Mutation pattern in Go

mutation_pattern.go

package main

import (
	"fmt"
)

// This mutation pattern allows mutating an object without having to instantiate
// it first. This is specially useful in tests where you need to do small changes
// in the object's state based on a default initializtion value.

func main() {
	fmt.Println(NewDefaultBicycle())
	fmt.Println(NewDefaultBicycle(func(b *Bicycle) {
		b.FrameSize = "54"
	}))
}

type Bicycle struct {
	WheelSize string
	FrameSize string
	RearCogs  []int
	FrontCogs []int
}

// NewDefaultBicycle creates a new Bicycle with default values but allows mutation
// via the func argument.
func NewDefaultBicycle(mutate ...func(*Bicycle)) Bicycle {
	b := Bicycle{
		WheelSize: "700c",
		FrameSize: "M",
		RearCogs:  []int{11, 12, 13, 14, 15, 17, 19, 21, 23, 25, 28},
		FrontCogs: []int{42, 55},
	}

	if mutate != nil {
		for _, m := range mutate {
			m(&b)
		}
	}

	return b
}