[9rx]

> By that definition a void* pointer in C is a sum type.

No. That doesn't make any sense. void* is essentially equivalent to any in Go, which isn't sum types either.

You can construct sum types in C by combining structs, enums, and unions, but it is not an out of the box feature like in Go. Sum types are a first-class citizen in Go.

> Ergonomics matter.

Math doesn't care about ergonomics. You might care about ergonomics, but logically when talking about those ergonomics you'd call those ergonomics by name, not by some unrelated thing from type theory.

[josephg]

> You can construct sum types in C by combining structs and unions, but it is not an out of the box feature like in Go. Sum types are a first-class citizen in Go.

Maybe I'm misunderstanding what you mean. Can you give me an example? How would you write a Result type in Go? (Result is defined as either Ok(val) or Err(err))

[9rx]

C:

    enum result_type {
        OK,
        ERROR
    };

    union result_value {    
        int value;
        char *error_message;
    };

    struct result {
        enum result_type tag;
        union result_value value;
    };

This is not technically closed, but does offer a close enough approximation. Again, not a first-class feature, so there is no expectation if it being true sum types.

Go:

    type Result interface {
        isResult()
    }

    type OK[T any] struct {
        Value T
    }
    func (OK[T]) isResult() {}

    type Error struct {
        Message string
    }
    func (Error) isResult() {}

This one is closed. It perfectly satisfies being sum types. It may not satisfy your opinion of what makes for good ergonomics, but if you want to talk about ergonomics let's use ergonomic words, not type theory words.