go.uber.org/zap v1.28.0: zapcore.LowercaseLevelEncoder

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: verify go.uber.org/zap/zapcore.LowercaseLevelEncoder in pkg:golang/go.uber.org/zap@v1.28.0
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/go.uber.org/zap@v1.28.0
Demonstrate these symbols/APIs:
  - go.uber.org/zap/zapcore.LowercaseLevelEncoder

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:3e95da505ae1bc2c84a87cb47aac458b7809b5e36f5c0802a6d8810dbb4cb115","contract":["LowercaseLevelEncoder serializes zapcore log levels to lowercase strings via PrimitiveArrayEncoder","NewLowercaseEncoderConfig configures LowercaseLevelEncoder as the level encoder in EncoderConfig","Loggers configured with LowercaseLevelEncoder serialize log levels in lowercase within JSON output"],"goal":"verify go.uber.org/zap/zapcore.LowercaseLevelEncoder in pkg:golang/go.uber.org/zap@v1.28.0","kind":"HOW","packages":["pkg:golang/go.uber.org/zap@v1.28.0"],"schemaVersion":1,"symbols":["go.uber.org/zap/zapcore.LowercaseLevelEncoder"]},"contractCommand":["go","run","./test"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"golang","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"go","schemaVersion":1},"license":"MIT-0","packages":["pkg:golang/go.uber.org/zap@v1.28.0"],"schemaVersion":1,"subject":"pkg:golang/go.uber.org/zap@v1.28.0","symbols":["go.uber.org/zap/zapcore.LowercaseLevelEncoder"],"verifierAdapter":"golang@1"}

module example.com/sample

go 1.22.0

require go.uber.org/zap v1.28.0

require go.uber.org/multierr v1.10.0 // indirect

github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
github.com/stretchr/testify v1.8.1 h1:w7B6lhMri9wdJUVmEZPGGhZzrYTPvgJArz7wNPgYKsk=
github.com/stretchr/testify v1.8.1/go.mod h1:w2LPCIKwWwSfY2zedu0+kehJoqGctiVI29o6fzry7u4=
go.uber.org/goleak v1.3.0 h1:2K3zAYmnTNqV73imy9J1T3WC+gmCePx2hEGkimedGto=
go.uber.org/goleak v1.3.0/go.mod h1:CoHD4mav9JJNrW/WLlf7HGZPjdw8EucARQHekz1X6bE=
go.uber.org/multierr v1.10.0 h1:S0h4aNzvfcFsC3dRF1jLoaov7oRaKqRGC/pUEJ2yvPQ=
go.uber.org/multierr v1.10.0/go.mod h1:20+QtiLqy0Nd6FdQB9TLXag12DsQkrbs3htMFfDN80Y=
go.uber.org/zap v1.28.0 h1:IZzaP1Fv73/T/pBMLk4VutPl36uNC+OSUh3JLG3FIjo=
go.uber.org/zap v1.28.0/go.mod h1:rDLpOi171uODNm/mxFcuYWxDsqWSAVkFdX4XojSKg/Q=
go.yaml.in/yaml/v3 v3.0.4 h1:tfq32ie2Jv2UxXFdLJdh3jXuOzWiL1fo0bu/FbuKpbc=
go.yaml.in/yaml/v3 v3.0.4/go.mod h1:DhzuOOF2ATzADvBadXxruRBLzYTpT36CKvDb3+aBEFg=
gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

package sample

import (
	"go.uber.org/zap"
	"go.uber.org/zap/zapcore"
)

// StringArrayEncoder is an in-memory PrimitiveArrayEncoder that records strings.
type StringArrayEncoder struct {
	Strings []string
}

// AppendString appends a string to the slice of recorded strings.
func (s *StringArrayEncoder) AppendString(v string) {
	s.Strings = append(s.Strings, v)
}

func (s *StringArrayEncoder) AppendBool(bool)             {}
func (s *StringArrayEncoder) AppendByteString([]byte)     {}
func (s *StringArrayEncoder) AppendComplex128(complex128) {}
func (s *StringArrayEncoder) AppendComplex64(complex64)   {}
func (s *StringArrayEncoder) AppendFloat64(float64)       {}
func (s *StringArrayEncoder) AppendFloat32(float32)       {}
func (s *StringArrayEncoder) AppendInt(int)               {}
func (s *StringArrayEncoder) AppendInt64(int64)           {}
func (s *StringArrayEncoder) AppendInt32(int32)           {}
func (s *StringArrayEncoder) AppendInt16(int16)           {}
func (s *StringArrayEncoder) AppendInt8(int8)             {}
func (s *StringArrayEncoder) AppendUint(uint)             {}
func (s *StringArrayEncoder) AppendUint64(uint64)         {}
func (s *StringArrayEncoder) AppendUint32(uint32)         {}
func (s *StringArrayEncoder) AppendUint16(uint16)         {}
func (s *StringArrayEncoder) AppendUint8(uint8)           {}
func (s *StringArrayEncoder) AppendUintptr(uintptr)       {}

// EncodeLevelToString encodes the given Level to lowercase using zapcore.LowercaseLevelEncoder.
func EncodeLevelToString(l zapcore.Level) string {
	enc := &StringArrayEncoder{}
	zapcore.LowercaseLevelEncoder(l, enc)
	if len(enc.Strings) > 0 {
		return enc.Strings[0]
	}
	return ""
}

// NewLowercaseEncoderConfig returns an EncoderConfig configured to use LowercaseLevelEncoder.
func NewLowercaseEncoderConfig() zapcore.EncoderConfig {
	return zapcore.EncoderConfig{
		MessageKey:     "msg",
		LevelKey:       "level",
		NameKey:        "logger",
		EncodeLevel:    zapcore.LowercaseLevelEncoder,
		EncodeTime:     zapcore.ISO8601TimeEncoder,
		EncodeDuration: zapcore.StringDurationEncoder,
	}
}

// NewLogger creates a zap.Logger configured with LowercaseLevelEncoder writing to the given WriteSyncer.
func NewLogger(ws zapcore.WriteSyncer, level zapcore.LevelEnabler) *zap.Logger {
	core := zapcore.NewCore(
		zapcore.NewJSONEncoder(NewLowercaseEncoderConfig()),
		ws,
		level,
	)
	return zap.New(core)
}

{
  "schemaVersion": 1,
  "goal": "verify go.uber.org/zap/zapcore.LowercaseLevelEncoder in pkg:golang/go.uber.org/zap@v1.28.0",
  "kind": "HOW",
  "packages": [
    "pkg:golang/go.uber.org/zap@v1.28.0"
  ],
  "symbols": [
    "go.uber.org/zap/zapcore.LowercaseLevelEncoder"
  ]
}

package main

import (
	"bytes"
	"encoding/json"
	"fmt"
	"os"

	"go.uber.org/zap/zapcore"

	"example.com/sample"
)

type syncer struct {
	bytes.Buffer
}

func (s *syncer) Sync() error {
	return nil
}

type logEntry struct {
	Level string `json:"level"`
	Msg   string `json:"msg"`
}

func main() {
	// Assertion 1: LowercaseLevelEncoder serializes zapcore log levels to lowercase strings via PrimitiveArrayEncoder
	testLevels := []struct {
		level    zapcore.Level
		expected string
	}{
		{zapcore.DebugLevel, "debug"},
		{zapcore.InfoLevel, "info"},
		{zapcore.WarnLevel, "warn"},
		{zapcore.ErrorLevel, "error"},
		{zapcore.DPanicLevel, "dpanic"},
		{zapcore.PanicLevel, "panic"},
		{zapcore.FatalLevel, "fatal"},
	}

	for _, tc := range testLevels {
		var enc sample.StringArrayEncoder
		zapcore.LowercaseLevelEncoder(tc.level, &enc)
		if len(enc.Strings) != 1 {
			fmt.Fprintf(os.Stderr, "FAIL: expected 1 encoded string for %v, got %d\n", tc.level, len(enc.Strings))
			os.Exit(1)
		}
		if enc.Strings[0] != tc.expected {
			fmt.Fprintf(os.Stderr, "FAIL: expected lowercase string %q for level %v, got %q\n", tc.expected, tc.level, enc.Strings[0])
			os.Exit(1)
		}
		fromHelper := sample.EncodeLevelToString(tc.level)
		if fromHelper != tc.expected {
			fmt.Fprintf(os.Stderr, "FAIL: helper returned %q, expected %q\n", fromHelper, tc.expected)
			os.Exit(1)
		}
	}

	// Assertion 2: NewLowercaseEncoderConfig configures LowercaseLevelEncoder as the level encoder in EncoderConfig
	cfg := sample.NewLowercaseEncoderConfig()
	if cfg.EncodeLevel == nil {
		fmt.Fprintln(os.Stderr, "FAIL: expected non-nil EncodeLevel in config")
		os.Exit(1)
	}
	var directEnc sample.StringArrayEncoder
	cfg.EncodeLevel(zapcore.InfoLevel, &directEnc)
	if len(directEnc.Strings) != 1 || directEnc.Strings[0] != "info" {
		fmt.Fprintf(os.Stderr, "FAIL: cfg.EncodeLevel did not serialize to lowercase info: %v\n", directEnc.Strings)
		os.Exit(1)
	}

	// Assertion 3: Loggers configured with LowercaseLevelEncoder serialize log levels in lowercase within JSON output
	ws := &syncer{}
	logger := sample.NewLogger(ws, zapcore.DebugLevel)
	if logger == nil {
		fmt.Fprintln(os.Stderr, "FAIL: expected non-nil logger")
		os.Exit(1)
	}

	logger.Debug("debug event")
	logger.Info("info event")
	logger.Warn("warn event")
	logger.Error("error event")

	decoder := json.NewDecoder(ws)
	expectedLevels := []string{"debug", "info", "warn", "error"}
	for _, expectedLevel := range expectedLevels {
		var entry logEntry
		if err := decoder.Decode(&entry); err != nil {
			fmt.Fprintf(os.Stderr, "FAIL: failed decoding json log entry: %v\n", err)
			os.Exit(1)
		}
		if entry.Level != expectedLevel {
			fmt.Fprintf(os.Stderr, "FAIL: expected level %q in json output, got %q\n", expectedLevel, entry.Level)
			os.Exit(1)
		}
	}

	fmt.Println("PASS: LowercaseLevelEncoder contract verified successfully")
}