package main

import "fmt"

const defaultA = 100
const defaultB = 200
const defaultC = 300

type thing struct {
    // Mandatory parameters
    x int
    y int
    z int

    // Optional parameters with defaults
    a int
    b int
    c int
}

type thingOpt func(*thing)

func newThing(x, y, z int, opts ...thingOpt) *thing {
    new := &thing{
        x: x,
        y: y,
        z: z,
        a: defaultA,
        b: defaultB,
        c: defaultC,
    }

    for _, o := range opts {
        o(new)
    }

    return new
}

func withA(a int) func(*thing) {
    return func(t *thing) {
        t.a = a
    }
}

func withB(b int) func(*thing) {
    return func(t *thing) {
        t.b = b
    }
}

func withC(c int) func(*thing) {
    return func(t *thing) {
        t.c = c
    }
}

func main() {
    t := newThing(1, 2, 3, withA(10), withB(20))
    fmt.Println("Thing: ", *t)
}