a function named String in go lang [duplicate]

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Closed 19 mins ago.

I'm trying to learn GO and naturally I have difficulty in basic stuff. I came across this recently in an example regarding IOTA however my question isn't about IOTA but about whats going on regarding the String() receiver function with a capital S.

Is there a technical / programming term for what we are doing with the function named String in this example? It has something to do with data type "string" but cannot understand. Why doesn't this code work when I change the name from String to anything else, even to string() with non capital letters?

I would like some programming terms regarding what we are doing so that I can google further, since all I have at the moment is "String function" which doesnt bring much on google.

 package main

import "fmt"

type Direction byte

const (
    North Direction = iota
    East
    South
    West
)

func (d Direction) String() string {
    switch d {
    case North:
        return fmt.Sprintf("North")
    case South:
        return fmt.Sprintf("South")
    case East:
        return fmt.Sprintf("East")
    case West:
        return fmt.Sprintf("West")
    default:
        return "other direction"
    }

}
func main() {
    north := North
    fmt.Println(north)

}