github.com/imdario/mergo v0.3.11: 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 github.com/imdario/mergo.Merge in pkg:golang/github.com/imdario/mergo@v0.3.11
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/github.com/imdario/mergo@v0.3.11
Demonstrate these symbols/APIs:
  - github.com/imdario/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:ef502029c8426d8a6378068a78567daeafe468fea47438c114e0bf20c8aa2c28","contract":["mergo.Merge copies non-zero fields from a source struct into zero-value fields of a destination struct pointer without overwriting existing non-zero destination fields.","mergo.Merge with mergo.WithOverride overwrites existing non-zero destination fields with non-zero source fields.","mergo.Merge with mergo.WithAppendSlice appends source slice elements onto existing destination slice elements.","mergo.Merge populates missing keys in a destination map from a source map while preserving existing destination keys.","mergo.Merge returns ErrNilArguments when given a nil destination or source interface and ErrDifferentArgumentsTypes when destination and source struct types differ."],"goal":"verify github.com/imdario/mergo.Merge in pkg:golang/github.com/imdario/mergo@v0.3.11","kind":"HOW","packages":["pkg:golang/github.com/imdario/mergo@v0.3.11"],"schemaVersion":1,"symbols":["github.com/imdario/mergo.Merge"]},"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/github.com/imdario/mergo@v0.3.11"],"schemaVersion":1,"subject":"pkg:golang/github.com/imdario/mergo@v0.3.11","symbols":["github.com/imdario/mergo.Merge"],"verifierAdapter":"golang@1"}

module sample

go 1.22

require github.com/imdario/mergo v0.3.11

github.com/imdario/mergo v0.3.11 h1:3tnifQM4i+fbajXKBHXWEH+KvNHqojZ778UH75j3bGA=
github.com/imdario/mergo v0.3.11/go.mod h1:jmQim1M+e3UYxmgPu/WyfjB3N3VflVyUjjjwH0dnCYA=
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=

{
  "schemaVersion": 1,
  "goal": "verify github.com/imdario/mergo.Merge in pkg:golang/github.com/imdario/mergo@v0.3.11",
  "kind": "HOW",
  "packages": [
    "pkg:golang/github.com/imdario/mergo@v0.3.11"
  ],
  "symbols": [
    "github.com/imdario/mergo.Merge"
  ]
}

package main

import (
	"errors"
	"fmt"
	"os"
	"reflect"

	"github.com/imdario/mergo"
)

type ServerOptions struct {
	Address string
	Port    int
	Workers int
	Roles   []string
}

type UnrelatedOptions struct {
	Label string
}

func main() {
	// 1. Default struct merge fills zero-value destination fields from non-zero source fields while keeping existing non-zero destination fields intact.
	dstDefault := ServerOptions{Port: 8443}
	srcDefault := ServerOptions{Address: "localhost", Port: 8080, Workers: 4, Roles: []string{"ingress", "worker"}}
	if err := mergo.Merge(&dstDefault, srcDefault); err != nil {
		fmt.Fprintf(os.Stderr, "default struct merge failed: %v\n", err)
		os.Exit(1)
	}
	if dstDefault.Address != "localhost" || dstDefault.Port != 8443 || dstDefault.Workers != 4 || !reflect.DeepEqual(dstDefault.Roles, []string{"ingress", "worker"}) {
		fmt.Fprintf(os.Stderr, "unexpected default merge result: %+v\n", dstDefault)
		os.Exit(1)
	}

	// 2. Merging with mergo.WithOverride replaces existing non-zero destination fields with non-zero source fields.
	dstOverride := ServerOptions{Address: "example.com", Port: 8443, Workers: 2}
	srcOverride := ServerOptions{Address: "localhost", Port: 9000}
	if err := mergo.Merge(&dstOverride, srcOverride, mergo.WithOverride); err != nil {
		fmt.Fprintf(os.Stderr, "WithOverride merge failed: %v\n", err)
		os.Exit(1)
	}
	if dstOverride.Address != "localhost" || dstOverride.Port != 9000 || dstOverride.Workers != 2 {
		fmt.Fprintf(os.Stderr, "unexpected WithOverride result: %+v\n", dstOverride)
		os.Exit(1)
	}

	// 3. Merging with mergo.WithAppendSlice appends source slice elements onto existing destination slice elements.
	dstSlice := ServerOptions{Roles: []string{"primary"}}
	srcSlice := ServerOptions{Roles: []string{"replica", " edge"}}
	if err := mergo.Merge(&dstSlice, srcSlice, mergo.WithAppendSlice); err != nil {
		fmt.Fprintf(os.Stderr, "WithAppendSlice merge failed: %v\n", err)
		os.Exit(1)
	}
	if !reflect.DeepEqual(dstSlice.Roles, []string{"primary", "replica", " edge"}) {
		fmt.Fprintf(os.Stderr, "unexpected WithAppendSlice result: %+v\n", dstSlice.Roles)
		os.Exit(1)
	}

	// 4. Merging maps populates missing keys in the destination map without overwriting existing keys.
	dstMap := map[string]int{"alpha": 10, "beta": 20}
	srcMap := map[string]int{"beta": 99, "gamma": 30}
	if err := mergo.Merge(&dstMap, srcMap); err != nil {
		fmt.Fprintf(os.Stderr, "map merge failed: %v\n", err)
		os.Exit(1)
	}
	if dstMap["alpha"] != 10 || dstMap["beta"] != 20 || dstMap["gamma"] != 30 {
		fmt.Fprintf(os.Stderr, "unexpected map merge result: %+v\n", dstMap)
		os.Exit(1)
	}

	// 5. Passing nil arguments or mismatched struct types returns the corresponding mergo errors.
	if err := mergo.Merge(nil, srcDefault); !errors.Is(err, mergo.ErrNilArguments) {
		fmt.Fprintf(os.Stderr, "expected ErrNilArguments for nil destination, got: %v\n", err)
		os.Exit(1)
	}
	validDst := ServerOptions{}
	if err := mergo.Merge(&validDst, UnrelatedOptions{Label: "mismatch"}); !errors.Is(err, mergo.ErrDifferentArgumentsTypes) {
		fmt.Fprintf(os.Stderr, "expected ErrDifferentArgumentsTypes, got: %v\n", err)
		os.Exit(1)
	}
}