Linear types are merged in GHC

19 June 2020 — by Arnaud Spiwack

It’s been almost 4 years. And what 4 years they were! We learned so much. I am exhausted, but mostly, I’m happy, I’m thankful, and I’m hopeful.

How we got there


The journey towards linear types in GHC started in the fall of 2016. At this point in time, Jean-Philippe Bernardy and I had been working on distributed storage R&D for a year or so. Prior to that we had both fairly freshly published research into linear logic. And, we started seeing opportunities for linear types pretty much everywhere: for memory management, to generalise protocol types, to manage buffers… It had become a recurring joke in this project: “You know what would help with this? Linear types!”.

I guess that after repeating a joke enough, you start wondering whether maybe it shouldn’t be a joke. Maybe linear types would really be helpful. Why not after all? And so we decided to shave that yak.

I’ll be honest though: I didn’t think it was possible. I really thought that to add linear types to a language, you would need to redesign it from the ground up. That is, really create a new language. But, fortunately, Jean-Philippe knew better. He came up with a convincing design to add linear types to GHC. I was on board.

In the fall of 2016, together with my colleague Mathieu Boespflug, we brainstormed for a couple of months, refined our design. In the winter, we were joined by Simon Peyton Jones and Ryan Newton, in the effort of turning our idea, still quite rough, into a design solid enough to warrant an academic publication. By the fall of 2017, our article was accepted for publication at POPL 2018 and accompanied by a prototype implementation.


The prototype only modified the front end of GHC and didn’t touch Core yet. Yet we felt confident enough to write a proposal for extending GHC with linear types in November 2017. Over the course of the three following weeks, this proposal attracted 200 comments. We eventually closed the proposal PR for heavy revision. Because 200 comments sounded so massive at the time, we elected to reopen the proposal as a separate PR, with the memorable number 111, in February 2018. This second iteration of the proposal was accepted in October 2018. Altogether, the proposal has gathered almost 600 comments, comparable only to the record dot-syntax proposal. What a ride!

While the proposal was being hotly debated, we carried on with the implementation. An implementation doesn’t lie: we started it to find out what we might have missed in our design. And, indeed, we found some holes in the design. Matthew Pickering joined us from April to July 2018 as part of an internship. He undertook the frankly thankless task of making the first implementation of linear types inside Core. Just making GHC compile again took about 2 months. In the process, Matthew found bugs in our backward compatibility story. And so we updated our design, and the proposal.

Merge request

In October 2018, my colleague Krzysztof Gogolewski joined the project. And, to give an idea of how unglamorous working on linear types could be at times, proceeded to merge the current state of linear types with GHC’s master. A task which took upwards of two weeks of full-time work. Over the course of the next year, Krzysztof relentlessly squashed the remaining bugs and infelicities to the proposal.

Why a year? Well, it turns out that one innocent looking design choice: 𝜂-expanding data constructors, ended up being a massive undertaking. We have had to fight bona fide bugs in GHC, or simply implicit, unconscious, assumptions in the code which required quite a bit of work to unravel.

It was the summer of 2019, and we were ready, or so we thought. We arranged for a focused review session in the Fall, in Cambridge, with Simon Peyton Jones and Richard Eisenberg. And glitches were found. In the course of this implementation, a lot of research had happened: it turns out that teaching linear types to Core is not as simple as just implementing linear logic, or even the calculus from our article, but this is a story for another time. We had to do more research on the spot.

We addressed the glitches by January 2020. The rest of the year has been focused on hunting performance regressions.

Let me be sure not to forget Richard Eisenberg’s role in all this. Richard shepherded the proposal, reviewed the entire 3000 line patch (several times!), shepherded the merge, and helped out crucially with performance. No mean task to be sure!

Special thanks go to Andreas Klebinger, who benchmarked the linear types branch several times to help pinpoint the performance regression, and to Ben Gamari, who ran and analysed an 8h benchmark on the linear types patch.

And here we are. 4 years, nearly 30 bugs found in GHC (most we fixed ourselves)1, and over 200 internal pull requests later. This is very much a collective work, and I’m hugely thankful to everyone involved.

In GHC 8.12

Linear types

There will be linear types in GHC 8.12. But don’t expect a finished product. This is our very first iteration, an MVP as it were. This is as minimal a set of features we think can be useful for anybody. But it’s still very much aimed at early adopters and eager tinkerers.

Turn on -XLinearTypes, and the first thing you will notice, probably, is that the error messages are typically unhelpful: you will get typing errors saying that you promised to use a variable linearly, but didn’t. How hasn’t it been used linearly? Well, it’s for you to puzzle out. And while what went wrong is sometimes egregiously obvious, it can often be tricky to figure the mistake out.

Plenty of things are missing from the proposal too. In no particular order:

  • There is no infix syntax for multiplicity polymorphism. You can use the prefix form FUN p a b for an arrow with multiplicity p. However:

  • Multiplicity polymorphism is mostly unsupported, and you can expect it to misbehave most of the time.

  • Record fields are always linear.

  • let and where bindings are never linear.

  • The scrutinee of a case expression is always considered as being used non-linearly.

    A trick, found by my colleague Divesh Otwani, which you can use to work around this is to replace

    case x of {}


    x & \case {}

    For a linear version of (&)

  • Inference of linear types is very limited.

There are other missing bits; they are all documented in the manual.

And, of course, there will be bugs. Probably many. We will be happy to receive your bug reports on the GHC bug tracker.

Linear base

When GHC 8.12 is released, we will release the first version of linear-base, a toolkit to get you started with linear types. It contains linearised versions of many functions from the Haskell base library, two kinds of functors, mutable data structures with pure API, a monad for safe management of resources, linear optics, an API for allocation-free array pipelines, …

The linear-base library is currently being developed by my colleague Divesh Otwani, and previously by Bhavik Mehta, during his summer internship at Tweag.

What’s to come

Evidently, we still have a lot of work ahead of us to make linear types in GHC a smooth experience. Completing the design of the proposal, and, probably, going beyond the proposal.

But what’s next, above all, is you. Until now, linear typing has been mostly a subject for experts, who did propose many applications. But what I, personally, find most exciting about this enterprise is that adding linear types to a mainstream compiler makes them available to many more people. And as clever as the experts are, they are no match against many times more programmers armed with a type checker. Not even close.

So by all means, play, tinker, experiment, come up with ideas, build libraries. This will all be extremely fascinating. We provide you with linear-base to get you started, but maybe it’s not the right thing for you, and then you can build your own. There is a huge design space to explore. It’s all ahead of us. And I, for one, can’t wait to see what the Haskell community comes up with.

See you all in 8.12!

About the author

Arnaud Spiwack

Arnaud is Tweag's head of R&D. He described himself as a multi-classed Software Engineer/Constructive Mathematician. He can regularly be seen in the Paris office, but he doesn't live in Paris as he much prefers the calm and fresh air of his suburban town.

If you enjoyed this article, you might be interested in joining the Tweag team.

This article is licensed under a Creative Commons Attribution 4.0 International license.


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