Resources for newcomers to GHC
This page is intended to serve as the first stop for those people who say, "I want to contribute to GHC, but I don't know quite where to begin." Begin here. 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 page 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
Prepare your machine, clone the git repo, and Building/QuickStart GHC. For the short, short version, which may or may not work for your machine, you can try this:
# needed only once, URL rewrite rule is persisted in ${HOME}/.gitconfig git config --global url."git://github.com/ghc/packages-".insteadOf git://github.com/ghc/packages/ # clone GHC's main Git repository (creates './ghc' folder in CWD) git clone --recursive git://github.com/ghc/ghc cd ghc/ # configure build cp mk/build.mk.sample mk/build.mk ## edit mk/build.mk to remove the comment marker # on the line "BuildFlavour = devel2" ./boot ./configure # NOTE: On Windows you need to download some binary distributables before being able to build # This only has to be done once and can be done by adding a flag to the call to configure: ./configure --enable-tarballs-autodownload # build GHC make -j8 # parallelize to at most 8 parallel jobs; adapt to actual number of cpu cores
If your machine has all the prerequisites, this might just work. Expect it all to take roughly an hour.
- 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.
Fast rebuilding
There are 4 things to remember:
- Select BuildFlavour = devel2 in your mk/build.mk file (copy mk/build.mk.sample to mk/build.mk first), to make GHC build more quickly.
- Don't run make directly in the ghc root directory (unless you just pulled in changes from others). Instead, first change to the directory (compiler, utils, ghc or libraries) where you're making your changes. See Building a single sub-component.
- Set stage=2 in your mk/build.mk file, to freeze the stage 1 compiler. This makes sure that only the stage-2 compiler will be rebuilt after this.
- Use make fast to skip dependency building (except after pulling in changes from others).
A good first sanity check is to twiddle some error message in the code, just to see that changed error message pop up when you compile a file. Write some Haskell code with an error in it, and look at the error message. Search through the code for that error message. Change the message, rebuild ghc (run make fast in the ghc directory), and recompile your file again with ./inplace/bin/ghc-stage2. If you see the changed message, you're good to go.
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-devs mailing 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.
Bugs:
- #11068
- Make Generic/Generic1 methods inlinable
- #11777
- RTS source code issues
- #11789
- Flag suggestion does not always work
- #12056
- Too aggressive `-w` option
- #12379
- WARN pragma gives warning `warning: [-Wdeprecations]'
- #12488
- Explicit namespaces doesn't enforce namespaces
- #12502
- Reject wrong find utility
- #12576
- Large Address space is not supported on Windows
- #12636
- ProfHeap's printf modifiers are incorrect
- #12978
- User guide section on AllowAmbiguousTypes should mention TypeApplications
Feature requests:
- #7169
- Warning for incomplete record field label used as function
- #7401
- Can't derive instance for Eq when datatype has no constructor, while it is trivial do do so.
- #8501
- Improve error message when using rec/mdo keyword without RecursiveDo extention.
- #9793
- Some as-patterns could be accepted in pattern synonyms
- #10789
- Notify user when a kind mismatch holds up a type family reduction
- #11799
- Legend for hpc markup colors
Tasks:
- #4960
- Better inlining test in CoreUnfold
- #11024
- Fix strange parsing of BooleanFormula
- #11031
- Record Pattern Synonym Cleanup
- #11392
- Decide and document how semicolons are supposed to work in GHCi
- #11767
- Add @since annotations for base instances
- #11791
- Remove the `isInlinePragma prag` test
- #12687
- Add a flag to control constraint solving trace
- #12822
- Cleanup GHC verbosity flags
- #12891
- Automate symbols inclusion in RtsSymbols.c from Rts.h
- #12979
- -fspecialise-aggressively is not documented
Practical advice
- Read up on the steps you are expected to take for contributing a patch to GHC.
Less practical advice
- 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.
- Hacking on GHC (is not that hard) by Andrew Gibiansky
Need help?
You can email the ghc-devs list, or ask on IRC in #ghc.
Happy hacking!
