README.md

# Mergo

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

Also a lovely [comune](http://en.wikipedia.org/wiki/Mergo) (municipality) in the Province of Ancona in the Italian region of Marche.

## Status

It is ready for production use. [It is used in several projects by Docker, Google, The Linux Foundation, VMWare, Shopify, etc](https://github.com/imdario/mergo#mergo-in-the-wild).

[![GoDoc][3]][4]
[![GoCard][5]][6]
[![Build Status][1]][2]
[![Coverage Status][7]][8]
[![Sourcegraph][9]][10]

[1]: https://travis-ci.org/imdario/mergo.png
[2]: https://travis-ci.org/imdario/mergo
[3]: https://godoc.org/github.com/imdario/mergo?status.svg
[4]: https://godoc.org/github.com/imdario/mergo
[5]: https://goreportcard.com/badge/imdario/mergo
[6]: https://goreportcard.com/report/github.com/imdario/mergo
[7]: https://coveralls.io/repos/github/imdario/mergo/badge.svg?branch=master
[8]: https://coveralls.io/github/imdario/mergo?branch=master
[9]: https://sourcegraph.com/github.com/imdario/mergo/-/badge.svg
[10]: https://sourcegraph.com/github.com/imdario/mergo?badge

### Latest release

[Release v0.3.4](https://github.com/imdario/mergo/releases/tag/v0.3.4).

### Important note

Please keep in mind that in [0.3.2](//github.com/imdario/mergo/releases/tag/0.3.2) Mergo changed `Merge()`and `Map()` signatures to support [transformers](#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 github.com/imdario/mergo```. 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).

### Donations

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. :heart_eyes:

<a href='https://ko-fi.com/B0B58839' target='_blank'><img height='36' style='border:0px;height:36px;' src='https://az743702.vo.msecnd.net/cdn/kofi1.png?v=0' border='0' alt='Buy Me a Coffee at ko-fi.com' /></a>
[![Beerpay](https://beerpay.io/imdario/mergo/badge.svg)](https://beerpay.io/imdario/mergo)
[![Beerpay](https://beerpay.io/imdario/mergo/make-wish.svg)](https://beerpay.io/imdario/mergo)
<a href="https://liberapay.com/dario/donate"><img alt="Donate using Liberapay" src="https://liberapay.com/assets/widgets/donate.svg"></a>

### Mergo in the wild

- [moby/moby](https://github.com/moby/moby)
- [kubernetes/kubernetes](https://github.com/kubernetes/kubernetes)
- [vmware/dispatch](https://github.com/vmware/dispatch)
- [Shopify/themekit](https://github.com/Shopify/themekit)
- [imdario/zas](https://github.com/imdario/zas)
- [matcornic/hermes](https://github.com/matcornic/hermes)
- [OpenBazaar/openbazaar-go](https://github.com/OpenBazaar/openbazaar-go)
- [kataras/iris](https://github.com/kataras/iris)
- [michaelsauter/crane](https://github.com/michaelsauter/crane)
- [go-task/task](https://github.com/go-task/task)
- [sensu/uchiwa](https://github.com/sensu/uchiwa)
- [ory/hydra](https://github.com/ory/hydra)
- [sisatech/vcli](https://github.com/sisatech/vcli)
- [dairycart/dairycart](https://github.com/dairycart/dairycart)
- [projectcalico/felix](https://github.com/projectcalico/felix)
- [resin-os/balena](https://github.com/resin-os/balena)
- [go-kivik/kivik](https://github.com/go-kivik/kivik)
- [Telefonica/govice](https://github.com/Telefonica/govice)
- [supergiant/supergiant](supergiant/supergiant)
- [SergeyTsalkov/brooce](https://github.com/SergeyTsalkov/brooce)
- [soniah/dnsmadeeasy](https://github.com/soniah/dnsmadeeasy)
- [ohsu-comp-bio/funnel](https://github.com/ohsu-comp-bio/funnel)
- [EagerIO/Stout](https://github.com/EagerIO/Stout)
- [lynndylanhurley/defsynth-api](https://github.com/lynndylanhurley/defsynth-api)
- [russross/canvasassignments](https://github.com/russross/canvasassignments)
- [rdegges/cryptly-api](https://github.com/rdegges/cryptly-api)
- [casualjim/exeggutor](https://github.com/casualjim/exeggutor)
- [divshot/gitling](https://github.com/divshot/gitling)
- [RWJMurphy/gorl](https://github.com/RWJMurphy/gorl)
- [andrerocker/deploy42](https://github.com/andrerocker/deploy42)
- [elwinar/rambler](https://github.com/elwinar/rambler)
- [tmaiaroto/gopartman](https://github.com/tmaiaroto/gopartman)
- [jfbus/impressionist](https://github.com/jfbus/impressionist)
- [Jmeyering/zealot](https://github.com/Jmeyering/zealot)
- [godep-migrator/rigger-host](https://github.com/godep-migrator/rigger-host)
- [Dronevery/MultiwaySwitch-Go](https://github.com/Dronevery/MultiwaySwitch-Go)
- [thoas/picfit](https://github.com/thoas/picfit)
- [mantasmatelis/whooplist-server](https://github.com/mantasmatelis/whooplist-server)
- [jnuthong/item_search](https://github.com/jnuthong/item_search)
- [bukalapak/snowboard](https://github.com/bukalapak/snowboard)

## Installation

    go get github.com/imdario/mergo

    // use in your .go code
    import (
        "github.com/imdario/mergo"
    )

## Usage

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](https://golang.org/ref/spec#The_zero_value) either. Also maps will be merged recursively except for structs inside maps (because they are not addressable using Go reflection).

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

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

```go
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.

```go
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](http://godoc.org/github.com/imdario/mergo).

### Nice example

```go
package main

import (
	"fmt"
	"github.com/imdario/mergo"
)

type Foo struct {
	A string
	B int64
}

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

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

    go get gopkg.in/yaml.v2

### Transformers

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`?

```go
package main

import (
	"fmt"
	"github.com/imdario/mergo"
        "reflect"
        "time"
)

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() {
					dst.Set(src)
				}
			}
			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{}))
	fmt.Println(dest)
	// 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](https://twitter.com/im_dario)

## About

Written by [Dario Castañé](http://dario.im).

## License

[BSD 3-Clause](http://opensource.org/licenses/BSD-3-Clause) license, as [Go language](http://golang.org/LICENSE).