zapslog: Support for a custom mapping of slog.Level to zap.Level

Signed-off-by: junya koyama <arukiidou@yahoo.co.jp>

exp/zapslog/handler.go

@@ -39,6 +39,7 @@ type Handler struct {
 	addCaller  bool
 	addStackAt slog.Level
 	callerSkip int
+	leveler    ConvertLeveler
 
 	// List of unapplied groups.
 	//
@@ -54,6 +55,7 @@ func NewHandler(core zapcore.Core, opts ...HandlerOption) *Handler {
 	h := &Handler{
 		core:       core,
 		addStackAt: slog.LevelError,
+		leveler:    &DefaultConvertLeveler{},
 	}
 	for _, v := range opts {
 		v.apply(h)
@@ -113,31 +115,15 @@ func convertAttrToField(attr slog.Attr) zapcore.Field {
 	}
 }
 
-// convertSlogLevel maps slog Levels to zap Levels.
-// Note that there is some room between slog levels while zap levels are continuous, so we can't 1:1 map them.
-// See also https://go.googlesource.com/proposal/+/master/design/56345-structured-logging.md?pli=1#levels
-func convertSlogLevel(l slog.Level) zapcore.Level {
-	switch {
-	case l >= slog.LevelError:
-		return zapcore.ErrorLevel
-	case l >= slog.LevelWarn:
-		return zapcore.WarnLevel
-	case l >= slog.LevelInfo:
-		return zapcore.InfoLevel
-	default:
-		return zapcore.DebugLevel
-	}
-}
-
 // Enabled reports whether the handler handles records at the given level.
 func (h *Handler) Enabled(ctx context.Context, level slog.Level) bool {
-	return h.core.Enabled(convertSlogLevel(level))
+	return h.core.Enabled(h.leveler.ConvertLevel(level))
 }
 
 // Handle handles the Record.
 func (h *Handler) Handle(ctx context.Context, record slog.Record) error {
 	ent := zapcore.Entry{
-		Level:      convertSlogLevel(record.Level),
+		Level:      h.leveler.ConvertLevel(record.Level),
 		Time:       record.Time,
 		Message:    record.Message,
 		LoggerName: h.name,

exp/zapslog/leveler.go

@@ -0,0 +1,65 @@
+// Copyright (c) 2023 Uber Technologies, Inc.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+//go:build go1.21
+
+package zapslog
+
+import (
+	"log/slog"
+
+	"go.uber.org/zap/zapcore"
+)
+
+// ConvertLeveler maps from [log/slog.Level] to [go.uber.org/zap/zapcore.Level].
+// Note that there is some room between slog levels while zap levels are continuous, so we can't 1:1 map them.
+// See also [structured logging proposal]
+//
+// [structured logging proposal]: https://go.googlesource.com/proposal/+/master/design/56345-structured-logging.md?pli=1#levels
+type ConvertLeveler interface {
+	ConvertLevel(l slog.Level) zapcore.Level
+}
+
+// DefaultConvertLeveler maps.
+// implements: [go.uber.org/zap/exp/zapslog.ConvertLeveler]
+type DefaultConvertLeveler struct{}
+
+// ConvertLevel static maps from [log/slog.Level] to [go.uber.org/zap/zapcore.Level].
+//   - [log/slog.LevelError] to [go.uber.org/zap/zapcore.ErrorLevel]
+//   - [log/slog.LevelWarn] to [go.uber.org/zap/zapcore.WarnLevel]
+//   - [log/slog.LevelInfo] to [go.uber.org/zap/zapcore.InfoLevel]
+//   - [log/slog.LevelDebug] or default to [go.uber.org/zap/zapcore.DebugLevel]
+//
+// Note that there is some room between slog levels while zap levels are continuous, so we can't 1:1 map them.
+// See also [structured logging proposal]
+//
+// [structured logging proposal]: https://go.googlesource.com/proposal/+/master/design/56345-structured-logging.md?pli=1#levels
+func (c *DefaultConvertLeveler) ConvertLevel(l slog.Level) zapcore.Level {
+	switch {
+	case l >= slog.LevelError:
+		return zapcore.ErrorLevel
+	case l >= slog.LevelWarn:
+		return zapcore.WarnLevel
+	case l >= slog.LevelInfo:
+		return zapcore.InfoLevel
+	default:
+		return zapcore.DebugLevel
+	}
+}