proposal: sum types as a non-interface union type

This is an issue to discuss yet another form of sum type to Go. This is not a full proposal, and not a language change that I expect will be adopted in the near future. I am filing this issue as a point of reference to discuss this particular approach. A sum type, tagged union, variant, variant record, choice type, discriminated union, disjoint union, or coproduct is a type which can hold a value of one of a fixed set of types. (Thanks [Wikipedia](https://en.wikipedia.org/wiki/Tagged_union) for the list.) Rust calls these enums, which means that when someone says Go should support "enums" they might be asking for a C-style enumeration of integers or a Rust-style enumeration of types. I posit that most people who want this feature in Go are looking for something along the lines of a Rust enum: https://doc.rust-lang.org/book/ch06-01-defining-an-enum.html To guide what follows, I'm going to also propose a few motivating examples which any design should support well. 1. A Direction type holding one of four cardinal directions: North, South, East, and West. 2. A Maybe type which either holds a value of some type or nothing. 3. An IP type holding either a `[4]byte` IPv4 address or a `[16]byte` IPv6 address. ### A digression: Can we just use interfaces? Many proposals for sum types in Go aim to extend interface types to cover this case. Since interfaces already support listing a set of type variants for use in type constraints, it's tempting to say that we could just permit so

Posted by neild on Dec 18, 2025, 10:03 PM

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