Function within a structure. What for?

I think the best answer would be an example.

Have a look Client.CheckRedirect

in the documentation.

type Client struct {
    // (...)
    CheckRedirect func(req *Request, via []*Request) error
}

      

        
        
        
      

    

This is the function that gets called whenever it http.Client

has a redirect response. Since this function is public, you can set it when creating an object Client

or afterwards, and thus you can define custom behavior in such a case.

client := &http.Client{
    CheckRedirect: func(req *http.Request, via []*http.Request) error {
        return http.ErrUseLastResponse
    }
}

      

        
        
        
      

    

Functional properties are just delegates for custom behavior (and more!).

Another example would be creating an object with an event.

type Example struct {
    EventHandler func(params []interface{})
}

      

        
        
        
      

    

You can specify the behavior of this event by setting the property Example.EventHandler

.