A Guide to GHC's Extensions

The latest official version of the Haskell standard was published in 2010. Since then, GHC, the most popular Haskell compiler, has grown almost 100 extensions which modify the language, sometimes quite drastically. Many of them have proven popular, to the point where it’s rare to find a library on Hackage that doesn’t use any extensions.

This is a problem for students of Haskell since most teaching materials only cover the base standard, leaving learners to fend for themselves in the extension jungle. This guide aims to make the journey a little easier. To that end, I sort GHC’s extensions into four tracks:

• Thecontains extensions that are generally simple and either commonly used or particularly useful for Haskell beginners. I also consider most of these extensions future-proof, in the sense that a future Haskell standard would probably include them without major changes.
• Thecontains extensions that are useful, but either more complex or less popular than the ones in the basic track. This includes a bunch of ‘special interest’ extensions that are only relevant if you want to do particular things with Haskell.
• Thecontains extensions that you probably shouldn’t use – either because they are actively unhelpful (badly designed or deprecated), or because they have failed to gain much adoption.
• Thecontains extensions that wouldn’t fit in anywhere else.

Each track is subdivided into topics that are mostly independent, so you can pick and choose according to your interests. Which extension belongs into which track is of course a matter of opinion, so take mine with a grain of salt.

For each extension, I provide the following information:

• The GHC version which first introduced the extension. Note that some extensions have been changed considerably in later GHC versions, so there may be surprises lurking if you use extensions with older GHCs. When in doubt, consult the User’s Guide for your GHC version.
• An estimate of how stable the extension is. GHC doesn’t provide formal stability guarantees for extensions, so this is just my outside view of the situation. Possible classifications are:
  • Stable : the extension is well-established and I don’t expect its design to change much anymore.
  • Mostly stable : The extension’s core design is stable, but details are somewhat likely to change.
  • Unstable : it is at least possible that the extension will be changed significantly in future GHC versions.
• A link to the relevant section of the GHC User’s Guide. This is your primary reference, but it often contains a bunch of technical details that may obscure the ‘essence’ of an extension.
• Links to external tutorials where available. You should take a look at these before diving into the User’s Guide.
• A brief explanation of what the extension is all about. This is intended as a primer which allows you to determine if the extension is something worth knowing about. It’s not a full tutorial, and certainly not a reference.

Pleaselet me know if I’ve made any mistakes. I don’t know many of these extensions too well myself, so there’s a good chance that some of the explanations are wrong. Also, if you have a suggestion for how to explain things more clearly (structure, choice of examples, etc.), I’d be grateful for that as well.

Of course, I’m not the first to cover the topic of GHC extensions. Some alternative takes:

• Stephen Diehl’s excellent, exhaustive and exhausting collection of knowledge, What I Wish I Knew When Learning Haskell , includes a list of extensions similar in spirit to this guide.
• Doug Beardsley provides a list of extensions sorted by how viable they are in a commercial setting.
• Alexis King’s Opinionated Guide to Haskell in 2018 contains a section with extensions that should be enabled by default.
• Alexander Altman has written a Guide to GHC Extensions for the School of Haskell, providing a mini-tutorial for each extension. For all extensions he covers, I provide a link in the relevant section of this guide.
• Oliver Charles (and guest authors) ran a blog post series on GHC extensions, called 24 Days of GHC Extensions , which I also frequently link to.

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