How Iota works in Go

Have you ever wondered how iota works in Go?

Basically, iota is the index of the constant inside a multi-constant (const (...)) declaration. So the value of iota for the first constant is 0, the second constant is 1, and so on. This only works for constants, not vars.

This is probably best explained with an example.

package main

import "fmt"

type day int

const (
	// Start counting days from 1, so ignore 0.
	_ day = iota

	// evaluates to the same as the RHS of the previous expression,
	// which is iota,
	// which is the index of this const expression in this const block,
	// which is 1
	monday

	// this time the index is 2
	tuesday

	// etc.
	wednesday
	thursday
	friday
	saturday
	sunday
)

func (d day) String() string {
	days := []string{
		"",
		"Monday",
		"Tuesday",
		"Wednesday",
		"Thursday",
		"Friday",
		"Saturday",
		"Sunday",
	}
	return days[d]
}

func main() {
	// Let's do some wat maths. Waths if you will.
	// Test your understanding by guessing the answers:
	printDay(monday + tuesday)
	printDay(sunday - wednesday)
	printDay(tuesday * wednesday)
	printDay(saturday / tuesday)
	printDay(sunday % wednesday)
	printDay(friday & thursday)
	printDay(sunday ^ thursday)
}

func printDay(d day) {
	fmt.Println(d)
}

You can run the above code in a Go Playground if you want to check your answers.