4 ms·

My biggest gripe with slog is that there is no clear guidance on supported types of attributes. One could argue that supported types are the ones provided by A

• 0x696C6961 1y ago
  All values are supported.
  • aleksi 1y ago
    Well, is fmt.Stringer supported? The result might surprise you: req := expvar.NewInt("requests") req.Add(1) attr := slog.Any("requests", req) slog.New(slog.NewTextHandler(os.Stderr, nil)).Info("text", attr) slog.New(slog.NewJSONHandler(os.Stderr, nil)).Info("json", attr) This code produces time=2025-09-12T13:15:42.125+02:00 level=INFO msg=text requests=1 {"time":"2025-09-12T13:15:42.125555+02:00","level":"INFO","msg":"json","requests":{}} So the code that uses slog but does not know what handler will be used can't rely on it lazily calling the `String() string` method: half of the standard handlers do that, half don't.
    • 0x696C6961 1y ago
      If you need more control, you can create a wrapper type that implements `slog.LogValuer` type StringerValue struct { fmt.Stringer } func (v StringerValue) LogValue() slog.Value { return slog.StringValue(v.String()) } Usage example: slog.Any("requests", StringerValue{req}) There might be a case for making the expvar types implement `slog.LogValuer` directly.
      • aleksi 1y ago
        So clearly not all values are supported. And I know that I can create a wrapper for unsupported types. My problem is exactly that – I don't know what types are supported. Is error supported, for example? Should I create a wrapper for it? And, as a handler author, should I support it directly or not?
        • 0x696C6961 1y ago
          Not sure what your definition of "supported" is, but I'm afraid you're going to have to bite the bullet and ... gasp ... read the documentation https://pkg.go.dev/log/slog https://pkg.go.dev/log/slog
          • aleksi 1y ago
            Not sure I understand your sarcasm. I read the documentation, source code, handler writing guide, and issues in the Go repository multiple times over two years, and I use slog extensively. Go is my primary language since r60. I think I know how to read Go docs. Now, please point me to the place in the documentation that says if I can or can't use a value implementing the error interface as an attribute value, and will the handler or something else would call the `Error() string` method. My definition of "supported" is simple – I could pass a supported value to the logger and get a reasonable representation from any handler. In my example, the JSON handler does not provide it for the fmt.Stringer.
            • arccy 1y ago
              https://pkg.go.dev/log/slog#JSONHandler.Handle https://pkg.go.dev/log/slog#JSONHandler.Handle > Values are formatted as with an encoding/json.Encoder with SetEscapeHTML(false), with two exceptions. > First, an Attr whose Value is of type error is formatted as a string, by calling its Error method. Only errors in Attrs receive this special treatment, not errors embedded in structs, slices, maps or other data structures that are processed by the encoding/json package. So the json handler more or less works as if you called json.Marshal, which sounds pretty reasonable.
              • 1y ago
    • Philip-J-Fry 1y ago
      That seems to work as expected? The output of data is handled by the handler. Such behaviour is clearly outlined in the documentation by the JSONHandler. I wouldn't expect a JSONHandler to use Stringer. I'd expect it to use the existing JSON interfaces, which it does. I'd expect the Text handler to use TextMarshaller. Which it does. Or Stringer, which it does implicitly via fmt.Sprintf.
      • aleksi 1y ago
        My problem with that is that it makes it impossible to use slog logger safely without knowing what handler is being used. Which kind of defeats the purpose of defining the common structured logging interface.
        • deleted 1y ago
          [deleted]
        • 9rx 1y ago
          > Which kind of defeats the purpose of defining the common structured logging interface. Does it, though? Why would the log producer care about how the log entires are formatted? Only the log consumer cares about that.
          • skeezyboy 1y ago
            [flagged]
            • 9rx 1y ago
              A good, if a bit strange, example. A CPI logger wouldn't need to log the same thing as an access logger, but the producer need not care about who is consuming the logs. Consumers might even want to see both and, given the design, can have both.
              • skeezyboy 1y ago
                [flagged]