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).
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:
<ahref='https://ko-fi.com/B0B58839'target='_blank'><imgheight='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>
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`?
// { 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).
