Go for Object Oriented Programmers or Object Oriented Programming without Objects

Go for Object  
Oriented Programmers
!
or
!
Object Oriented Programming  
Without Objects

• Author of Hugo, Cobra,
Viper & More
• Chief Developer
Advocate for MongoDB
• Gopher
@spf13

— txxxxd
“Most of the appeal for me is not
the features that Go has, but
rather the features that have
been intentionally left out.”

— Rob Pike
“Why would you have a
language that is not
theoretically exciting?
Because it’s very useful.”

Does Go have Objects?
• Go lacks Classes
• Go lacks “Objects”

– Steve Francia
“An object is an
abstract data type that
has state (data) and
behavior (code).”

type Rect struct {
width int
height int
}
Type Declaration (Struct)

func (r *Rect) Area() int {
return r.width * r.height
}
Declaring a Method

func main() {
r := Rect{width: 10, height: 5}
fmt.Println("area: ", r.Area())
}
In Action

type Rects []Rect
Type Declaration (Slice)

func (rs Rects) Area() int {
var a int
for _, r := range rs {
a += r.Area()
}
return a
}
Declaring a Method

func main() {
r := Rect{width: 10, height: 5}
x := Rect{width: 7, height:10}
rs := Rects{r, x}
fmt.Println("r's area: ", r.Area())
fmt.Println("x's area: ", x.Area())
fmt.Println("total area: ", rs.Area())
}
In Action
http://play.golang.org/p/G1OWXPGvc3

type Foo func() int
Type Declaration (Func)

func (f Foo) Add(x int) int {
return f() + x
}
Declaring a Method

func main() {
var x Foo
!
x = func() int { return 1 }
!
fmt.Println(x())
fmt.Println(x.Add(3))
}
In Action
http://play.golang.org/p/YGrdCG3SlI

– Wikipedia
A language is usually considered object-based
if it includes the basic capabilities for an
object: identity, properties, and attributes.
 
A language is considered object-oriented if it
is object-based and also has the capability of
polymorphism and inheritance.

Go is Object Based.
!
Is it OO?

Inheritance
• Provides reuse of objects
• Classes are created in hierarchies
• Inheritance lets the structure and
methods in one class pass down the
hierarchy

Go’s approach
• Go explicitly avoided inheritance
• Go strictly follows the composition over
inheritance principle
• Composition through embedded types

Composition
• Provides reuse of Objects
• One object is declared by including other
objects
• Composition lets the structure and methods
in one class be pulled into another

– Steve Francia
Inheritance passes “knowledge” down
!
Composition pulls “knowledge” up

type Person struct {
Name string
Address
}
!
!
type Address struct {
Number string
Street string
City string
State string
Zip string
}
Embedding Types
Inner Type

func (a *Address) String() string {
return a.Number+" "+a.Street+"n"+ 
a.City+", "+a.State+" "+a.Zip+"n"
}
Declaring a Method

func main() {
p := Person{
Name: "Steve",
Address: Address{
Number: "13",
Street: "Main",
City: "Gotham",
State: "NY",
Zip: "01313",
},
}
}
Declare using Composite Literal

func main() {
p := Person{
Name: "Steve",
Address: Address{
Number: "13",
Street: "Main",
City: "Gotham",
State: "NY",
Zip: "01313",
},
}
fmt.Println(p.String())
In Action
http://play.golang.org/p/9beVY9jNlW

Promotion
• Promotion looks to see if a single inner type can
satisify the request and “promotes” it
• Embedded fields & methods are “promoted”
• Promotion only occurs during usage, not declaration
• Promoted methods are considered for interface
adherance

func (a *Address) String() string {
return a.Number+" "+a.Street+"n"+ 
a.City+", "+a.State+" "+a.Zip+"n"
}
!
func (p *Person) String() string {
return p.Name + "n" + p.Address.String()
}
Not Overloading

func main() {
p := Person{
Name: "Steve",
Address: Address{
Number: "13",
Street: "Main",
City: "Gotham",
State: "NY",
Zip: "01313",
},
}
!
fmt.Println(p.String())
fmt.Println(p.Address.String())
}
Both Methods Available
http://play.golang.org/p/Aui0nGa5Xi

func isValidAddress(a *Address) bool {
return a.Street != ""
}
!
func main() {
p := Person{ Name: "Steve", Address: Address{ Number: "13", Street:
"Main", City: "Gotham", State: "NY", Zip: "01313"}}
!
fmt.Println(isValidAddress(p))  
// cannot use p (type Person) as type Address  
// in argument to isValidAddress
fmt.Println(isValidAddress(p.Address))
}
Types Remain Distinct
http://play.golang.org/p/KYjXZxNBcQ

Polymorphism
• “The provision of a single interface to
entities of different types”
• Typically implmented via Generics,
Overloading and/or Subtyping

Go’s approach
• Go explicitly avoided subtyping &
overloading
• Go does not provide Generics (yet)
• Go’s interface provide polymorphic
capabilities

Interfaces
• A list of required methods
• Structural vs Nominal typing
• ‘‘If something can do this, then it can be
used here”
• Convention is call it a Something-er

type Shaper interface {
Area() int
}
Interface Declaration

func Describe(s Shaper) {
fmt.Println("Area is:", s.Area())
}
Using Interface as Param Type

func main() {
r := &Rect{width: 10, height: 5}
x := &Rect{width: 7, height: 10}
rs := &Rects{r, x}
Describe(r)
Describe(x)
Describe(rs)
}
In Action
http://play.golang.org/p/WL77LihUwi

–James Gosling (creator of Java)
“If you could do Java over again, what would you
change?” “I’d leave out classes,” he replied. After the
laughter died down, he explained that the real
problem wasn’t classes per se, but rather
implementation inheritance (the extends
relationship). Interface inheritance (the implements
relationship) is preferable. You should avoid
implementation inheritance whenever possible.

Go Interfaces are
based on
implementation,
not declaration

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

io.Reader
• Interface
• Read reads up to len(p) bytes into p
• Returns the # of bytes read & any error
• Does not dictate how Read() is implemented
• Used by os.File, bytes.Buffer, net.Conn,
http.Request.Body, loads more

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

io.Writer
• Interface
• Write writes up to len(p) bytes into p
• Returns the # of bytes written & any error
• Does not dictate how Write() is implemented
• Used by os.File, bytes.Buffer, net.Conn,
http.Response.Body, loads more

func MarshalGzippedJSON(r io.Reader,  
v interface{}) error {
raw, err := gzip.NewReader(r)
if err != nil {
return err
}
return json.NewDecoder(raw).Decode(&v)
}
io.Reader in Action

f, err := os.Open("myfile.json.gz")
if err != nil {
log.Fatalln(err)
}
defer f.Close()
m = make(map[string]interface{})
MarshalGzippedJSON(f, &m)
Reading a json.gz file

Practical Interoperability
• Gzip.NewReader(io.Reader)
• Works on files, http requests, byte buffers,
network connections, …anything you create
• Nothing special needed in gzip to be able to
do this… Simply call Read(n) and leave the
abstracting to the implementor

func main() {
resp, err := http.Get("...")
if err != nil {
log.Fatalln(err)
}
defer resp.Body.Close()
out, err := os.Create("filename.ext")
if err != nil {
log.Fatalln(err)
}
defer out.Close()
io.Copy(out, resp.Body) // out io.Writer, resp.Body io.Reader
}
Pipe http response to file

— Steve Jobs
Simple can be harder than
complex: You have to work hard
to get your thinking clean to
make it simple. But it's worth it
in the end because once you get
there, you can move mountains.

Go is simple,
pratical &
wonderful

Go for Object Oriented Programmers or Object Oriented Programming without Objects

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