Zig Error Patterns

In my opinion Zig is elegant except for one thing: cannot attach data to error.

https://github.com/ziglang/zig/issues/2647

You have to workaround by things like passing into a pointer to error object.

The idiomatic way in Zig is to return the simple unadorned error, but return detailed error data through a pointer argument passed into the function, allowing the function to fill in extra information before returning an error.

    const MyError = error{ FileNotFound, PermissionDenied };

    fn readFile(path: []const u8, outErrInfo: *ErrorInfo) ![]const u8 {
      if (fileMissing) {
        if (outErrInfo) |info| {
            info.* = ErrorInfo{
                .code = MyError.FileNotFound,
                .message = "File missing",
                .line = @line(),
            };
        }
        return MyError.FileNotFound;
      }
      return data; // success
    }

The advantage of this is that everything is explicit, and it is up to the caller to arrange memory usage for error data; ie. the compiler does not trigger any implicit memory allocation to accommodate error returns. This is a fundamental element of Zig's design, that there are no hidden or implicit memory allocations

> Are libraries typically designed to accept an optional input to store potential errors?

Yes. Stdlib's JSON module has a separate diagnostics object [1]. IMO, this is the weakest part of Zig's error handling story, although the reasons for this are understandable.

[1] https://ziglang.org/documentation/master/std/#std.json.Scann...

I wrote an article about one possible pattern which is a concrete realization of your question -- though with more ceremony and complexity since the pathway is fully compiled out if you don't use it (vs a nullable pointer strategy):

> Are libraries typically designed to accept an optional input to store potential errors?

https://zig.news/ityonemo/sneaky-error-payloads-1aka

if you prefer video form:

https://www.youtube.com/watch?v=aFeqWWJP4LE

The answer is no, libraries are not typically designed with a standardized convention for payload return.

Thank you all for these great and detailed explanations, I've learned a lot! I like the approach with an optional pointer, it fits to zig's philosophy quite well. Although there's a bit of a disconnect between the unadorned error and the corresponding data struct. I could imagine it requires care when the data struct is a union, as one needs to know which error corresponds to which variant.

At a practical level, most of the language doesn't care about the distinction between errors and other types. You mostly just have to consider `try/catch/errdefer`. Your question then, mildly restated, is "how do people deal with cases where they want to use `try/catch/errdefer` but also want to return a payload?"

It's worth asking, at least a little, how often you want that in the first place.

Contrasting with Rust as an example, suppose you want Zig's "try" functionality with arbitrary payloads. Both functions need a compatible error type (a notable source of minor refactors bubbling into whole-project changes), or else you can accept a little more boilerplate and box everything with a library like `anyhow`. That's _fine_, but does it help you solve real problems? Opinions vary, but I think it mostly makes your life harder. You have stack unwinding available if you really need to see the source of a thing, and since the whole point of `try` is to bubble things up to callers who don't have the appropriate context to handle them, they likely don't really care about the metadata you're tacking on.

Suppose you want Zig's "catch" functionality with arbitrary payloads. That's just a `union` type. If you actually expect callers to inspect and care about the details of each possible return branch, you should provide a return type allowing them to do stuff with that information.

The odd duck out is `errdefer`. IMO it's reasonably common for libraries to want to do some sort of cleanup on "error" conditions, where that cleanup often doesn't depend on which error you hit, and you lose that functionality if you just return a union type. My usual workaround (in the few cases where I actually want that information returned and also have to do some sort of cleanup) is to have a private inner function and a public outer function. The inner function has some sort of `out` parameter where it sticks that unioned metadata. The outer function executes the code which might have to be cleaned up on errors, calls the inner function, and figures out what to do from there. Result location semantics make it as efficient as hand-rolled code for release builds. Not everything fits into that paradigm, but the exceptions are rare enough that the extra boilerplate really isn't bad on average (especially when comparing to an already very verbose language).

Depending on the API, your proposal of having a dedicated `out` parameter exposed further up the chain to callers might be appropriate. I'm sure somebody has done so.

Something I also do in a fair amount of my code is let the caller specify my return type, and I'll avoid work if they don't request a certain payload (e.g., not adding parse failure line numbers if not requested). It lets you write a reasonably generic API without a ton of code complexity, still allowing callers to get the information they want.

I think the idea is errors are for control flow. If you have other information to return from a function, you can just return it — whether directly as the return value or through an “out” parameter or setting it in some context.

If I understand the feature correctly, D has this via scope(failure) [1]

``` void func() { scope(failure) writeln("Something went wrong!"); } ```

[1] https://tour.dlang.org/tour/en/gems/scope-guards

Is it significantly different than a try-catch block?

It felt like a man page to me :)

The language reference has a section about naming identifiers https://ziglang.org/documentation/0.14.1/#Names