Polymorphism in Go - does it exist?

Question

I am trying to make something real simple on Go: to have an interface with getter and setter methods. And it seems setter methods are not allowed.

Given this code:

package main

import "fmt"

type MyInterfacer interface {
    Get() int
    Set(i int)
}

type MyStruct struct {
    data int
}

func (this MyStruct) Get() int {
    return this.data
}

func (this MyStruct) Set(i int) {
    this.data = i
}

func main() {
    s := MyStruct{123}
    fmt.Println(s.Get())

    s.Set(456)
    fmt.Println(s.Get())

    var mi MyInterfacer = s
    mi.Set(789)
    fmt.Println(mi.Get())
}

Set method does not work, because in func (this MyStruct) Set(i int), this MyStruct is not a pointer, and the changes are lost as soon at the function exits. But making it this *MyStruct would not compile. Is there any workaround?

Answer 1

Here is a corrected version of your code (playground). This isn't exactly Polymorphism, but the use of an interface is good Go style.

package main

import "fmt"

type MyInterfacer interface {
    Get() int
    Set(i int)
}

type MyStruct struct {
    data int
}

func (this *MyStruct) Get() int {
    return this.data
}

func (this *MyStruct) Set(i int) {
    this.data = i
}

func main() {
    s := &MyStruct{123}
    fmt.Println(s.Get())

    s.Set(456)
    fmt.Println(s.Get())

    var mi MyInterfacer = s
    mi.Set(789)
    fmt.Println(mi.Get())
}

Answer 2

I once found this example of how to do polymorphism in Go:

http://play.golang.org/p/6Ip9scm4c3

package main

import "fmt"

type Talker interface {
        Talk(words string)
}

type Cat struct {
        name string
}

type Dog struct {
        name string
}

func (c *Cat) Talk(words string) {
        fmt.Printf("Cat " + c.name + " here: " + words + "\n")
}

func (d *Dog) Talk(words string) {
        fmt.Printf("Dog " + d.name + " here: " + words + "\n")
}

func main() {
        var t1, t2 Talker

        t1 = &Cat{"Kit"}
        t2 = &Dog{"Doug"}

        t1.Talk("meow")
        t2.Talk("woof")
}