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

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 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

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:sha256:5d8ad612b9bd8f33ea8be54f51a658b94c584a93c13e0e74d652f0f1c995f75f","contract":["yaml.Unmarshal unmarshals valid YAML bytes into a struct with yaml struct tags","yaml.Unmarshal unmarshals nested maps and slices into typed struct fields","yaml.Unmarshal returns an error when parsing malformed YAML syntax","yaml.Unmarshal unmarshals custom type implementing yaml.Unmarshaler interface","yaml.Unmarshal returns TypeError when unmarshaling incompatible types"],"goal":"verify 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.Unmarshal"]},"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.Unmarshal"],"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 (
	"fmt"
	"os"

	"gopkg.in/yaml.v2"
)

// ServerConfig holds the application configuration parsed from YAML.
type ServerConfig struct {
	Host     string            `yaml:"host"`
	Port     int               `yaml:"port"`
	Enabled  bool              `yaml:"enabled"`
	Tags     []string          `yaml:"tags,omitempty"`
	Settings map[string]string `yaml:"settings,omitempty"`
}

// ParseConfig decodes YAML bytes into a ServerConfig.
func ParseConfig(data []byte) (*ServerConfig, error) {
	var cfg ServerConfig
	if err := yaml.Unmarshal(data, &cfg); err != nil {
		return nil, fmt.Errorf("failed to unmarshal yaml: %w", err)
	}
	return &cfg, nil
}

func main() {
	rawYAML := []byte(`
host: localhost
port: 8080
enabled: true
tags:
  - web
  - production
settings:
  timeout: "30s"
`)

	cfg, err := ParseConfig(rawYAML)
	if err != nil {
		fmt.Fprintf(os.Stderr, "Error parsing configuration: %v\n", err)
		os.Exit(1)
	}

	fmt.Printf("Parsed ServerConfig: host=%s, port=%d, enabled=%t\n", cfg.Host, cfg.Port, cfg.Enabled)
}

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

package main

import (
	"errors"
	"fmt"
	"os"

	"gopkg.in/yaml.v2"
)

type ServerConfig struct {
	Host     string            `yaml:"host"`
	Port     int               `yaml:"port"`
	Enabled  bool              `yaml:"enabled"`
	Tags     []string          `yaml:"tags,omitempty"`
	Settings map[string]string `yaml:"settings,omitempty"`
}

type PortNumber int

func (p *PortNumber) UnmarshalYAML(unmarshal func(interface{}) error) error {
	var raw int
	if err := unmarshal(&raw); err != nil {
		return err
	}
	if raw <= 0 || raw > 65535 {
		return fmt.Errorf("port must be between 1 and 65535, got %d", raw)
	}
	*p = PortNumber(raw)
	return nil
}

type CustomService struct {
	Name string     `yaml:"name"`
	Port PortNumber `yaml:"port"`
}

func main() {
	// Assertion 1: yaml.Unmarshal unmarshals valid YAML bytes into a struct with yaml struct tags
	validYAML := []byte("host: localhost\nport: 8080\nenabled: true\n")
	var srv ServerConfig
	if err := yaml.Unmarshal(validYAML, &srv); err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: yaml.Unmarshal failed: %v\n", err)
		os.Exit(1)
	}
	if srv.Host != "localhost" || srv.Port != 8080 || !srv.Enabled {
		fmt.Fprintf(os.Stderr, "FAIL: struct fields mismatch: %+v\n", srv)
		os.Exit(1)
	}

	// Assertion 2: yaml.Unmarshal unmarshals nested maps and slices into typed struct fields
	nestedYAML := []byte("host: localhost\nport: 8080\nenabled: true\ntags:\n  - api\n  - v1\nsettings:\n  region: us-east\n")
	var nestedSrv ServerConfig
	if err := yaml.Unmarshal(nestedYAML, &nestedSrv); err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: yaml.Unmarshal nested failed: %v\n", err)
		os.Exit(1)
	}
	if len(nestedSrv.Tags) != 2 || nestedSrv.Tags[0] != "api" || nestedSrv.Tags[1] != "v1" {
		fmt.Fprintf(os.Stderr, "FAIL: tags slice mismatch: %v\n", nestedSrv.Tags)
		os.Exit(1)
	}
	if nestedSrv.Settings["region"] != "us-east" {
		fmt.Fprintf(os.Stderr, "FAIL: settings map mismatch: %v\n", nestedSrv.Settings)
		os.Exit(1)
	}

	// Assertion 3: yaml.Unmarshal returns an error when parsing malformed YAML syntax
	malformedYAML := []byte("host: [unclosed sequence\nport: 8080\n")
	var malformedSrv ServerConfig
	if err := yaml.Unmarshal(malformedYAML, &malformedSrv); err == nil {
		fmt.Fprintf(os.Stderr, "FAIL: expected error for malformed YAML, got nil\n")
		os.Exit(1)
	}

	// Assertion 4: yaml.Unmarshal unmarshals custom type implementing yaml.Unmarshaler interface
	customYAML := []byte("name: proxy\nport: 8443\n")
	var customSvc CustomService
	if err := yaml.Unmarshal(customYAML, &customSvc); err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: custom unmarshaler failed: %v\n", err)
		os.Exit(1)
	}
	if customSvc.Name != "proxy" || customSvc.Port != 8443 {
		fmt.Fprintf(os.Stderr, "FAIL: custom unmarshaler values mismatch: %+v\n", customSvc)
		os.Exit(1)
	}

	// Assertion 5: yaml.Unmarshal returns TypeError when unmarshaling incompatible types
	incompatibleYAML := []byte("host: localhost\nport: not-a-number\n")
	var incompatibleSrv ServerConfig
	incompatibleErr := yaml.Unmarshal(incompatibleYAML, &incompatibleSrv)
	if incompatibleErr == nil {
		fmt.Fprintf(os.Stderr, "FAIL: expected error for incompatible type, got nil\n")
		os.Exit(1)
	}
	var typeErr *yaml.TypeError
	if !errors.As(incompatibleErr, &typeErr) {
		fmt.Fprintf(os.Stderr, "FAIL: expected *yaml.TypeError, got %T (%v)\n", incompatibleErr, incompatibleErr)
		os.Exit(1)
	}

	fmt.Println("PASS: pkg:golang/gopkg.in/yaml.v2@v2.3.0 contract passed")
}