The Next Step for Generics

Introduction

It’s been almost a year since we last wrote about the possibility of adding generics to Go . It’s time for an update.

Updated design

We’ve been continuing to refine the generics design draft . We’ve written a type checker for it: a program that can parse Go code that uses generics as described in the design draft and report any type errors. We’ve written example code. And we’ve collected feedback from many, many people—thanks for providing it!

Based on what we’ve learned, we’re releasing an updated design draft . The biggest change is that we are dropping the idea of contracts. The difference between contracts and interface types was confusing, so we’re eliminating that difference. Type parameters are now constrained by interface types. Interface types are now permitted to include type lists, though only when used as constraints; in the previous design draft type lists were a feature of contracts. More complex cases will use a parameterized interface type.

We hope that people will find this design draft simpler and easier to understand.

Experimentation tool

To help decide how to further refine the design draft, we are releasing a translation tool. This is a tool that permits people to type check and run code written using the version of generics described in the design draft. It works by translating generic code into ordinary Go code. This translation process imposes some limitations, but we hope that it will be good enough for people to get a feel for what generic Go code might look like. The real implementation of generics, if they are accepted into the language, will work differently. (We have only just begun to sketch out what a direct compiler implementation would look like.)

The tool is available on a variant of the Go playground at https://go2goplay.golang.org . This playground works just like the usual Go playground, but it supports generic code.

You can also build and use the tool yourself. It is available in a branch of the master Go repo. Follow the instructions on installing Go from source . Where those instructions direct you to check out the latest release tag, instead run git checkout dev.go2go . Then build the Go toolchain as directed.

The translation tool is documented in README.go2go .

Next steps

We hope that the tool will give the Go community a chance to experiment with generics. There are two main things that we hope to learn.

First, does generic code make sense? Does it feel like Go? What surprises do people encounter? Are the error messages useful?

Second, we know that many people have said that Go needs generics, but we don’t necessarily know exactly what that means. Does this draft design address the problem in a useful way? If there is a problem that makes you think “I could solve this if Go had generics,” can you solve the problem when using this tool?

We will use the feedback we gather from the Go community to decide how to move forward. If the draft design is well received and doesn’t need significant changes, the next step would be a formal language change proposal . To set expectations, if everybody is completely happy with the design draft and it does not require any further adjustments, the earliest that generics could be added to Go would be the Go 1.17 release, scheduled for August 2021. In reality, of course, there may be unforeseen problems, so this is an optimistic timeline; we can’t make any definite prediction.

Feedback

The best way to provide feedback for the language changes will be on the mailing list golang-nuts@googlegroups.com . Mailing lists are imperfect, but they seem like our best option for initial discussion. When writing about the design draft, please put [generics] at the start of the Subject line and to start different threads for different specific topics.

If you find bugs in the generics type checker or the translation tool, they should be filed in the standard Go issue tracker at https://golang.org/issue. Please start the issue title with cmd/go2go: . Note that the issue tracker is not the best place to discuss changes to the language, because it does not provide threading and it is not well suited to lengthy conversations.

We look forward to your feedback.

Acknowledgements

We’re not finished, but we’ve come a long way. We would not be here without a lot of help.

We’d like to thank Philip Wadler and his collaborators for thinking formally about generics in Go and helping us clarify the theoretical aspects of the design. Their paper Featherweight Go analyzes generics in a restricted version of Go, and they have developed a prototype on GitHub .

We would also like to thank the people who provided detailed feedback on an earlier version of the design draft.

And last but definitely not least, we’d like to thank many people on the Go team, many contributors to the Go issue tracker, and everybody else who shared ideas and feedback on earlier design drafts. We read all of it, and we’re grateful. We wouldn’t be here without you.