Functional config in go

README.md

Quick demo of how to initialize your data structure with optional configuration parameters ... with sane defaults if left unspecified.

This functional config technique ...

• lets you write APIs that can evolve without pain
• provides meaningful configuration parameters
• makes it easy to set default settings
• makes it possible to set complex conditional values

See the posts by Rob Pike ("Self-referential functions and the design of options") and Dave Cheney ("Functional options for friendly APIs") for more details on this technique.

main.go

package main

import (
    "fmt"
    "github.com/joyrexus/person"
)

func main() {

    bill, _ := person.New("Bill")  // sets defaults for Job, Age, Alive

    fmt.Printf("%#v\n", bill)
    // {Name:"Bill", Age:0, Job:"unknown", Alive:true}


    bob, _ := person.New("Bob", person.Job("programmer"),
                                person.Age(44))

    fmt.Printf("%#v\n", bob)
    // {Name:"Bob", Age:44, Job:"programmer", Alive:true}


    // change bob's configuration
    bob.Config(person.Age(33), 
               person.Job("masseuse"), 
               person.Alive(false))

    fmt.Printf("%#v\n", bob)
    // {Name:"Bob", Age:33, Job:"masseuse", Alive:false}
}

person.go

package person

import "fmt"

type Person struct{
    Name string
    Age int
    Job string
    Alive bool
}

func (p *Person) String() string {
    return fmt.Sprintf("%s, %v, %s, %v", p.Name, p.Age, p.Job, p.Alive)
}

type option func(*Person)

// Config sets the options specified.
func (p *Person) Config(opts ...option) {
    for _, opt := range opts {
        opt(p)
    }
}

func Age(yrs int) option {
    return func(p *Person) {
        p.Age = yrs
    }
}

func Job(job string) option {
    return func(p *Person) {
        p.Job = job
    }
}

func Alive(status bool) option {
    return func(p *Person) {
        p.Alive = status
    }
}

func New(name string, options ...option) (*Person, error) {
    p := Person{
        Name: name,
        Age: 0,
        Job: "unknown",
        Alive: true,
    }
    for _, opt := range options {
        opt(&p)
    }
    return &p, nil
}