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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 working conventions page has 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.
- To orient yourself to the general architecture of GHC, this article, written by two of the chief architects of GHC, Simon Marlow and Simon Peyton-Jones, is excellent and current (2012).
- While you're reading that article, download and build the sources. Prepare your machine, download the source, and build. For the short, short version, which may or may not work for your machine, you can try this (note that three commands are not meant to be taken literally!):
# clone GHC's main Git repository (creates './ghc' folder in CWD) git clone git://git.haskell.org/ghc.git # see note below if this fails cd ghc/ ./sync-all --testsuite get # configure build cd mk cp build.mk.sample build.mk ## edit build.mk to remove the comment marker # on the line "BuildFlavour = quick" cd .. perl boot ./configure # build GHC make -j8 # parallelize to at most 8 parallel jobs; adapt to actual number of cpu cores
http://in the instructions above if you're behind a firewall blocking port 9418
If your machine has all the prerequisites, this might just work. Expect it all to take roughly an hour.
- The first line of the above sequence creates a
ghcfolder. This is the root of your GHC source tree. After a successful build, you should have your brand new compiler in
ghc/inplace/bin/ghc-stage2. (GHCi is launched with
ghc/inplace/bin/ghc-stage2 --interactive). Try it out.
- 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 ghc code for that error message (almost all the relevant code is in the
ghc). Change the message, and then rebuild (run
make 2in the
ghc-- that is,
ghc/ghc). If you see the changed message, you're good to go.
- If you've made it this far, you're well on your way to becoming a GHC developer. You should subscribe to the ghc-devs mailing list.
Fixing a bug
Below is a list of bugs 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. This list was last updated on 18 September 2013.
- #2507 - Unicode quotes in error messages
- #2258 - ghc --cleanup / #4114 - Add a flag to remove/delete intermediate files generated by GHC
- #2450 - Data.Complex.magnitude squares using ^ (2 :: Int), which is slow
- #7021 - Tuple (and other exotic predicates) not yet handled in Template Haskell
- #7401 - Can't derive instance for Eq when datatype has no constructor, while it is trivial do do so.
- #8100 - Standalone deriving using template haskell
- #8308 - Resurrect ticky code for counting constructor arity
Once you fix the bug, make sure to write a test-case proving that you've done what you said. Then, use git to prepare patches and post these to the bug report. If the patch looks good, one of the committers will put it into the GHC codebase. Then, tackle another bug!
- After a small change, you just want to build the stage-2 compiler (see here about stages). Go into the
make 2. It's much faster than just using
- If, for some reason, you're working on the stage-1 compiler, you can use
make 1, but you must be in the
compilersubdirectory, not the
- I (Richard E) use emacs to edit the code, and I have a hotkey dedicated to searching the ghc codebase, and another one dedicated to compiling ghc. This makes work on ghc much more interactive. See the Emacs page for more info.
Less practical advice
- Don't get scared. GHC is a big codebase, but it makes sense when you stare at it long enough!
- Be forewarned that many pages on the GHC wiki are somewhat out-of-date. Always check the last modification date. Email if you're not sure.
- This page describes in more detail workflow for fixing bugs. When you decide to submit your first patch it will certainly be worthwile to take a look.
- This page gives more deatail about our git working conventions. It will tell you how to set up your working trees in a bit more sophisticated way and how to prepare patches. This is information you'll need to know when you get more serious about GHC development, but as long as you're just hacking you may skip this page.
You can email the ghc-devs list, or Richard Eisenberg (
eir at cis . upenn . edu), a newish GHC developer himself who would be happy to foster more participation and answer your emails.