Demonstrates a trivial sum monoid in Go.

sum_monoid.go

package main

import (
	"fmt"
	"log"
)

// Sum monoid
var identity int = 0
var mappend = func(a, b int) int {
	return a + b
}

func main() {

	// n could be replaced with any value of type int
	n := 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 := 1
	b := 9
	c := 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}
	sum := identity
	for _, n := range numbers {
		sum = mappend(sum, n)
	}
	fmt.Printf("The sum of %v is %d\n", numbers, sum)
}