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// Copyright 2022 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package slog
import (
// A Level is the importance or severity of a log event.
// The higher the level, the more important or severe the event.
type Level int
// Level numbers are inherently arbitrary,
// but we picked them to satisfy three constraints.
// Any system can map them to another numbering scheme if it wishes.
// First, we wanted the default level to be Info, Since Levels are ints, Info is
// the default value for int, zero.
// Second, we wanted to make it easy to use levels to specify logger verbosity.
// Since a larger level means a more severe event, a logger that accepts events
// with smaller (or more negative) level means a more verbose logger. Logger
// verbosity is thus the negation of event severity, and the default verbosity
// of 0 accepts all events at least as severe as INFO.
// Third, we wanted some room between levels to accommodate schemes with named
// levels between ours. For example, Google Cloud Logging defines a Notice level
// between Info and Warn. Since there are only a few of these intermediate
// levels, the gap between the numbers need not be large. Our gap of 4 matches
// OpenTelemetry's mapping. Subtracting 9 from an OpenTelemetry level in the
// DEBUG, INFO, WARN and ERROR ranges converts it to the corresponding slog
// Level range. OpenTelemetry also has the names TRACE and FATAL, which slog
// does not. But those OpenTelemetry levels can still be represented as slog
// Levels by using the appropriate integers.
// Names for common levels.
const (
LevelDebug Level = -4
LevelInfo Level = 0
LevelWarn Level = 4
LevelError Level = 8
// String returns a name for the level.
// If the level has a name, then that name
// in uppercase is returned.
// If the level is between named values, then
// an integer is appended to the uppercased name.
// Examples:
// LevelWarn.String() => "WARN"
// (LevelInfo+2).String() => "INFO+2"
func (l Level) String() string {
str := func(base string, val Level) string {
if val == 0 {
return base
return fmt.Sprintf("%s%+d", base, val)
switch {
case l < LevelInfo:
return str("DEBUG", l-LevelDebug)
case l < LevelWarn:
return str("INFO", l-LevelInfo)
case l < LevelError:
return str("WARN", l-LevelWarn)
return str("ERROR", l-LevelError)
// MarshalJSON implements [encoding/json.Marshaler]
// by quoting the output of [Level.String].
func (l Level) MarshalJSON() ([]byte, error) {
// AppendQuote is sufficient for JSON-encoding all Level strings.
// They don't contain any runes that would produce invalid JSON
// when escaped.
return strconv.AppendQuote(nil, l.String()), nil
// UnmarshalJSON implements [encoding/json.Unmarshaler]
// It accepts any string produced by [Level.MarshalJSON],
// ignoring case.
// It also accepts numeric offsets that would result in a different string on
// output. For example, "Error-8" would marshal as "INFO".
func (l *Level) UnmarshalJSON(data []byte) error {
s, err := strconv.Unquote(string(data))
if err != nil {
return err
return l.parse(s)
// MarshalText implements [encoding.TextMarshaler]
// by calling [Level.String].
func (l Level) MarshalText() ([]byte, error) {
return []byte(l.String()), nil
// UnmarshalText implements [encoding.TextUnmarshaler].
// It accepts any string produced by [Level.MarshalText],
// ignoring case.
// It also accepts numeric offsets that would result in a different string on
// output. For example, "Error-8" would marshal as "INFO".
func (l *Level) UnmarshalText(data []byte) error {
return l.parse(string(data))
func (l *Level) parse(s string) (err error) {
defer func() {
if err != nil {
err = fmt.Errorf("slog: level string %q: %w", s, err)
name := s
offset := 0
if i := strings.IndexAny(s, "+-"); i >= 0 {
name = s[:i]
offset, err = strconv.Atoi(s[i:])
if err != nil {
return err
switch strings.ToUpper(name) {
case "DEBUG":
*l = LevelDebug
case "INFO":
*l = LevelInfo
case "WARN":
*l = LevelWarn
case "ERROR":
*l = LevelError
return errors.New("unknown name")
*l += Level(offset)
return nil
// Level returns the receiver.
// It implements Leveler.
func (l Level) Level() Level { return l }
// A LevelVar is a Level variable, to allow a Handler level to change
// dynamically.
// It implements Leveler as well as a Set method,
// and it is safe for use by multiple goroutines.
// The zero LevelVar corresponds to LevelInfo.
type LevelVar struct {
val atomic.Int64
// Level returns v's level.
func (v *LevelVar) Level() Level {
return Level(int(v.val.Load()))
// Set sets v's level to l.
func (v *LevelVar) Set(l Level) {
func (v *LevelVar) String() string {
return fmt.Sprintf("LevelVar(%s)", v.Level())
// MarshalText implements [encoding.TextMarshaler]
// by calling [Level.MarshalText].
func (v *LevelVar) MarshalText() ([]byte, error) {
return v.Level().MarshalText()
// UnmarshalText implements [encoding.TextUnmarshaler]
// by calling [Level.UnmarshalText].
func (v *LevelVar) UnmarshalText(data []byte) error {
var l Level
if err := l.UnmarshalText(data); err != nil {
return err
return nil
// A Leveler provides a Level value.
// As Level itself implements Leveler, clients typically supply
// a Level value wherever a Leveler is needed, such as in HandlerOptions.
// Clients who need to vary the level dynamically can provide a more complex
// Leveler implementation such as *LevelVar.
type Leveler interface {
Level() Level