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defaults

Initialize structs with default values

• Supports almost all kind of types
• Scalar types
  • int/8/16/32/64, uint/8/16/32/64, float32/64
  • uintptr, bool, string
• Complex types
  • map, slice, struct
• Nested types
  • map[K1]map[K2]Struct, []map[K1]Struct[]
• Aliased types
  • time.Duration
  • e.g., type Enum string
• Pointer types
  • e.g., *SampleStruct, *int
• Recursively initializes fields in a struct
• Dynamically sets default values by:
• Implementing the defaults.Setter interface, or
• Implementing encoding.TextUnmarshaler, which takes precedence over defaults.Setter -- the tag is handed to UnmarshalText and SetDefaults is not called
• Preserves non-initial values from being reset with a default value
• A field is written only while it still holds its type's zero value. No scalar type can tell an unspecified value from its zero value — an int left alone is 0, a string is "", a bool is false — so a zero the caller set on purpose is indistinguishable from one never set, and the default replaces it.
• Use a pointer where that distinction matters. nil means unspecified, and a pointer to the zero value is preserved: *bool is the way to let false survive a default:"true".

Usage

package main

import (
    "encoding/json"
    "fmt"
    "math/rand"

    "github.com/creasty/defaults"
)

type Gender string

type Sample struct {
    Name    string `default:"John Smith"`
    Age     int    `default:"27"`
    Gender  Gender `default:"m"`
    Working bool   `default:"true"`

    SliceInt    []int    `default:"[1, 2, 3]"`
    SlicePtr    []*int   `default:"[1, 2, 3]"`
    SliceString []string `default:"[\"a\", \"b\"]"`

    MapNull            map[string]int          `default:"{}"`
    Map                map[string]int          `default:"{\"key1\": 123}"`
    MapOfStruct        map[string]OtherStruct  `default:"{\"Key2\": {\"Foo\":123}}"`
    MapOfPtrStruct     map[string]*OtherStruct `default:"{\"Key3\": {\"Foo\":123}}"`
    MapOfStructWithTag map[string]OtherStruct  `default:"{\"Key4\": {\"Foo\":123}}"`

    Struct    OtherStruct  `default:"{\"Foo\": 123}"`
    StructPtr *OtherStruct `default:"{\"Foo\": 123}"`

    NoTag    OtherStruct // Recurses into a nested struct by default
    NoOption OtherStruct `default:"-"` // no option
}

type OtherStruct struct {
    Hello  string `default:"world"` // Tags in a nested struct also work
    Foo    int    `default:"-"`
    Random int    `default:"-"`
}

// SetDefaults implements defaults.Setter interface
func (s *OtherStruct) SetDefaults() {
    if defaults.CanUpdate(s.Random) { // Check if it's a zero value (recommended)
        s.Random = rand.Int() // Set a dynamic value
    }
}

func main() {
    obj := &Sample{}
    if err := defaults.Set(obj); err != nil {
        panic(err)
    }

    out, err := json.MarshalIndent(obj, "", "   ")
    if err != nil {
        panic(err)
    }
    fmt.Println(string(out))

    // Output:
    // {
    //  "Name": "John Smith",
    //  "Age": 27,
    //  "Gender": "m",
    //  "Working": true,
    //  "SliceInt": [
    //      1,
    //      2,
    //      3
    //  ],
    //  "SlicePtr": [
    //      1,
    //      2,
    //      3
    //  ],
    //  "SliceString": [
    //      "a",
    //      "b"
    //  ],
    //  "MapNull": {},
    //  "Map": {
    //      "key1": 123
    //  },
    //  "MapOfStruct": {
    //      "Key2": {
    //          "Hello": "world",
    //          "Foo": 123,
    //          "Random": 5577006791947779410
    //      }
    //  },
    //  "MapOfPtrStruct": {
    //      "Key3": {
    //          "Hello": "world",
    //          "Foo": 123,
    //          "Random": 8674665223082153551
    //      }
    //  },
    //  "MapOfStructWithTag": {
    //      "Key4": {
    //          "Hello": "world",
    //          "Foo": 123,
    //          "Random": 6129484611666145821
    //      }
    //  },
    //  "Struct": {
    //      "Hello": "world",
    //      "Foo": 123,
    //      "Random": 4037200794235010051
    //  },
    //  "StructPtr": {
    //      "Hello": "world",
    //      "Foo": 123,
    //      "Random": 3916589616287113937
    //  },
    //  "NoTag": {
    //      "Hello": "world",
    //      "Foo": 0,
    //      "Random": 6334824724549167320
    //  },
    //  "NoOption": {
    //      "Hello": "",
    //      "Foo": 0,
    //      "Random": 0
    //  }
    // }
}