Configuring a slog handler with functional options

slogging.go

package slogging

import (
	"path/filepath"
	"strings"

	"golang.org/x/exp/slog"
)

type source int

const (
	SourceNone source = iota
	SourceFile
	SourcePath
)

type configuration struct {
	source   source
	stampFmt string
	level    slog.Level
}

type option func(*configuration)

var Options options

type options struct{}

func (options) Source(s source) option    { return func(c *configuration) { c.source = s } }
func (options) StampFmt(s string) option  { return func(c *configuration) { c.stampFmt = s } }
func (options) Level(l slog.Level) option { return func(c *configuration) { c.level = l } }

func HandlerOptions(options ...option) slog.HandlerOptions {
	var config configuration
	for _, option := range options {
		option(&config)
	}
	return slog.HandlerOptions{
		Level:     config.level,
		AddSource: config.source != SourceNone,
		ReplaceAttr: func(a slog.Attr) slog.Attr {
			switch a.Key {
			case "source":
				if config.source == SourceFile {
					fields := strings.Split(a.Value.String(), ":")
					path := filepath.Base(fields[0])
					return slog.String("source", path+":"+fields[1])
				}
			case "time":
				if len(config.stampFmt) > 0 {
					return slog.String("time", a.Value.Time().Format(config.stampFmt))
				}
			}
			return a
		},
	}
}

// SetScriptingLogger sets the default logger using an abbreviated timestamp and only basename of the source file.
// It also receives 0 or more io.Writers at which to emit logs (os.Stderr is hard-coded). I find these settings to
// work nicely in short-lived processes and scripts.
func SetScriptingLogger(outs ...io.Writer) {
	options := HandlerOptions(
		Options.StampFmt(time.TimeOnly),
		Options.Source(SourceFile),
	)
	out := io.MultiWriter(append(outs, os.Stderr)...)
	slog.SetDefault(slog.New(options.NewTextHandler(out)))
}

slogging_test.go

package slogging_test

import (
	"os"
	"time"

	"golang.org/x/exp/slog"

	"slogging"
)

func ExampleHandlerOptions() {
	options := slogging.HandlerOptions(
		slogging.Options.Level(slog.WarnLevel),
		slogging.Options.Source(slogging.SourceFile),
		slogging.Options.StampFmt(time.TimeOnly),
	)
	slog.SetDefault(slog.New(options.NewTextHandler(os.Stderr)))
	slog.Warn("hello", slog.String("recipient", "world"))
}