package module
Version: v0.3.9 Latest Latest

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Published: Mar 24, 2020 License: BSD-3-Clause Imports: 6 Imported by: 0



A helper to merge structs and maps in Golang. Useful for configuration default values, avoiding messy if-statements.

Also a lovely comune (municipality) in the Province of Ancona in the Italian region of Marche.


It is ready for production use. It is used in several projects by Docker, Google, The Linux Foundation, VMWare, Shopify, etc.

GoDoc GoCard Build Status Coverage Status Sourcegraph FOSSA Status

Latest release

Release v0.3.7.

Important note

Please keep in mind that in 0.3.2 Mergo changed Merge()and Map() signatures to support transformers. An optional/variadic argument has been added, so it won't break existing code.

If you were using Mergo before April 6th 2015, please check your project works as intended after updating your local copy with go get -u I apologize for any issue caused by its previous behavior and any future bug that Mergo could cause (I hope it won't!) in existing projects after the change (release 0.2.0).


If Mergo is useful to you, consider buying me a coffee, a beer or making a monthly donation so I can keep building great free software. 😍

Buy Me a Coffee at Beerpay Beerpay Donate using Liberapay

Mergo in the wild


go get

// use in your .go code
import (


You can only merge same-type structs with exported fields initialized as zero value of their type and same-types maps. Mergo won't merge unexported (private) fields but will do recursively any exported one. It won't merge empty structs value as they are not considered zero values either. Also maps will be merged recursively except for structs inside maps (because they are not addressable using Go reflection).

if err := mergo.Merge(&dst, src); err != nil {
    // ...

Also, you can merge overwriting values using the transformer WithOverride.

if err := mergo.Merge(&dst, src, mergo.WithOverride); err != nil {
    // ...

Additionally, you can map a map[string]interface{} to a struct (and otherwise, from struct to map), following the same restrictions as in Merge(). Keys are capitalized to find each corresponding exported field.

if err := mergo.Map(&dst, srcMap); err != nil {
    // ...

Warning: if you map a struct to map, it won't do it recursively. Don't expect Mergo to map struct members of your struct as map[string]interface{}. They will be just assigned as values.

More information and examples in godoc documentation.

Nice example
package main

import (

type Foo struct {
	A string
	B int64

func main() {
	src := Foo{
		A: "one",
		B: 2,
	dest := Foo{
		A: "two",
	mergo.Merge(&dest, src)
	// Will print
	// {two 2}

Note: if test are failing due missing package, please execute:

go get

Transformers allow to merge specific types differently than in the default behavior. In other words, now you can customize how some types are merged. For example, time.Time is a struct; it doesn't have zero value but IsZero can return true because it has fields with zero value. How can we merge a non-zero time.Time?

package main

import (

type timeTransfomer struct {

func (t timeTransfomer) Transformer(typ reflect.Type) func(dst, src reflect.Value) error {
	if typ == reflect.TypeOf(time.Time{}) {
		return func(dst, src reflect.Value) error {
			if dst.CanSet() {
				isZero := dst.MethodByName("IsZero")
				result := isZero.Call([]reflect.Value{})
				if result[0].Bool() {
			return nil
	return nil

type Snapshot struct {
	Time time.Time
	// ...

func main() {
	src := Snapshot{time.Now()}
	dest := Snapshot{}
	mergo.Merge(&dest, src, mergo.WithTransformers(timeTransfomer{}))
	// Will print
	// { 2018-01-12 01:15:00 +0000 UTC m=+0.000000001 }

Contact me

If I can help you, you have an idea or you are using Mergo in your projects, don't hesitate to drop me a line (or a pull request): @im_dario


Written by Dario Castañé.

Top Contributors

0 1 2 3 4 5 6 7


BSD 3-Clause license, as Go language.

FOSSA Status



Package mergo merges same-type structs and maps by setting default values in zero-value fields.

Mergo won't merge unexported (private) fields but will do recursively any exported one. It also won't merge structs inside maps (because they are not addressable using Go reflection).


From my own work-in-progress project:

type networkConfig struct {
	Protocol string
	Address string
	ServerType string `json: "server_type"`
	Port uint16

type FssnConfig struct {
	Network networkConfig

var fssnDefault = FssnConfig {
	networkConfig {

// Inside a function [...]

if err := mergo.Merge(&config, fssnDefault); err != nil {

// More code [...]



This section is empty.


View Source
var (
	ErrNilArguments                = errors.New("src and dst must not be nil")
	ErrDifferentArgumentsTypes     = errors.New("src and dst must be of same type")
	ErrNotSupported                = errors.New("only structs and maps are supported")
	ErrExpectedMapAsDestination    = errors.New("dst was expected to be a map")
	ErrExpectedStructAsDestination = errors.New("dst was expected to be a struct")

Errors reported by Mergo when it finds invalid arguments.


func Map

func Map(dst, src interface{}, opts ...func(*Config)) error

Map sets fields' values in dst from src. src can be a map with string keys or a struct. dst must be the opposite: if src is a map, dst must be a valid pointer to struct. If src is a struct, dst must be map[string]interface{}. It won't merge unexported (private) fields and will do recursively any exported field. If dst is a map, keys will be src fields' names in lower camel case. Missing key in src that doesn't match a field in dst will be skipped. This doesn't apply if dst is a map. This is separated method from Merge because it is cleaner and it keeps sane semantics: merging equal types, mapping different (restricted) types.

func MapWithOverwrite deprecated

This function has been deprecated.
func MapWithOverwrite(dst, src interface{}, opts ...func(*Config)) error

MapWithOverwrite will do the same as Map except that non-empty dst attributes will be overridden by non-empty src attribute values. Deprecated: Use Map(…) with WithOverride

func Merge

func Merge(dst, src interface{}, opts ...func(*Config)) error

Merge will fill any empty for value type attributes on the dst struct using corresponding src attributes if they themselves are not empty. dst and src must be valid same-type structs and dst must be a pointer to struct. It won't merge unexported (private) fields and will do recursively any exported field.

func MergeWithOverwrite deprecated

This function has been deprecated.
func MergeWithOverwrite(dst, src interface{}, opts ...func(*Config)) error

MergeWithOverwrite will do the same as Merge except that non-empty dst attributes will be overridden by non-empty src attribute values. Deprecated: use Merge(…) with WithOverride

func WithAppendSlice added in v0.3.4

func WithAppendSlice(config *Config)

WithAppendSlice will make merge append slices instead of overwriting it.

func WithOverride

func WithOverride(config *Config)

WithOverride will make merge override non-empty dst attributes with non-empty src attributes values.

func WithOverrideEmptySlice added in v0.3.8

func WithOverrideEmptySlice(config *Config)

WithOverrideEmptySlice will make merge override empty dst slice with empty src slice.

func WithOverwriteWithEmptyValue added in v0.3.9

func WithOverwriteWithEmptyValue(config *Config)

WithOverwriteWithEmptyValue will make merge override non empty dst attributes with empty src attributes values.

func WithTransformers

func WithTransformers(transformers Transformers) func(*Config)

WithTransformers adds transformers to merge, allowing to customize the merging of some types.

func WithTypeCheck added in v0.3.8

func WithTypeCheck(config *Config)

WithTypeCheck will make merge check types while overwriting it (must be used with WithOverride).


type Config

type Config struct {
	Overwrite    bool
	AppendSlice  bool
	TypeCheck    bool
	Transformers Transformers
	// contains filtered or unexported fields

type Transformers

type Transformers interface {
	Transformer(reflect.Type) func(dst, src reflect.Value) error

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