Slog handler for building a map representation of the JSON outputted by slog.JSONHandler

slog_map_handler.go

package maphandler
import (
	"context"
	"encoding/json"
	"fmt"
	"log/slog"
)

type group struct {
	name  string
	attrs []slog.Attr
	group *group
}

func (g *group) resolve() map[string]interface{} {
	m := make(map[string]interface{})

	if g.group == nil {
		for _, attr := range g.attrs {
			m[attr.Key] = attr.Value.Resolve().Any()
		}
		return m
	}

	m[g.group.name] = g.group.resolve()
	for _, attr := range g.attrs {
		m[attr.Key] = attr.Value.Resolve().Any()
	}
	return m
}

type MapHandler struct {
	attrs []slog.Attr
	group *group
}

func (h *MapHandler) Enabled(_ context.Context, _ slog.Level) bool {
	return true
}

func (h *MapHandler) Handle(_ context.Context, r slog.Record) error {
	m := make(map[string]interface{})
	m["message"] = r.Message

	for _, attr := range h.attrs {
		m[attr.Key] = attr.Value.Resolve().Any()
	}

	if h.group != nil {
		m[h.group.name] = h.group.resolve()
	}

	b, err := json.MarshalIndent(m, "", "  ")
	if err == nil {
		fmt.Println(string(b))
	}
	return nil
}

func (h *MapHandler) WithGroup(name string) slog.Handler {
	h2 := *h
	copy(h2.attrs, h.attrs)
	if h.group != nil {
		h2.group = &group{name: h.group.name, attrs: h.group.attrs, group: &group{name: name}}
	} else {
		h2.group = &group{name: name}
	}
	return &h2
}

func (h *MapHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
	h2 := *h
	copy(h2.attrs, h.attrs)
	if h.group != nil {
		if h.group.group != nil {
			h2.group = &group{attrs: h.group.attrs, name: h.group.name, group: &group{attrs: attrs, name: h.group.group.name}}
		} else {
			h2.group = &group{attrs: attrs, name: h.group.name}
		}
	} else {
		h2.attrs = append(h2.attrs, attrs...)
	}
	return &h2
}