Go功能字段设置器为什么不保留功能？

Given this short program:

package main

import "fmt"

type Foo struct {
    doer func()
}

func (f Foo) SetDoer(doer func()) {
    f.doer = doer
}

func main() {
    foo := Foo{func() { fmt.Println("original") }}
    foo.doer()
    foo.SetDoer(func() { fmt.Println("replacement") })
    foo.doer()
}

The output is:

original
original

I had expected it to be:

original
replacement

Why isn't it? Note that the output is as expected if I set foo.doer directly in main(). Just not if I use the SetDoer method.

In Go, the item on the left of the function name is the receiving type. This is the type from which a function can be called. However, receiver can be both pointers or value types. In this case, it is a value. The receiver is purely for the purpose of organization, under the covers, it is passed to the function like any other argument. You're passing by value so a copy of foo is passed into SetDoer, the value is modified, then the setter returns, the value goes out of scope and in the calling scope you're working with the original.

Try this;

// make the receiver a pointer
func (f *Foo) SetDoer(doer func()) {
    f.doer = doer
}
// instantiate as pointer
foo := &Foo{func() { fmt.Println("original") }}
foo.SetDoer(func() { fmt.Println("replacement") })
// now the version of doer on foo has been updated.

playground example; https://play.golang.org/p/ZQlvKiluu3