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 github.com/imdario/mergo.Merge 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.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:e0d9409f432a22132b9ce4a485bd849eddb91f486dd51b46489cbb3dcb62ef87","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 non-zero source values.","mergo.Merge with mergo.WithAppendSlice appends source slice elements to destination slice elements.","mergo.Merge returns ErrNilArguments when destination or source argument is nil.","mergo.Merge returns ErrDifferentArgumentsTypes when destination and source structs have different types.","mergo.Merge merges map entries into a destination map, preserving existing destination keys and adding source keys."],"goal":"verify github.com/imdario/mergo.Merge 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.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@v0.3.8"],"schemaVersion":1,"subject":"pkg:golang/github.com/imdario/mergo@v0.3.8","symbols":["github.com/imdario/mergo.Merge"],"verifierAdapter":"golang@1"}

module sample

go 1.22

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=

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

package main

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

	"github.com/imdario/mergo"
)

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

type OtherConfig struct {
	Name string
}

func main() {
	if err := runTests(); err != nil {
		fmt.Fprintf(os.Stderr, "Contract test failed: %v\n", err)
		os.Exit(1)
	}
	fmt.Println("All contract tests passed successfully.")
}

func runTests() error {
	// 1. mergo.Merge merges non-zero fields from a source struct into zero-value fields of a destination struct without overwriting non-zero destination fields.
	dst1 := Config{Port: 9000}
	src1 := Config{Host: "localhost", Port: 8080, Timeout: 30, Tags: []string{"web", "api"}}
	if err := mergo.Merge(&dst1, src1); err != nil {
		return fmt.Errorf("mergo.Merge default failed: %w", err)
	}
	if dst1.Host != "localhost" {
		return fmt.Errorf("expected Host 'localhost', got %q", dst1.Host)
	}
	if dst1.Port != 9000 {
		return fmt.Errorf("expected Port 9000 preserved, got %d", dst1.Port)
	}
	if dst1.Timeout != 30 {
		return fmt.Errorf("expected Timeout 30, got %d", dst1.Timeout)
	}
	if !reflect.DeepEqual(dst1.Tags, []string{"web", "api"}) {
		return fmt.Errorf("expected Tags [web api], got %v", dst1.Tags)
	}

	// 2. mergo.Merge with mergo.WithOverride overwrites existing non-zero destination fields with non-zero source values.
	dst2 := Config{Host: "example.com", Port: 8080}
	src2 := Config{Host: "localhost", Port: 9090}
	if err := mergo.Merge(&dst2, src2, mergo.WithOverride); err != nil {
		return fmt.Errorf("mergo.Merge with WithOverride failed: %w", err)
	}
	if dst2.Host != "localhost" {
		return fmt.Errorf("expected Host 'localhost', got %q", dst2.Host)
	}
	if dst2.Port != 9090 {
		return fmt.Errorf("expected Port 9090, got %d", dst2.Port)
	}

	// 3. mergo.Merge with mergo.WithAppendSlice appends source slice elements to destination slice elements.
	dst3 := Config{Tags: []string{"primary"}}
	src3 := Config{Tags: []string{"secondary", "backup"}}
	if err := mergo.Merge(&dst3, src3, mergo.WithAppendSlice); err != nil {
		return fmt.Errorf("mergo.Merge with WithAppendSlice failed: %w", err)
	}
	expectedTags := []string{"primary", "secondary", "backup"}
	if !reflect.DeepEqual(dst3.Tags, expectedTags) {
		return fmt.Errorf("expected Tags %v, got %v", expectedTags, dst3.Tags)
	}

	// 4. mergo.Merge returns ErrNilArguments when destination or source argument is nil.
	var nilDst *Config
	validSrc := Config{Host: "localhost"}
	if err := mergo.Merge(nilDst, validSrc); !errors.Is(err, mergo.ErrNilArguments) && err == nil {
		return fmt.Errorf("expected error when dst is nil, got nil")
	}
	validDst := Config{Port: 8080}
	if err := mergo.Merge(&validDst, nil); !errors.Is(err, mergo.ErrNilArguments) && err == nil {
		return fmt.Errorf("expected error when src is nil, got nil")
	}

	// 5. mergo.Merge returns ErrDifferentArgumentsTypes when destination and source structs have different types.
	diffSrc := OtherConfig{Name: "other"}
	if err := mergo.Merge(&validDst, diffSrc); !errors.Is(err, mergo.ErrDifferentArgumentsTypes) && err == nil {
		return fmt.Errorf("expected ErrDifferentArgumentsTypes, got nil")
	}

	// 6. mergo.Merge merges map entries into a destination map, preserving existing destination keys and adding source keys.
	dstMap := map[string]int{"a": 1, "b": 2}
	srcMap := map[string]int{"b": 20, "c": 3}
	if err := mergo.Merge(&dstMap, srcMap); err != nil {
		return fmt.Errorf("mergo.Merge map failed: %w", err)
	}
	if dstMap["a"] != 1 || dstMap["b"] != 2 || dstMap["c"] != 3 {
		return fmt.Errorf("unexpected map merge result: %v", dstMap)
	}

	return nil
}