1 // Copyright 2013 Dario Castañé. All rights reserved. 2 // Copyright 2009 The Go Authors. All rights reserved. 3 // Use of this source code is governed by a BSD-style 4 // license that can be found in the LICENSE file. 5 6 /* 7 A helper to merge structs and maps in Golang. Useful for configuration default values, avoiding messy if-statements. 8 9 Mergo merges same-type structs and maps by setting default values in zero-value fields. Mergo won't merge unexported (private) fields. It will do recursively any exported one. It also won't merge structs inside maps (because they are not addressable using Go reflection). 10 11 # Status 12 13 It is ready for production use. It is used in several projects by Docker, Google, The Linux Foundation, VMWare, Shopify, etc. 14 15 # Important notes 16 17 1.0.0 18 19 In 1.0.0 Mergo moves to a vanity URL `dario.cat/mergo`. 20 21 0.3.9 22 23 Please keep in mind that a problematic PR broke 0.3.9. We reverted it in 0.3.10. We consider 0.3.10 as stable but not bug-free. . Also, this version adds suppot for go modules. 24 25 Keep in mind that in 0.3.2, Mergo changed Merge() and Map() signatures to support transformers. We added an optional/variadic argument so that it won't break the existing code. 26 27 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 dario.cat/mergo. I apologize for any issue caused by its previous behavior and any future bug that Mergo could cause in existing projects after the change (release 0.2.0). 28 29 # Install 30 31 Do your usual installation procedure: 32 33 go get dario.cat/mergo 34 35 // use in your .go code 36 import ( 37 "dario.cat/mergo" 38 ) 39 40 # Usage 41 42 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 zero values too. Also, maps will be merged recursively except for structs inside maps (because they are not addressable using Go reflection). 43 44 if err := mergo.Merge(&dst, src); err != nil { 45 // ... 46 } 47 48 Also, you can merge overwriting values using the transformer WithOverride. 49 50 if err := mergo.Merge(&dst, src, mergo.WithOverride); err != nil { 51 // ... 52 } 53 54 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. 55 56 if err := mergo.Map(&dst, srcMap); err != nil { 57 // ... 58 } 59 60 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. 61 62 Here is a nice example: 63 64 package main 65 66 import ( 67 "fmt" 68 "dario.cat/mergo" 69 ) 70 71 type Foo struct { 72 A string 73 B int64 74 } 75 76 func main() { 77 src := Foo{ 78 A: "one", 79 B: 2, 80 } 81 dest := Foo{ 82 A: "two", 83 } 84 mergo.Merge(&dest, src) 85 fmt.Println(dest) 86 // Will print 87 // {two 2} 88 } 89 90 # Transformers 91 92 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? 93 94 package main 95 96 import ( 97 "fmt" 98 "dario.cat/mergo" 99 "reflect" 100 "time" 101 ) 102 103 type timeTransformer struct { 104 } 105 106 func (t timeTransformer) Transformer(typ reflect.Type) func(dst, src reflect.Value) error { 107 if typ == reflect.TypeOf(time.Time{}) { 108 return func(dst, src reflect.Value) error { 109 if dst.CanSet() { 110 isZero := dst.MethodByName("IsZero") 111 result := isZero.Call([]reflect.Value{}) 112 if result[0].Bool() { 113 dst.Set(src) 114 } 115 } 116 return nil 117 } 118 } 119 return nil 120 } 121 122 type Snapshot struct { 123 Time time.Time 124 // ... 125 } 126 127 func main() { 128 src := Snapshot{time.Now()} 129 dest := Snapshot{} 130 mergo.Merge(&dest, src, mergo.WithTransformers(timeTransformer{})) 131 fmt.Println(dest) 132 // Will print 133 // { 2018-01-12 01:15:00 +0000 UTC m=+0.000000001 } 134 } 135 136 # Contact me 137 138 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): https://twitter.com/im_dario 139 140 # About 141 142 Written by Dario Castañé: https://da.rio.hn 143 144 # License 145 146 BSD 3-Clause license, as Go language. 147 */ 148 package mergo 149