Go: What is rune? – Hossein Naderi

One of the new concepts developers encounter when working with strings in Go is the rune. As shown in the code below, s[0] is a byte, and its ASCII code is 72.

package main

import "fmt"

func main() {
	s := "HELP!"
	var b byte
	b = s[0]
	fmt.Println(b) // prints 72
}

However, the code below results in a compilation error, stating that ch is of type rune, not byte. If you examine Go’s internal libraries, you’ll see that rune is an alias for int32, which is 4 bytes.

package main

import "fmt"

func main() {
	s := "HELP!"
	var b byte
	for _, ch := range s {
		b = ch
		fmt.Println(b)
	}
}

error: cannot use ch (variable of type rune) as byte value in assignment

The issue is that in UTF-8, not all characters have the same encoded length. Common characters use fewer bytes, while less common ones use more. This results in compression within the encoding. Go is designed with UTF-8 in mind, so when you iterate over a string, you get runes, even though a string is actually a slice of bytes.