mergo

README

Mergo

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 Marche.

Status

It is ready for production use. It works fine after extensive use in the wild.

Important note

Mergo is intended to assign only zero value fields on destination with source value. Since April 6th it works like this. Before it didn't work properly, causing some random overwrites. After some issues and PRs I found it didn't merge as I designed it. Thanks to imdario/mergo#8 overwriting functions were added and the wrong behavior was clearly detected.

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).

Mergo in the wild

• docker/docker
• kubernetes/kubernetes
• imdario/zas
• soniah/dnsmadeeasy
• EagerIO/Stout
• lynndylanhurley/defsynth-api
• russross/canvasassignments
• rdegges/cryptly-api
• casualjim/exeggutor
• divshot/gitling
• RWJMurphy/gorl
• andrerocker/deploy42
• elwinar/rambler
• tmaiaroto/gopartman
• jfbus/impressionist
• Jmeyering/zealot
• godep-migrator/rigger-host
• Dronevery/MultiwaySwitch-Go
• thoas/picfit
• mantasmatelis/whooplist-server
• jnuthong/item_search
• Iris Web Framework

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. 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 {
    // ...
}

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

type Foo struct {
	A string
	B int64
}

func main() {
	src := Foo{
		A: "one",
	}

	dest := Foo{
		A: "two",
		B: 2,
	}

	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.v1

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

About

Written by Dario Castañé.

License

BSD 3-Clause license, as Go language.

Documentation

Overview

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).

Usage

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 {
		"tcp",
		"127.0.0.1",
		"http",
		31560,
	},
}

// Inside a function [...]

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

// More code [...]

Index

• Variables
• func Map(dst, src interface{}) error
• func MapWithOverwrite(dst, src interface{}) error
• func Merge(dst, src interface{}) error
• func MergeWithOverwrite(dst, src interface{}) error

Variables

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.

Functions

func Map

func Map(dst, src interface{}) 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

func MapWithOverwrite(dst, src interface{}) error

MapWithOverwrite will do the same as Map except that non-empty dst attributes will be overriden by non-empty src attribute values.

func Merge

func Merge(dst, src interface{}) 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

func MergeWithOverwrite(dst, src interface{}) error

MergeWithOverwrite will do the same as Merge except that non-empty dst attributes will be overriden by non-empty src attribute values.