gopkg.in/yaml.v2 v2.3.0: yaml.Encoder

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 yaml.Encoder in pkg:golang/gopkg.in/yaml.v2@v2.3.0
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/gopkg.in/yaml.v2@v2.3.0
Demonstrate these symbols/APIs:
  - yaml.Encoder

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:sha256:0fc79444eaa7b964f09001a7bbf69136c75877747960f835afb2d2fe1b36ec88","contract":["yaml.NewEncoder writes serialized YAML data to an io.Writer","yaml.Encoder encodes multiple documents sequentially separated by document markers","yaml.Encoder encodes struct fields observing yaml struct tags including omitempty and ignored fields","yaml.Encoder Close flushes output and subsequent Encode calls return an error"],"goal":"verify yaml.Encoder in pkg:golang/gopkg.in/yaml.v2@v2.3.0","kind":"HOW","packages":["pkg:golang/gopkg.in/yaml.v2@v2.3.0"],"schemaVersion":1,"symbols":["yaml.Encoder"]},"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/gopkg.in/yaml.v2@v2.3.0"],"schemaVersion":1,"subject":"pkg:golang/gopkg.in/yaml.v2@v2.3.0","symbols":["yaml.Encoder"],"verifierAdapter":"golang@1"}

module example.com/sample

go 1.26.6

require gopkg.in/yaml.v2 v2.3.0

gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
gopkg.in/yaml.v2 v2.3.0 h1:clyUAQHOM3G0M3f5vQj7LuJrETvjVot3Z5el9nffUtU=
gopkg.in/yaml.v2 v2.3.0/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=

package main

import (
	"bytes"
	"fmt"
	"io"
	"os"

	"gopkg.in/yaml.v2"
)

// Document represents a generic configuration entry to encode into YAML.
type Document struct {
	Kind     string            `yaml:"kind"`
	Name     string            `yaml:"name"`
	Metadata map[string]string `yaml:"metadata,omitempty"`
}

// EncodeDocuments serializes multiple Document values to the destination writer using yaml.Encoder.
func EncodeDocuments(w io.Writer, docs []Document) error {
	enc := yaml.NewEncoder(w)
	defer enc.Close()

	for _, doc := range docs {
		if err := enc.Encode(doc); err != nil {
			return fmt.Errorf("failed to encode document: %w", err)
		}
	}
	return nil
}

func main() {
	docs := []Document{
		{
			Kind: "Service",
			Name: "web-server",
			Metadata: map[string]string{
				"env": "production",
			},
		},
		{
			Kind: "ConfigMap",
			Name: "app-config",
		},
	}

	var buf bytes.Buffer
	if err := EncodeDocuments(&buf, docs); err != nil {
		fmt.Fprintf(os.Stderr, "Error: %v\n", err)
		os.Exit(1)
	}

	fmt.Print(buf.String())
}

{
  "schemaVersion": 1,
  "goal": "verify yaml.Encoder in pkg:golang/gopkg.in/yaml.v2@v2.3.0",
  "kind": "HOW",
  "packages": [
    "pkg:golang/gopkg.in/yaml.v2@v2.3.0"
  ],
  "symbols": [
    "yaml.Encoder"
  ]
}

package main

import (
	"bytes"
	"fmt"
	"os"
	"strings"

	"gopkg.in/yaml.v2"
)

type ResourceConfig struct {
	Title  string   `yaml:"title"`
	Port   int      `yaml:"port"`
	Tags   []string `yaml:"tags,omitempty"`
	Secret string   `yaml:"-"`
}

func main() {
	// Assertion 1: yaml.NewEncoder writes serialized YAML data to an io.Writer
	var buf1 bytes.Buffer
	enc1 := yaml.NewEncoder(&buf1)
	item := ResourceConfig{
		Title: "web",
		Port:  8080,
	}
	if err := enc1.Encode(item); err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 1 encode failed: %v\n", err)
		os.Exit(1)
	}
	if err := enc1.Close(); err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 1 close failed: %v\n", err)
		os.Exit(1)
	}
	expectedSingle := "title: web\nport: 8080\n"
	if buf1.String() != expectedSingle {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 1 expected %q, got %q\n", expectedSingle, buf1.String())
		os.Exit(1)
	}

	// Assertion 2: yaml.Encoder encodes multiple documents sequentially separated by document markers
	var buf2 bytes.Buffer
	enc2 := yaml.NewEncoder(&buf2)
	doc1 := map[string]string{"name": "doc1"}
	doc2 := map[string]string{"name": "doc2"}
	if err := enc2.Encode(doc1); err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 2 encode doc1 failed: %v\n", err)
		os.Exit(1)
	}
	if err := enc2.Encode(doc2); err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 2 encode doc2 failed: %v\n", err)
		os.Exit(1)
	}
	if err := enc2.Close(); err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 2 close failed: %v\n", err)
		os.Exit(1)
	}
	expectedMulti := "name: doc1\n---\nname: doc2\n"
	if buf2.String() != expectedMulti {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 2 expected %q, got %q\n", expectedMulti, buf2.String())
		os.Exit(1)
	}

	// Assertion 3: yaml.Encoder encodes struct fields observing yaml struct tags including omitempty and ignored fields
	var buf3 bytes.Buffer
	enc3 := yaml.NewEncoder(&buf3)
	tagged := ResourceConfig{
		Title:  "gateway",
		Port:   443,
		Tags:   nil,
		Secret: "do-not-serialize",
	}
	if err := enc3.Encode(tagged); err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 3 encode failed: %v\n", err)
		os.Exit(1)
	}
	if err := enc3.Close(); err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 3 close failed: %v\n", err)
		os.Exit(1)
	}
	out3 := buf3.String()
	if strings.Contains(out3, "secret") || strings.Contains(out3, "do-not-serialize") {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 3 secret was not ignored: %s\n", out3)
		os.Exit(1)
	}
	if strings.Contains(out3, "tags") {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 3 empty tags was not omitted: %s\n", out3)
		os.Exit(1)
	}
	if !strings.Contains(out3, "title: gateway") || !strings.Contains(out3, "port: 443") {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 3 expected fields missing: %s\n", out3)
		os.Exit(1)
	}

	// Assertion 4: yaml.Encoder Close flushes output and subsequent Encode calls return an error
	var buf4 bytes.Buffer
	enc4 := yaml.NewEncoder(&buf4)
	if err := enc4.Encode(map[string]string{"status": "ok"}); err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 4 initial encode failed: %v\n", err)
		os.Exit(1)
	}
	if err := enc4.Close(); err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 4 close failed: %v\n", err)
		os.Exit(1)
	}
	errAfterClose := enc4.Encode(map[string]string{"status": "another"})
	if errAfterClose == nil {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 4 expected error encoding after close, got nil\n")
		os.Exit(1)
	}

	fmt.Println("PASS: yaml.Encoder contract passed")
}