Getting Started With GHC Hacking

In my opinion, GHC may very well be the best documented open source project. And despite this, lots of folks still ask “How can I get started?” or “Has anyone written a guide on how to contribute to GHC?” and similar questions.

The goal of this article is two fold:

• Get the message out that there is documentation for getting started with hacking on GHC and contributing back to GHC HQ.
• Provide a template for boostrapping yourself into GHC hacking.

I assume that as a reader you are motivated, enthusiastic, and you know Haskell. Recall that we learn best by doing. You have to dive in. So before we go any further:

git clone https://github.com/ghc/ghc.git

While we wait for that to finish, let’s dive in :)

Drinking from the firehose

GHC has been the subject of many academic papers. These papers are good for understanding the intuition and motivation behind the source code you’ll find in the GHC repository. The vast majority of the papers about GHC are very approachable and well written, if you have the right background. As such, you will need to find a way to develop that background and “level up” before the papers will be useful to you. Read the section below, “Finding your way” for more information about this.

In addition to research publications (and perhaps, more widely approachable), the GHC source code is very well commented and the GHC trac has a wealth of information. Let’s try to enumerate all the places you can find out about GHC:

• Simon Peyton-Jones’ research publications
• GHC Trac
  • Tickets
  • Wiki
• The excellently commented source code itself
• #ghc on freenode
• GHC mailing lists
  • ghc-devs
  • ghc-users
• Books
  • “The Architecture of Open Source Applications”
  • “Implementing functional languages: a tutorial”

The commentary page of the GHC Trac is specifically design to answer the question, “How do I get started hacking on GHC?”. If there is only one link that you follow from this post, follow this one: http://ghc.haskell.org/trac/ghc/wiki/Commentary

Finding your way

Now for the second half of this post. How to “level up” so that you have sufficient skills to contribute to GHC in a meaningful way. Here is a list of things that will help you or anyone else become a GHC contributor. Remeber, you really can’t start too small:

• Get a local build working
• Write some code to add a trivial feature (change the ghci banner for example)
• Build your modified version and see it work
• Find an expert who is willing to help you (try IRC or the mailing lists)
• With the help of your expert friend, find an easy bug and fix it

If you get this far, you’ll have made it further than most haskellers! At this point, you’ll have the skills and knowledge necessary to fix bugs. Keep in mind that bug fix contributions are very valuable in the open source world where core contributors would rather spend their time on new features (or similar tasks where their deep expertise is required).

If your ambition is to develop true GHC wizardry (eg., understanding the research publications and the source code deeply) you’ll have a long road ahead of you. A road where continuing to fix bugs and other modest contributions will accelerate you down the path.

What you’ll need to get there is time to play with the ideas in the publications. For example, I wanted to understand how to make a new backend for ghc. So I started reading papers about the intermediate representation that GHC uses and also I started doing the exercises in the book, “Implementing functional languages: a tutorial”.

Read everything you can and implement the ideas (even in a “toy” way) and play with it. Doing this gives you much deep understanding that just reading. Then when you go back to the GHC sources much more of it will make intuitive sense to you because you now understand the principles it is derived from.

Parting words

Fire up your editor and get to work :)