New to Zig, dont understand this error message | Zig Programming Language | Page 1

nova trout Jan 9, 2024, 5:04 AM

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Hello, I just started Zig like 15 minutes ago, and im currently learning about tagged enums. I am familiar with sum types because i've written rust code before, but i dont understnad the error im getting. this is my code:

test "tagged unions" {
    const RustOptionInt = union(enum) {
        none,
        some: i32,
    };

    var maybe_int = RustOptionInt.none;
    switch (maybe_int) {
        .none => {},
        .some => |*value| value.* += 1,
    }
    try expect(maybe_int == RustOptionInt.none);
}

and this is the error i get:

main.zig:75:35: error: incompatible types: '@typeInfo(main.test.tagged unions.RustOptionInt).Union.tag_type.?' and 'comptime_int'
        .some => |*value| value.* += 1,
                          ~~~~~~~~^~~~
main.zig:75:32: note: type '@typeInfo(main.test.tagged unions.RustOptionInt).Union.tag_type.?' here
        .some => |*value| value.* += 1,
                          ~~~~~^~
main.zig:75:38: note: type 'comptime_int' here
        .some => |*value| value.* += 1,
                                     ^

can someone please explain to me what is going on?

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also,can you give tagged unions methods like you can with enums?

mossy fossil Jan 9, 2024, 5:34 AM

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wow this is the most confused i've been in a while staring at a zig error message.

nova trout Jan 9, 2024, 5:34 AM

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wow. and its my first one too lol

mossy fossil Jan 9, 2024, 5:34 AM

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it turns out that the way you inititalized maybe_int gives it the tag type of none, not the expected integer type.

lament palm Jan 9, 2024, 5:34 AM

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You use var maybe_int: RustOptionInt = .none; because it thinks that maybe_int is something like @typeInfo(main.test.tagged unions.RustOptionInt).Union.tag_type.? (Was also confused why it said that). v like that down there.

mossy fossil Jan 9, 2024, 5:35 AM

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in order to init properly:

var maybe_int: RustOptionInt = .none;

nova trout Jan 9, 2024, 5:35 AM

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this fixed it, but why does this example work?

test "switch on tagged union" {
    var value = Tagged{ .b = 1.5 };
    switch (value) {
        .a => |*byte| byte.* += 1,
        .b => |*float| float.* *= 2,
        .c => |*b| b.* = !b.*,
    }
    try expect(value.b == 3);
}

value is not given an explicit type

mossy fossil Jan 9, 2024, 5:37 AM

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that is another fine way to init value. it doesn't need an explicit type, a comptime_int is fine as long as it coerces to the payload type.

nova trout Jan 9, 2024, 5:38 AM

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ok i see. so just to clarify, the zig compiler inferred my variable as a tag type

mossy fossil Jan 9, 2024, 5:38 AM

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that way of initializing is similar to this, which actually works for void payload types:

var maybe_int = RustOptionInt{ .none = {} };

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yeah, it was telling you that the capture was actually an enum type (the tag type).

nova trout Jan 9, 2024, 5:39 AM

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ah ok. so type inference works best when a payload can be coerced. in this case {} coerced to a void type

mossy fossil Jan 9, 2024, 5:39 AM

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it almost seems like its worthy of opening an issue. that was very confusing even for someone like me who has seen quite a bit of zig code

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yeah the void payload init syntax can be a little confusing i guess

nova trout Jan 9, 2024, 5:41 AM

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another clarification: .{} is for tuples, and {} is for void?

mossy fossil Jan 9, 2024, 5:41 AM

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but in general, coercion works great for unions. you can always just use bare literals w/ out needing to use @as

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yes that is correct. .{} used to work void void literals too, but that was changed

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and now only {} works

nova trout Jan 9, 2024, 5:42 AM

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ok i understand. thank you for all your help!

mossy fossil Jan 9, 2024, 5:42 AM

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np zeroLike

nova trout Jan 9, 2024, 5:43 AM

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nova trout also,can you give tagged unions methods like you can with enums?

one last thing!

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this question^^^

mossy fossil Jan 9, 2024, 5:43 AM

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oh yeah i forgot. yes you can.

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any container type can have methods and other declarations.

nova trout Jan 9, 2024, 5:44 AM

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hell yeah

mossy fossil Jan 9, 2024, 5:44 AM

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like container level const/vars

nova trout Jan 9, 2024, 5:44 AM

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nice. thank you

mossy fossil Jan 9, 2024, 5:45 AM

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i think you should consider opening an issue for that. it was very confusing.

nova trout Jan 9, 2024, 5:45 AM

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i will consider that

atomic hinge Jan 9, 2024, 6:10 AM

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Huh, this one took me a second too - I thought if you referenced a void field it would give a union value by default which could decay to the tag where appropriate. I'm not sure whether this behaviour is intentional or not

mossy fossil Jan 9, 2024, 6:24 AM

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the thing that confused me is thinking the type of maybe_int was RustOptionInt rather than the tag type

night crane Jan 9, 2024, 1:33 PM

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nova trout another clarification: `.{}` is for tuples, and `{}` is for `void`?

{} is just an empty block of code, and a block of code that doesn't yield anything yields a value of type void.
You could do void{} too, IIRC.

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.{} is a structure literal, which can coerce into a fixed array, struct, or union.

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&.{} can coerce into a slice, or pointer to struct.

night crane Jan 9, 2024, 1:35 PM

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night crane `&.{}` can coerce into a slice, or pointer to struct.

Never tried that as a pointer to a union, but maybe that too.

#New to Zig, dont understand this error message