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

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.WithAppendSlice 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.WithAppendSlice

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:sha256:6b4f814cc4c282e42a745667f977045783583cb9b9643e5bc2afe89f4a291c73","contract":["mergo.WithAppendSlice appends source slice elements to destination slice in a struct","mergo.WithAppendSlice appends slice values for matching keys in map types","mergo.WithAppendSlice populates nil destination slices from source slices","mergo.WithAppendSlice preserves destination slice order when appending multiple elements","mergo.WithAppendSlice combines with WithOverride to update scalar fields while appending slice fields"],"goal":"verify github.com/imdario/mergo.WithAppendSlice 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.WithAppendSlice"]},"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.WithAppendSlice"],"verifierAdapter":"golang@1"}

module sample

go 1.26.6

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 ServiceConfig struct {
	Name      string
	Endpoints []string
	Tags      []string
}

func main() {
	dst := ServiceConfig{
		Name:      "sample-service",
		Endpoints: []string{"https://example.com/v1"},
		Tags:      []string{"primary"},
	}

	src := ServiceConfig{
		Name:      "sample-service-v2",
		Endpoints: []string{"https://example.com/v2"},
		Tags:      []string{"v2-ready"},
	}

	// Merge src into dst using WithAppendSlice so slices are concatenated rather than kept or overwritten
	if err := mergo.Merge(&dst, src, mergo.WithAppendSlice); err != nil {
		fmt.Printf("merge failed: %v\n", err)
		return
	}

	fmt.Printf("Service: %s\n", dst.Name)
	fmt.Printf("Endpoints: %v\n", dst.Endpoints)
	fmt.Printf("Tags: %v\n", dst.Tags)
}

{
  "schemaVersion": 1,
  "goal": "verify github.com/imdario/mergo.WithAppendSlice 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.WithAppendSlice"
  ]
}

package main

import (
	"fmt"
	"os"
	"reflect"

	"github.com/imdario/mergo"
)

func main() {
	// Assertion 1: mergo.WithAppendSlice appends source slice elements to destination slice in a struct
	{
		type Config struct {
			Items []string
		}
		dst := Config{Items: []string{"first", "second"}}
		src := Config{Items: []string{"third", "fourth"}}

		if err := mergo.Merge(&dst, src, mergo.WithAppendSlice); err != nil {
			fmt.Fprintf(os.Stderr, "assertion 1 failed: %v\n", err)
			os.Exit(1)
		}

		expected := []string{"first", "second", "third", "fourth"}
		if !reflect.DeepEqual(dst.Items, expected) {
			fmt.Fprintf(os.Stderr, "assertion 1 failed: expected %v, got %v\n", expected, dst.Items)
			os.Exit(1)
		}
	}

	// Assertion 2: mergo.WithAppendSlice appends slice values for matching keys in map types
	{
		dst := map[string][]int{
			"scores": {10, 20},
		}
		src := map[string][]int{
			"scores": {30, 40},
		}

		if err := mergo.Merge(&dst, src, mergo.WithAppendSlice); err != nil {
			fmt.Fprintf(os.Stderr, "assertion 2 failed: %v\n", err)
			os.Exit(1)
		}

		expected := []int{10, 20, 30, 40}
		if !reflect.DeepEqual(dst["scores"], expected) {
			fmt.Fprintf(os.Stderr, "assertion 2 failed: expected %v, got %v\n", expected, dst["scores"])
			os.Exit(1)
		}
	}

	// Assertion 3: mergo.WithAppendSlice populates nil destination slices from source slices
	{
		type Config struct {
			Items []string
		}
		dst := Config{Items: nil}
		src := Config{Items: []string{"alpha", "beta"}}

		if err := mergo.Merge(&dst, src, mergo.WithAppendSlice); err != nil {
			fmt.Fprintf(os.Stderr, "assertion 3 failed: %v\n", err)
			os.Exit(1)
		}

		expected := []string{"alpha", "beta"}
		if !reflect.DeepEqual(dst.Items, expected) {
			fmt.Fprintf(os.Stderr, "assertion 3 failed: expected %v, got %v\n", expected, dst.Items)
			os.Exit(1)
		}
	}

	// Assertion 4: mergo.WithAppendSlice preserves destination slice order when appending multiple elements
	{
		type Sequence struct {
			Steps []int
		}
		dst := Sequence{Steps: []int{1, 2, 3}}
		src := Sequence{Steps: []int{4, 5, 6}}

		if err := mergo.Merge(&dst, src, mergo.WithAppendSlice); err != nil {
			fmt.Fprintf(os.Stderr, "assertion 4 failed: %v\n", err)
			os.Exit(1)
		}

		expected := []int{1, 2, 3, 4, 5, 6}
		if !reflect.DeepEqual(dst.Steps, expected) {
			fmt.Fprintf(os.Stderr, "assertion 4 failed: expected %v, got %v\n", expected, dst.Steps)
			os.Exit(1)
		}
	}

	// Assertion 5: mergo.WithAppendSlice combines with WithOverride to update scalar fields while appending slice fields
	{
		type Config struct {
			Name  string
			Ports []int
		}
		dst := Config{Name: "initial", Ports: []int{80}}
		src := Config{Name: "updated", Ports: []int{443}}

		if err := mergo.Merge(&dst, src, mergo.WithOverride, mergo.WithAppendSlice); err != nil {
			fmt.Fprintf(os.Stderr, "assertion 5 failed: %v\n", err)
			os.Exit(1)
		}

		if dst.Name != "updated" {
			fmt.Fprintf(os.Stderr, "assertion 5 failed: expected name 'updated', got %q\n", dst.Name)
			os.Exit(1)
		}
		expectedPorts := []int{80, 443}
		if !reflect.DeepEqual(dst.Ports, expectedPorts) {
			fmt.Fprintf(os.Stderr, "assertion 5 failed: expected ports %v, got %v\n", expectedPorts, dst.Ports)
			os.Exit(1)
		}
	}

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