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In Zig, error codes are fundamentally a control flow construct, not a reporting mechanism.

The pattern here is to return something like error.Diagnostics for all errors that have been reported via diagnostics.

The only reason you'd have a different error code would be if the control flow should be different. For example error.OutOfMemory makes sense to be separate because it's a retryable error, which means it should be handled by different control flow.

That is indeed more or less what you commonly do when you need something more than just error codes.

As an example: https://github.com/kristoff-it/ziggy/blob/852053b09a5f8f5b79...

The part about mapping every error kind to different error code in Zig is debatable. It might be useful in some cases maybe (I don't have the confidence to fully exclude it), but at the very least in my experience I never ever needed that.

In general if you have the (IMO sensible) approach of taking as few dependencies as possible and not treating them like a black box, then for any error you can simply look at the call stack and figure out the problem from reading the code during development.

Outside of that, error codes are useful for debugging code that is running on other people's machines (i.e. in production) for and for reporting reasons.

Does the Zig language not have useful exceptions/stacktraces that can be propagated out?

At a high level, all non-trivial programming is composition. And (see principle of encapsulation) the vast majority of errors shouldn't be recovered and just need to be propagated out.

Then, to be useful, errors need enough information to be diagnosed or investigated. It seems like this should have been a straight-forward requirement in the language design.

Zig has error unions on return types which are basically a special cased (rust enum aka sum type) of the return type with a special cased (c enum aka enumerated type), and stacktraces are only available in certain build modes.

A "diagnostics" pattern has emerged in the community to optionally request extra information about a failure. You can pass a pointer to the diagnostic (it can be on the stack) and get the extra info back. It's just a more explicit version of what would otherwise happen in a language with error payloads.

> stacktraces are only available in certain build modes.

Minor correction: stack traces are available on all build modes, but different build modes have different defaults. See: std.Options.allow_stack_tracing

Note that this "diagnostics" pattern is only meant for handling a error locally with potential extra information, or showing a more useful error to a end user of the software. For software bugs, crashes, or developer facing errors, you often don't have to do anything as zig has pretty good error traces by default.

> stacktraces are only available in certain build modes

> zig has pretty good error traces by default

These seem rather conditional. If I need to run release-fast in prod, say, do we loose these error traces (for bugs)?

You can enable error traces for release-fast builds as well, without enabling full debug info. Though the quality of call stack of course vary depending on optimization level.

You do, to a significant extent. Though there is always the option of running ReleaseSafe.

> Does the Zig language not have useful exceptions/stacktraces that can be propagated out?

Yes, it has error return traces.

It wasn't clear from the examples, and the gist doesn't have a `deinit` method, so what happens if an error needs to own data?

> Here, sqlite.ErrorPayload.init saves 500 bytes of error message from sqlite

Who owns those 500 bytes and where are they being freed?

It's just stored as a [256]u8 in the struct.

  // sqlite.zig
  pub const ErrorPayload = struct {
      message: [256]u8,
  
      pub fn init(db: *c.sqlite3) @This() {
          var self = std.mem.zeroes(@This());
          var fw = std.Io.Writer.fixed(self.message[0..]);
          _ = fw.writeAll(std.mem.span(c.sqlite3_errmsg(db))) catch |err| switch (err) {
              error.WriteFailed => return self, // full
          };
          return self;
      }
  };

And what pattern would you recommend if you needed to allocate?

It's worth every effort to avoid situations where a function creates extra clean up responsibilities to the caller only on error conditions.

If I really needed a large error payload, too big to fit on the stack, I'd probably want to do something like this:

  const errbuf = try alloc.alloc(u8, 1024*1024*1024);
  module.setErrorBuf(&errbuf)
  defer {
    module.setErrorBuf(null);
    alloc.free(errbuf);
  }

  var func_diag = diagnostics.OfFunction(module.func){};
  module.func(foo, bar, &func_diag) catch |err| switch (err) {
    error.BigPayload => {
      const payload = func_diag.get(error.BigPayload);

      // The payload can reference the 1MiB of data safely here,
      // and it's cleaned up automatically.
    }
  }

The diagnostic struct could contain a caller-provided allocator field which the callee can use, and a deinit() function on the diagnostic struct which frees everything.

Continues to be a point of annoyance that Zig doesn't properly support payloads in errors.

What more do you want than what's covered in the post?

To me, this post is proof zig doesn't need "proper support". You can already have extremely ergonomic error payloads with existing language features.

Earlier version of this post had some language feature ideas, but then I realized Zig already had all the capabilities, so I just removed that section.

For example, I used to think it'd be nice for functions to be a namespace, so I didn't have `myFunc` and `MyFuncDiagnostics`. But then I realized that the Diagnostics type doesn't need its own name; you can just put the type in the function signature, and use a function like `diagnostics.OfFunction(fn)` to extract the type from the function definition, so you can just write this:

  var diag: diagnostics.OfFunction(myFunc) = .{};
  const res = myFunc(foo, bar, &diag) catch |err| ...;

Do you want automatic inference of the error payload type? So zig creates a special tagged union error payloads type for you automatically? It seems complicated and maybe not a good idea. Do you really want your function to return an invisible 20 elements union on error? Do you want to call a someone else's function which returns an invisible 20 elements union on error? You know, maybe it's not a good idea.

More than anything I want "Here's the current officially sanctioned best practice of how to report errors with payloads". For a language that's highly opinionated about everything it's strangely unopinionated here and worse off for it because many libraries just swallow useful diagnostic information, some of my own projects included.

There's a barrier to setting up the diagnostic pattern. When you're in the greenfield phase it's easy to search for information about error reporting, discover various different approaches and long threads of people arguing about what better and just say "ah screw it, I have more important things to do right now" and postpone the decision.

Your approach is fine, I don't love how verbose it is but it could probably be tweaked. If this is the way forward then it should be included in stdlib, documented, and promoted as the recommended way of reporting errors.

I agree. You understand why I wrote this post. It's what I wanted to read 3 weeks ago. We're told "Use Diagnostics like the json stdlib module!" but then you realize the json module is way too simplistic for a complicated application.

But also, I'm sure this method has flaws and can be greatly improved, so hopefully we can come to the right solution.

> You can already have extremely ergonomic error payloads with existing language features.

I think you meant extremely unergonomic. If you take a dev poll, 8/10 developers would not find the solution ergonomic at all.

Are you sure? Just because you feel that way doesn't mean you can just make up a statistic that supports your viewpoint.

In my experience, ergonomics in Zig means something other than ergonomics in many other languages. In plenty of languages, ergonomic code is basically just writing as few characters as possible and being able to achieve complex logic with little boilerplate. In Zig, it feels good to be able to compose the conceptually simple language features into an optimal solution to your problem, even if it results in code that is maybe not aesthetically pleasing on first glance.

I'm not going to declare that this solution is ergonomic or not, you can't always tell just by looking at Zig code whether using it is ergonomic, but by that same logic we shouldn't dismiss it off hand.

> In my experience, ergonomics in Zig means something other than ergonomics in many other languages.

So an opinion...

Obviously

You can't really easily have useful user defined payloads in errors without implicit allocations.

Best way to do it is to pass a payload pointer into the function call. You can put it into an options struct with a default null pointer that noops if you prefer the ergonomics of a kwarg.

I thought so too at first, coming from a language (Hare) where they are very easy and common, but the Diagnostics pattern isn't that bad once you expect it. Various examples: https://ziggit.dev/search?q=Diagnostics

Right. It's one thing to build the equivalent of Result into the language -- great. It's another to make it only support simple enum variants and not be extensible.

the 'error payload' pattern feels like a good compromise between (a) enums and (b) full exceptions, esp when you want to preserve structured context. in zig specifically, do you find yourself standardizing payload shapes per module, or does it devolve into ad-hoc structs? would be interesting to see guidance on when to box/allocate vs keep payload trivially copyable.

Why need error payloads when:

You can do validation at user interface and report actual nice errors.

And if something happens after that, you can save the stack trace into somewhere so the developer can see it. And you report unexpected error to user as that is exactly that

What zig really needs is a stack trace that works properly on release mode. And a way to collect the stack trace into a string at runtime so it can be saved as a log or into a db

Alternative title; instead of learning a new language, I adapted it into something I'm more familiar with.

Depending on your language preference; Zig has issues[citation needed], but 1) it's still version 0 and 2) and this is the important part: who cares?

I get you have a pattern and shape you like a lot. But there's less value in that existing shape, than there is in being mildly uncomfortable, and expanding your world view and trying things in ways you wouldn't normally do.

If you tried it and didn't like it, cool, go back to the language you're already used to. But for everyone else. I'd encourage you to try doing things "wrong" for a while and seeing if you can't learn something from forcing yourself to do so.

Something especially true for a language that HN likes to pretend is just a toy and can't compete with [ language you already decided won ]

This is just a "complex real world app code" extension of the stdlib Diagnostics pattern.

  % rg Diagnostics zig/lib/std | wc -l
  165

Note the person behind your link isn't Andrew Kelley

Oh thanks a bunch. That’s confusing. Removed that.

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