zlog

h := NewHandler(os.Stdout, &ExampleOptions)
slog.New(h).With("a", "b").Info("test", "c", "d")

Output:

{"level":"INFO","msg":"test","a":"b","c":"d"}

In this example, some values are extracted from the OpenTelemetry baggage and inserted into the record.

opts := ExampleOptions
opts.Baggage = func(_ string) bool { return true }
b := must(baggage.New(
	must(baggage.NewMember("test_kind", "example")),
))
ctx := baggage.ContextWithBaggage(context.Background(), b)
h := NewHandler(os.Stdout, &opts)
slog.New(h).InfoContext(ctx, "test")

Output:

{"level":"INFO","msg":"test","baggage":{"test_kind":"example"}}

In this example, some values are extracted from the pprof labels and inserted into the record.

ctx := pprof.WithLabels(context.Background(), pprof.Labels("test_kind", "example"))
pprof.SetGoroutineLabels(ctx)
h := NewHandler(os.Stdout, &ExampleOptions)
slog.New(h).InfoContext(ctx, "test")

Output:

{"level":"INFO","msg":"test","goroutine":{"test_kind":"example"}}

In this example, there are values stored in the Context at a known key and then automatically retrieved and integrated into the record by the handler.

// With is a helper function to add values to the Context at the known key.
//
// Typically, a module would provide a helper to do this, and do it with
// less garbage. Any ordering or replacement semantics need to happen here;
// this example does not implement being able to remove keys from the
// Context.
with := func(ctx context.Context, args ...any) context.Context {
	var s []slog.Attr
	if v, ok := ctx.Value(SetAttrs).(slog.Value); ok {
		s = v.Group()
	}
	s = append(s, slog.Group("", args...).Value.Group()...)
	seen := make(map[string]struct{}, len(s))
	del := func(a slog.Attr) bool {
		_, ok := seen[a.Key]
		seen[a.Key] = struct{}{}
		return ok
	}
	slices.Reverse(s)
	s = slices.DeleteFunc(s, del)
	slices.Reverse(s)
	return context.WithValue(ctx, SetAttrs, slog.GroupValue(s...))
}
// Setup:
ctx := context.Background()
h := NewHandler(os.Stdout, &ExampleOptions)
l := slog.New(h)

// Usage:
l.InfoContext(ctx, "without ctx attrs", "a", "b")
{
	ctx := with(ctx, "contextual", "value")
	l.InfoContext(ctx, "with ctx attrs", "a", "b")
	{
		ctx := context.WithValue(ctx, SetLevel, slog.LevelDebug)
		ctx = with(ctx, "contextual", "level")
		l.DebugContext(ctx, "with ctx attrs", "a", "b")
	}
	ctx = with(ctx, "appended", "value")
	l.InfoContext(ctx, "with more ctx attrs")
}
l.InfoContext(ctx, "without ctx attrs", "a", "b")

Output:

{"level":"INFO","msg":"without ctx attrs","a":"b"}
{"level":"INFO","msg":"with ctx attrs","contextual":"value","a":"b"}
{"level":"DEBUG","msg":"with ctx attrs","contextual":"level","a":"b"}
{"level":"INFO","msg":"with more ctx attrs","contextual":"value","appended":"value"}
{"level":"INFO","msg":"without ctx attrs","a":"b"}

In this example, the handler is configured with a very high minimum level, so without the per-record level filtering there would be no log messages.

// Per-record filter levels.
filters := []slog.Level{slog.LevelDebug, slog.LevelInfo, slog.LevelWarn, slog.LevelError}
// Levels of records to emit.
levels := []slog.Level{slog.LevelDebug - 4, slog.LevelDebug, slog.LevelInfo, slog.LevelWarn, slog.LevelError}
// With is a helper function to add the log level to the Context at the known key.
//
// Typically, a module would provide a helper to do this.
with := func(ctx context.Context, l slog.Level) context.Context {
	return context.WithValue(ctx, SetLevel, l)
}

// Setup:
ctx := context.Background()
opts := ExampleOptions
opts.Level = slog.Level(100)
h := NewHandler(os.Stdout, &opts)
log := slog.New(h)

// Usage:
a := slog.String("filter", "NONE")
for _, l := range levels {
	log.LogAttrs(ctx, l, "", a)
}
for _, l := range filters {
	a = slog.String("filter", l.String())
	ctx := with(ctx, l)
	for _, l := range levels {
		log.LogAttrs(ctx, l, "", a)
	}
}

Output:

{"level":"DEBUG","msg":"","filter":"DEBUG"}
{"level":"INFO","msg":"","filter":"DEBUG"}
{"level":"WARN","msg":"","filter":"DEBUG"}
{"level":"ERROR","msg":"","filter":"DEBUG"}
{"level":"INFO","msg":"","filter":"INFO"}
{"level":"WARN","msg":"","filter":"INFO"}
{"level":"ERROR","msg":"","filter":"INFO"}
{"level":"WARN","msg":"","filter":"WARN"}
{"level":"ERROR","msg":"","filter":"WARN"}
{"level":"ERROR","msg":"","filter":"ERROR"}