average_monoid.go

package main

import (
	"fmt"
	"log"
)

// A monoid that combines sum and count to calculate an average
type average struct {
	sum   int
	count int
}

var identity = average{}

func mappend(a, b average) average {
	return average{
		sum:   a.sum + b.sum,
		count: a.count + b.count,
	}
}

func toAverage(i int) average {
	return average{sum: i, count: 1}
}

func (a average) Float64() float64 {
	return float64(a.sum) / float64(a.count)
}

func main() {

	// n could be replaced with any value of type average
	n := toAverage(42)
	identityVerified := mappend(identity, n) == n && mappend(n, identity) == n
	if !identityVerified {
		log.Fatal("invalid identity")
	}

	// a, b, and c could be any values of our given type
	a := toAverage(1)
	b := toAverage(9)
	c := toAverage(40)
	associativityVerified := mappend(mappend(a, b), c) == mappend(a, mappend(b, c))
	if !associativityVerified {
		log.Fatal("not associative")
	}

	fmt.Println("You've got a monoid")

	numbers := []int{1, 2, 3, 4, 5, 6, 7, 8, 9}
	result := identity
	for _, n := range numbers {
		result = mappend(result, toAverage(n))
	}
	fmt.Printf("The average of %v is %f\n", numbers, result.Float64())
}