# 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]
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[4]: https://godoc.org/github.com/imdario/mergo
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### Latest release
[Release v0.3.7](https://github.com/imdario/mergo/releases/tag/v0.3.7).
### 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:
[](https://beerpay.io/imdario/mergo)
[](https://beerpay.io/imdario/mergo)
### 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).
## Top Contributors
[](https://sourcerer.io/fame/imdario/imdario/mergo/links/0)
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## License
[BSD 3-Clause](http://opensource.org/licenses/BSD-3-Clause) license, as [Go language](http://golang.org/LICENSE).
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