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

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.Config in 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:
  - github.com/imdario/mergo.Config

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:bd035cc2c472fd9bf0591ee0a861c74df254b2b3c4dffbaed7364587d05a4498","contract":["mergo.Config controls struct merge behavior via functional options modifying Config fields","mergo.Config.Overwrite set to true overwrites non-empty destination fields with source values","mergo.Config.AppendSlice set to true appends source slices to destination slices","mergo.Config fields Overwrite and AppendSlice can be applied together to replace scalars while appending slices","mergo.Config struct settings can be adapted into functional options to configure mergo.Merge"],"goal":"verify github.com/imdario/mergo.Config in pkg:golang/github.com/imdario/mergo@v0.3.8","kind":"HOW","packages":["pkg:golang/github.com/imdario/mergo@v0.3.8"],"schemaVersion":1,"symbols":["github.com/imdario/mergo.Config"]},"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@v0.3.8"],"schemaVersion":1,"subject":"pkg:golang/github.com/imdario/mergo@v0.3.8","symbols":["github.com/imdario/mergo.Config"],"verifierAdapter":"golang@1"}

module sample

go 1.26.6

require github.com/imdario/mergo v0.3.8

github.com/imdario/mergo v0.3.8 h1:CGgOkSJeqMRmt0D9XLWExdT4m4F1vd3FV3VPt+0VxkQ=
github.com/imdario/mergo v0.3.8/go.mod h1:2EnlNZ0deacrJVfApfmtdGgDfMuh/nq6Ok1EcJh5FfA=

package main

import (
	"fmt"

	"github.com/imdario/mergo"
)

type ServerConfig struct {
	Address string
	Port    int
	Tags    []string
}

func main() {
	dst := ServerConfig{
		Address: "localhost",
		Port:    8080,
		Tags:    []string{"primary"},
	}

	src := ServerConfig{
		Address: "example.com",
		Port:    9090,
		Tags:    []string{"secondary"},
	}

	// Configure merge behavior using *mergo.Config functional option
	applyConfig := func(c *mergo.Config) {
		c.Overwrite = true
		c.AppendSlice = true
	}

	if err := mergo.Merge(&dst, src, applyConfig); err != nil {
		fmt.Printf("merge error: %v\n", err)
		return
	}

	fmt.Printf("Address: %s, Port: %d, Tags: %v\n", dst.Address, dst.Port, dst.Tags)
}

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

package main

import (
	"fmt"
	"os"
	"reflect"

	"github.com/imdario/mergo"
)

type ConfigItem struct {
	Name    string
	Port    int
	Tags    []string
	Enabled bool
}

func main() {
	// Assertion 1: mergo.Config controls struct merge behavior via functional options modifying Config fields
	// With Overwrite=false and AppendSlice=false, preserve existing non-zero destination fields and fill zero values.
	{
		dst := ConfigItem{
			Name: "initial",
			Port: 0,
			Tags: []string{"tag1"},
		}
		src := ConfigItem{
			Name: "replacement",
			Port: 8080,
			Tags: []string{"tag2"},
		}
		applyDefault := func(c *mergo.Config) {
			c.Overwrite = false
			c.AppendSlice = false
		}
		if err := mergo.Merge(&dst, src, applyDefault); err != nil {
			fmt.Fprintf(os.Stderr, "Assertion 1 failed: %v\n", err)
			os.Exit(1)
		}
		if dst.Name != "initial" || dst.Port != 8080 || len(dst.Tags) != 1 || dst.Tags[0] != "tag1" {
			fmt.Fprintf(os.Stderr, "Assertion 1 unexpected state: %+v\n", dst)
			os.Exit(1)
		}
	}

	// Assertion 2: mergo.Config.Overwrite set to true overwrites non-empty destination fields with source values
	{
		dst := ConfigItem{
			Name: "original",
			Port: 3000,
		}
		src := ConfigItem{
			Name: "overwritten",
			Port: 9000,
		}
		applyOverwrite := func(c *mergo.Config) {
			c.Overwrite = true
		}
		if err := mergo.Merge(&dst, src, applyOverwrite); err != nil {
			fmt.Fprintf(os.Stderr, "Assertion 2 failed: %v\n", err)
			os.Exit(1)
		}
		if dst.Name != "overwritten" || dst.Port != 9000 {
			fmt.Fprintf(os.Stderr, "Assertion 2 unexpected state: %+v\n", dst)
			os.Exit(1)
		}
	}

	// Assertion 3: mergo.Config.AppendSlice set to true appends source slices to destination slices
	{
		dst := ConfigItem{
			Tags: []string{"first", "second"},
		}
		src := ConfigItem{
			Tags: []string{"third", "fourth"},
		}
		applyAppend := func(c *mergo.Config) {
			c.AppendSlice = true
		}
		if err := mergo.Merge(&dst, src, applyAppend); err != nil {
			fmt.Fprintf(os.Stderr, "Assertion 3 failed: %v\n", err)
			os.Exit(1)
		}
		expectedTags := []string{"first", "second", "third", "fourth"}
		if !reflect.DeepEqual(dst.Tags, expectedTags) {
			fmt.Fprintf(os.Stderr, "Assertion 3 unexpected tags: %v\n", dst.Tags)
			os.Exit(1)
		}
	}

	// Assertion 4: mergo.Config fields Overwrite and AppendSlice can be applied together to replace scalars while appending slices
	{
		dst := ConfigItem{
			Name: "service-a",
			Port: 80,
			Tags: []string{"prod"},
		}
		src := ConfigItem{
			Name: "service-b",
			Port: 443,
			Tags: []string{"canary"},
		}
		applyBoth := func(c *mergo.Config) {
			c.Overwrite = true
			c.AppendSlice = true
		}
		if err := mergo.Merge(&dst, src, applyBoth); err != nil {
			fmt.Fprintf(os.Stderr, "Assertion 4 failed: %v\n", err)
			os.Exit(1)
		}
		expectedTags := []string{"prod", "canary"}
		if dst.Name != "service-b" || dst.Port != 443 || !reflect.DeepEqual(dst.Tags, expectedTags) {
			fmt.Fprintf(os.Stderr, "Assertion 4 unexpected state: %+v\n", dst)
			os.Exit(1)
		}
	}

	// Assertion 5: mergo.Config struct settings can be adapted into functional options to configure mergo.Merge
	{
		cfg := mergo.Config{
			Overwrite:   true,
			AppendSlice: true,
		}
		fromConfig := func(c *mergo.Config) {
			c.Overwrite = cfg.Overwrite
			c.AppendSlice = cfg.AppendSlice
		}
		dst := ConfigItem{
			Name: "base",
			Tags: []string{"alpha"},
		}
		src := ConfigItem{
			Name: "custom",
			Tags: []string{"beta"},
		}
		if err := mergo.Merge(&dst, src, fromConfig); err != nil {
			fmt.Fprintf(os.Stderr, "Assertion 5 failed: %v\n", err)
			os.Exit(1)
		}
		expectedTags := []string{"alpha", "beta"}
		if dst.Name != "custom" || !reflect.DeepEqual(dst.Tags, expectedTags) {
			fmt.Fprintf(os.Stderr, "Assertion 5 unexpected state: %+v\n", dst)
			os.Exit(1)
		}
	}

	fmt.Println("All contract assertions passed successfully.")
}