Why can't I provide default values for tagged union?

Help
1

Recently, I wanted to specify default values for a tagged union but got some strange error messages:

const Foo = struct {
    bar: union(enum) {
        f1: ?bool = null,
        f2: ?bool = null,
    },
};

Throws:

howtos/q_taggedUnionDefVal.zig:4:10: error: explicitly valued tagged union missing integer tag type
    bar: union(enum) {
         ^~~~~
howtos/q_taggedUnionDefVal.zig:5:21: note: tag value specified here
        f1: ?bool = null,
                    ^~~~

Changing union(enum) to enum(u1) or to a separately defined enum throws something contradictory:

error: explicitly valued tagged union requires inferred enum tag type

I found an open issue, Incorrect error when assigning default value to tagged union in struct · Issue #14561 · ziglang/zig · GitHub, that reports the error message is indeed confusing. I thought this is a compiler bug, but after I looked closely, the issue says that it is just the error message that is incorrect, not the behavior of the compiler itself, and it should have reported something like “error: unions do not support default values” instead. Are default values for tagged unions not going to be supported? If so, why?

2

How would you expect default union fields to work? For structs it’s simple, you just do .{} and all fields get initialized, this makes intuitive sense to write and is easy for the compiler to parse.
But for unions we need a tag field Union{.tag = value}. Should a new syntax be introduced for a feature like this that almost no one will actually use?

4 Likes
3

Ok, I think I asked a bit silly question :blush:. I can’t because there is no way to decide which field is going to be initialized. So what I needed is just to move the defaults in the right place:

const Foo = struct {
    bar: union(u1) {
        f1: ?bool,
        f2: ?bool,
    } = .{ .f1 = true },
};
8 Likes