What does "it is no longer possible to reify error sets" mean?

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1

The Zig 0.16.0 release notes say:

There is no @ErrorSet builtin. To simplify the language, it is no longer possible to reify error sets. Instead, declare your error sets explicitly using error{ … } syntax.

That makes me think I can’t return an error from a function without declaring it in an error set first, e.g. putting return BogusError; in a function without declaring BogusError somewhere should fail. However, that doesn’t appear to be the case…

const std = @import("std");

pub fn main() !void {
	return error.MadeUpStuff;
}

quin@Minix~$ zig build-exe test.zig                                                                                     

quin@Minix~$ ./test                                                                                                     

error: MadeUpStuff                                                                                                      

/home/quin/test.zig:4:2: 0x11d39fc in main (test.zig)                                                                   

return error.MadeUpStuff;                                                                                       

^

Same behavior with 0.16.0 and 0.17.0-dev.607+456b2ec07. Am I missing something here?

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2

There used to be a built-in, @Type, that let you create types at comptime including errorsets from comptime known string arrays. However, it was replaced with @struct, @union, @enum, etc builtins. But there is no @errorset. So you can’t reify errorsets like that anymore.

You can still

const MyErrors = error{
    MissingThing,
    BadStuff,
};

There is still a hack that let’s you build errorsets from strings, but its not intended.

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3

Oh, so reify in this context doesn’t mean the implicit error behavior? It said to simplify the language, so I assumed it was actually a language change, not just removing a builtin. If you just can’t build errors from strings anymore though that makes sense.

4

builtins are part of the language, they’re not defined in the stdlib and they’re not functions (even though they do look like functions). as an example @ptrCast can observe the type of the variable it’s being assigned to, something that you cannot do in a zig function.

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5

Touche, makes sense.

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