embed.slide

Embedding

Matthew Heidemann
Pivotal Tracker
@heidmotron

* Definition

Go does not provide the typical, type-driven notion of subclassing, but it does have the ability to “borrow” pieces of an implementation by embedding types within a struct or interface.

* Where have you seen this before?

  type ReadWriter interface {
    Reader
    Writer
  }

  type Reader interface {
    Read(p []byte) (n int, err error)
  }

  type Writer interface {
    Write(p []byte) (n int, err error)
  }

* Basics

- How do we define types?

  type Story struct {
    Id int64
    Title string
    State string
  }

  type Foo int64

* Defining methods
  type Story struct {
    Id int64
    Title string
  }

  // Value Receivers
  func (s Story) isNew() bool {
    return s.Id == 0
  }

  // Pointer Receivers
  func (s *Story) Url() string {
    url := "/services/v5/stories"
    if !s.IsNew() {
      url = fmt.Sprintf("%s/%d", url, s.Id)
    }
    return url
  }

* So now what?
Let's say we have 3 more models that need IsNew behavior:

  type Project struct {
    Id int64
  }

  type Task struct {
    Id int64
  }

  type Comment struct {
    Id int64
  }

How should we do this? Copy-Pasta

* In other languages ruby, java, js

Define some abstract Model class with `IsNew` behavior

  // backbone.js
  Backbone.Model = function(attributes, options) {
    ...
    isNew: function() {
      return this.id == null;
    },
    ...
  }



* Let's embed

It's about composition, we now get all of `Id's` methods on the `Story`

.play embed/1.go

* This will work for more complex types

.play embed/c.go


* How do we override

.play embed/2.go