string_concatenation_monoid.go

package main

import (
	"fmt"
	"log"
)

// String Concatenation Monoid
var identity string = ""
var mappend = func(a, b string) string {
	return a + b
}

func main() {

	// s could be replaced with any value of type string
	s := "foo"
	identityVerified := mappend(identity, s) == s && mappend(s, identity) == s
	if !identityVerified {
		log.Fatal("invalid identity")
	}

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

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

	strings := []string{"Hello", " Gophers!",
		" Can", " I", " sell", " you",
		" some", " monoids?"}
	concatenated := identity
	for _, s := range strings {
		concatenated = mappend(concatenated, s)
	}
	fmt.Println(concatenated)
}