Similar to rust's Default trait; just make sure you call the defaults.Default() function against pointer versions of your structs. It will only fill values that are not already at their golang-defined defaults, meaning you can use this after setting struct elements and it will do the right thing.

Note that you can overwrite the CONVERSIONS map in the package after initialization to adjust how things are parsed. Reasonable defaults have been supplied, custom types are not supported with the struct tag; you must use the Default function. Only those that support reflect.Type as a constant.

You can also define custom types by writing a func (t *Type) Default() error function which modifies your struct directly. It will be called if it exists for all structs in the tree including the top level. This is very similar to the Rust Default trait.

Example:

import (
    "github.com/erikh/go-defaults"
)

var DefaultDBConfig = DatabaseConfig{
    Username: "scott",
    Password: "tiger",
    Host: "localhost:1234",
}

type Config struct {
    Listen string `default:"localhost:3000"`
    Listeners uint `default:"5"`
    DB *DatabaseConfig
}

type DatabaseConfig {
    Username string
    Password string
    Host string
}

func (d *DatabaseConfig) Default() error {
    *d = DefaultDBConfig
    return nil
}

func main() {
    config := &Config{}
    if err := defaults.Default(config); err != nil {
        panic(err)
    }

    if config.DB.Username == "scott" && config.DB.Password == "tiger" {
        fmt.Println("success")
    }
}

For hopefully obvious reasons, defaults only works on public struct members.

$ go get github.com/erikh/go-defaults
<sprinkle in code>

import (
    "github.com/erikh/go-defaults"
)

type Config struct {
    Listen string `default:"localhost:3000"`
    Listeners uint `default:"5"`
}

func main() {
    config := &Config{}
    // for config files, pepper your defaults in, then parse your document
    if err := defaults.Default(config); err != nil {
        panic(err)
    }

    if config.Listeners == 5 {
        fmt.Println("it works!")
    }
}

Erik Hollensbe erik@hollensbe.org

This library is considered stable and complete, only situations which panic the library or result in obviously broken behavior will be fixed.

BSD 3 Clause