github.com/imdario/mergo v0.3.8: Merge

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/github.com/imdario/mergo@v0.3.8
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/github.com/imdario/mergo@v0.3.8
Demonstrate these symbols/APIs:
  - mergo.Merge

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:cf0fffe452267b3fbff232836f9cdcf060d84b0b07a3df66339306001f91e2ec","contract":["mergo.Merge merges non-zero fields from a source struct into zero-value fields of a destination struct without overwriting non-zero destination fields.","mergo.Merge with mergo.WithOverride overwrites existing non-zero destination fields with source values.","mergo.Merge with mergo.WithAppendSlice appends source slice elements to destination slice elements.","mergo.Merge returns an error when destination or source argument is nil."],"goal":"verify pkg:golang/github.com/imdario/mergo@v0.3.8","kind":"HOW","packages":["pkg:golang/github.com/imdario/mergo@v0.3.8"],"schemaVersion":1,"symbols":["mergo.Merge"]},"contractCommand":["go","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/github.com/imdario/mergo@v0.3.8"],"schemaVersion":1,"subject":"pkg:golang/github.com/imdario/mergo@v0.3.8","symbols":["mergo.Merge"],"verifierAdapter":"golang@1"}

module example.com/mergomerge

go 1.22

require github.com/imdario/mergo v0.3.8

require (
	github.com/stretchr/testify v1.12.1 // indirect
	gopkg.in/yaml.v2 v2.4.0 // indirect
)

github.com/imdario/mergo v0.3.8 h1:CGgOkSJeqMRmt0D9XLWExdT4m4F1vd3FV3VPt+0VxkQ=
github.com/imdario/mergo v0.3.8/go.mod h1:2EnlNZ0deacrJVfApfmtdGgDfMuh/nq6Ok1EcJh5FfA=
github.com/stretchr/testify v1.12.1 h1:EuwCh5fleGS7H32xRwO3wRGT7DxrDhLAT6FF8MpWDWE=
github.com/stretchr/testify v1.12.1/go.mod h1:MDEgiDPPsNp5cuIrHPPCyornHKgEVbtFUmoNlxoYthg=
go.yaml.in/yaml/v3 v3.0.5 h1:N6y/pJk8buWs9NY5ERU2HSMfm+IuD/OtfdAnq6kESPw=
go.yaml.in/yaml/v3 v3.0.5/go.mod h1:HVTZu1O7/Vkt2N+BFy8Zza+lnLsABggaTM2ZpNIGuKg=
gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
gopkg.in/yaml.v2 v2.4.0 h1:D8xgwECY7CYvx+Y2n4sBz93Jn9JRvxdiyyo8CTfuKaY=
gopkg.in/yaml.v2 v2.4.0/go.mod h1:RDklbk79AGWmwhnvt/jBztapEOGDOx6ZbXqjP6csGnQ=

package main

import (
	"fmt"

	"github.com/imdario/mergo"
)

type Config struct {
	Host    string
	Port    int
	Timeout int
	Tags    []string
}

func main() {
	defaultCfg := Config{
		Host:    "localhost",
		Port:    8080,
		Timeout: 30,
		Tags:    []string{"default"},
	}

	userCfg := Config{
		Port: 9000,
	}

	if err := mergo.Merge(&userCfg, defaultCfg); err != nil {
		panic(err)
	}

	fmt.Printf("Host: %s, Port: %d, Timeout: %d, Tags: %v\n", userCfg.Host, userCfg.Port, userCfg.Timeout, userCfg.Tags)
}

{
  "schemaVersion": 1,
  "goal": "verify pkg:golang/github.com/imdario/mergo@v0.3.8",
  "kind": "HOW",
  "packages": [
    "pkg:golang/github.com/imdario/mergo@v0.3.8"
  ],
  "symbols": [
    "mergo.Merge"
  ]
}

package test

import (
	"reflect"
	"testing"

	"github.com/imdario/mergo"
)

type ServerConfig struct {
	Host    string
	Port    int
	Timeout int
	Tags    []string
}

func TestMergeDefaultWithoutOverwrite(t *testing.T) {
	dst := ServerConfig{
		Port: 9000,
	}
	src := ServerConfig{
		Host:    "localhost",
		Port:    8080,
		Timeout: 30,
		Tags:    []string{"web", "api"},
	}

	if err := mergo.Merge(&dst, src); err != nil {
		t.Fatalf("mergo.Merge failed: %v", err)
	}

	if dst.Host != "localhost" {
		t.Errorf("expected Host 'localhost', got %q", dst.Host)
	}
	if dst.Port != 9000 {
		t.Errorf("expected Port 9000 (preserved), got %d", dst.Port)
	}
	if dst.Timeout != 30 {
		t.Errorf("expected Timeout 30, got %d", dst.Timeout)
	}
	if !reflect.DeepEqual(dst.Tags, []string{"web", "api"}) {
		t.Errorf("expected Tags [web api], got %v", dst.Tags)
	}
}

func TestMergeWithOverride(t *testing.T) {
	dst := ServerConfig{
		Host: "example.com",
		Port: 8080,
	}
	src := ServerConfig{
		Host: "localhost",
		Port: 9090,
	}

	if err := mergo.Merge(&dst, src, mergo.WithOverride); err != nil {
		t.Fatalf("mergo.Merge with WithOverride failed: %v", err)
	}

	if dst.Host != "localhost" {
		t.Errorf("expected Host 'localhost', got %q", dst.Host)
	}
	if dst.Port != 9090 {
		t.Errorf("expected Port 9090, got %d", dst.Port)
	}
}

func TestMergeWithAppendSlice(t *testing.T) {
	dst := ServerConfig{
		Tags: []string{"primary"},
	}
	src := ServerConfig{
		Tags: []string{"secondary", "backup"},
	}

	if err := mergo.Merge(&dst, src, mergo.WithAppendSlice); err != nil {
		t.Fatalf("mergo.Merge with WithAppendSlice failed: %v", err)
	}

	expectedTags := []string{"primary", "secondary", "backup"}
	if !reflect.DeepEqual(dst.Tags, expectedTags) {
		t.Errorf("expected Tags %v, got %v", expectedTags, dst.Tags)
	}
}

func TestMergeNilArguments(t *testing.T) {
	var nilDst *ServerConfig
	src := ServerConfig{Host: "localhost"}

	err := mergo.Merge(nilDst, src)
	if err == nil {
		t.Errorf("expected error when merging into nil dst, got nil")
	}
}