9872c
    TcPlugin_RewritePerf
Metric Increase:
    T9872a
5d65393e
History

The Glasgow Haskell Compiler

pipeline status

This is the source tree for GHC, a compiler and interactive environment for the Haskell functional programming language.

For more information, visit GHC's web site.

Information for developers of GHC can be found on the GHC issue tracker, and you can also view proposals for new GHC features.

Getting the Source

There are two ways to get a source tree:

  1. Download source tarballs

    Download the GHC source distribution:

    ghc-<version>-src.tar.xz

    which contains GHC itself and the "boot" libraries.

  2. Check out the source code from git

    $ git clone --recurse-submodules git@gitlab.haskell.org:ghc/ghc.git

    Note: cloning GHC from Github requires a special setup. See Getting a GHC repository from Github.

See the GHC team's working conventions regarding how to contribute a patch to GHC. First time contributors are encouraged to get started by just sending a Merge Request.

Building & Installing

For full information on building GHC, see the GHC Building Guide. Here follows a summary - if you get into trouble, the Building Guide has all the answers.

Before building GHC you may need to install some other tools and libraries. See, Setting up your system for building GHC.

NB. In particular, you need GHC installed in order to build GHC, because the compiler is itself written in Haskell. You also need Happy, Alex, and Cabal. For instructions on how to port GHC to a new platform, see the GHC Building Guide.

For building library documentation, you'll need Haddock. To build the compiler documentation, you need Sphinx and Xelatex (only for PDF output).

Quick start: GHC is built using the Hadrian build system. The following gives you a default build:

$ ./boot
$ ./configure
$ hadrian/build         # can also say '-jX' for X number of jobs

On Windows, you need an extra repository containing some build tools. These can be downloaded for you by configure. This only needs to be done once by running:

$ ./configure --enable-tarballs-autodownload

Additionally, on Windows, to run Hadrian you should run hadrian/build.bat instead of hadrian/build.

(NB: Do you have multiple cores? Be sure to tell that to hadrian! This can save you hours of build time depending on your system configuration, and is almost always a win regardless of how many cores you have. As a simple rule, you should have about N+1 jobs, where N is the amount of cores you have.)

The ./boot step is only necessary if this is a tree checked out from git. For source distributions downloaded from GHC's web site, this step has already been performed.

These steps give you the default build, which includes everything optimised and built in various ways (eg. profiling libs are built). It can take a long time. To customise the build, see the file HACKING.md.

Filing bugs and feature requests

If you've encountered what you believe is a bug in GHC, or you'd like to propose a feature request, please let us know! Submit an issue and we'll be sure to look into it. Remember: Filing a bug is the best way to make sure your issue isn't lost over time, so please feel free.

If you're an active user of GHC, you may also be interested in joining the glasgow-haskell-users mailing list, where developers and GHC users discuss various topics and hang out.

Hacking & Developing GHC

Once you've filed a bug, maybe you'd like to fix it yourself? That would be great, and we'd surely love your company! If you're looking to hack on GHC, check out the guidelines in the HACKING.md file in this directory - they'll get you up to speed quickly.

Governance and Acknowledgements

GHC is a community project developed by a team of highly-talented researchers, individual contributors, and full-time developers. We are indebted to the many people whose work has brought GHC to its current state.

Some larger decisions are made by a smaller group of core contributors, as described in our governance documentation.