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contributing

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Contributing to GHC

GHC is a BSD-licensed open-source project, and we welcome your help in making it better. This page and the side bar on the left have pointers to information you'll need.

Working conventions

Tips and Tricks

  • Loading GHC into GHCi can provide a more iterative development experience.

  • To have an easier time looking up tickets and searching trac, use the browser tips page to make your search and lookups for Trac tickets substantially easier.

  • If you use Emacs, see Emacs for some useful stuff to put in your .emacs file.

  • If you have lots of Haskell installations, you may find Edsko's blog post Comprehensive Haskell Sandboxes useful.

Newcomers to GHC

While the building guide, working conventions, commentary and debugging pages (always linked from the left sidebar) have great information that can come in handy while you're working on your first, or first several patches, this section is intended to have the details you will need to get rolling.

If you have any questions along the way don't hesitate to reach out to the community. There are people on the mailing lists and IRC who will gladly help you (although you may need to be patient). Don't forget that all GHC developers are still learning; your question is never too silly to ask.

First steps

  • See Building/QuickStart to get started building GHC. Expect it all to take roughly between 30 minutes and a couple of hours, depending on your CPU speed, and the number of jobs you can run in parallel. Note that building older versions of GHC may require having an older version of GHC on your path.

  • While you are waiting for your build to finish, orient yourself to the general architecture of GHC. This article is written by two of the chief architects of GHC, Simon Marlow and Simon Peyton-Jones, is excellent and current (2012).

  • After a successful build, you should have your brand new compiler in ./inplace/bin/ghc-stage2. (GHCi is launched with ./inplace/bin/ghc-stage2 --interactive). Try it out.

Finding a ticket

Now that you can build GHC, let's get hacking. But first, you'll need to identify a goal. GHC's Trac tickets are a great place to find starting points. You are encouraged to ask for a starting point on IRC or the ghc-devsmailing list. There someone familiar with the process can help you find a ticket that matches your expertise and help you when you get stuck.

If you want to get a taste for possible starting tasks, below is a list of tickets that appear to be "low-hanging fruit" -- things that might be reasonable for a newcomer to GHC hacking. Of course, we can't ever be sure of how hard a task is before doing it, so apologies if one of these is too hard.

You can add tickets to this list by giving them the newcomer Trac keyword.

Bugs:

#8316 GHCi debugger panics when trying force a certain variable
#10346 Cross-module SpecConstr
#11068 Make Generic/Generic1 methods inlinable
#12488 Explicit namespaces doesn't enforce namespaces
#12576 Large Address space is not supported on Windows
#12636 ProfHeap's printf modifiers are incorrect
#13165 Speed up the RTS hash table
#13193 Integer (gmp) performance regression?
#13452 Lock .tix file
#13795 :kind! is not expanding type synonyms anymore
#14899 Significant compilation time regression between 8.4 and HEAD due to coverage checking
#15252 syn_arg_wraps and syn_res_wrap are only populated after typechecking
#15402 The settings and behaviour of idle GC are very confusing
#15540 GHCi does not follow the XDG Base Directory Specification
#15603 ref6 example from StaticPointers documentation doesn't type check
#15660 source file modify race leads to inconsistent error message
#15784 :doc shouldn't report \ for a data constructor when it can show docs for the type constructor of the same name and type
#15820 Document the proposals process in the GHC manual
#15836 ghc-in-ghci script fails when there is a Main.hs in the top-level directory
#15839 DerivingStrategies defaulting warning has no associated enable/suppress flag
#15843 Tuple sections can't be quoted
#15932 DeriveFunctor and GeneralizedNewtypeDeriving instances never reporting as covered
#15935 TYPE is not generated by genprimops
#15963 Test suite should report timeouts as timeouts
#15973 Int used to represent target integer literals
#16167 -ddump-json doesn't work with -e
#16196 Update README.md to reflect gitlab
#16235 Hadrian devel2 builds Haddock
#16273 Hadrian turns on \`-Wno-unused-imports\` for text when using integer-simple
#16282 all-missed-specializations doesn't suggest warning
#16360 GHC fails when GHC_PACKAGE_PATH contains trailing slash

Feature requests:

#7275 Give more detailed information about PINNED data in a heap profile
#8109 Type family patterns should support as-patterns.
#12178 Allow inline pragmas on pattern synonyms
#12982 Missed constant folding oportunities
#15461 Machine accessible interface to GHCi
#15483 ghc -M requires -dep-suffix for no good reason
#16155 Pattern Synonym for Ratio
#16164 Provide bitreverse primop

Tasks:

#4960 Better inlining test in CoreUnfold
#10068 Make the runtime reflection API for names, modules, locations more systematic
#11610 Remove IEThingAll constructor from IE datatype
#12619 Allow users guide to be built independently from GHC
#12687 Add a flag to control constraint solving trace
#12822 Cleanup GHC verbosity flags
#13698 Add a more complete example for the special SPEC argument to the user guide
#13892 Add some benchmarks to nofib from Andras Kovac's Eff benchmarks
#13923 Add a suppression flag to stop Typeable bindings being emitted with -ddump-simpl
#14023 Split up glasgow_exts.rst
#14099 Document fundeps
#15651 Check if some auto apply code is dead and remove if appropriate.
#15821 Implement more constant folding for Naturals
#15929 Explore whether adding XRay attributes to LLVM IR is worthwhile
#16052 Core optimizations for memset on a small range
#16062 Improve -dynamic-too progress messages
#16126 Make -threaded the default
#16277 Make JUnit report stdout/stderr in more cases

Advice

  • Read up on the steps you are expected to take for contributing a patch to GHC.

  • Need help? You can email the ghc-devs list, or ask on IRC in #ghc.

  • Don't get scared. GHC is a big codebase, but it makes sense when you stare at it long enough!

  • Don't hesitate to ask questions. We have all been beginners at some point and understand that diving in to GHC can be a challenge. Asking questions will help you make better use of your hacking time.

  • Be forewarned that many pages on the GHC Wiki are somewhat out-of-date. Always check the last modification date. Email ghc-devs if you're not sure.

  • You may want to look at these "how it went for me" blog posts.

  • There is a blog post series by Stephen Diehl that provides an overview of many important data structures and contains links to other sources of information: Dive into GHC

Happy hacking!