Duck Typing

2018-02-16T00:00:00+00:00

My primary development language at the moment is Ruby, but I’ve been learning Go. Languages have varying ways of dealing with different types of objects in the same way, and I always find myself thinking about these kinds of variation when learning a new language. The concept of duck typing (based on the idea that if it quacks like a duck, it’s a duck) is used in Ruby:

class Mallard
  def quack
    puts 'QUACK!'
  end
end

class Pekin
  def quack
    puts 'quack.'
  end
end

[Mallard.new, Pekin.new].each(&:quack)

Mallard and Pekin both quack, so even though they’re different classes, we can use them in the same way. In Java, this would be accomplished through the use of an interface:

interface Duck {
   public void quack();
}

class Mallard implements Duck {
  public void quack() {
    System.out.println("QUACK!");
  }
}

class Pekin implements Duck {
  public void quack() {
    System.out.println("quack.");
  }
}

public class Quack {
  public static void main(String arg[]) {
    Duck[] ducks = {new Mallard(), new Pekin()};
    for (int i = 0; i < ducks.length; i++) {
      ducks[i].quack();
    }
  }
}

Go uses interfaces too, but slightly differently:

package main

import "fmt"

type Duck interface {
  Quack()
}

type Mallard struct{}
type Pekin struct{}

func (m Mallard) Quack() {
  fmt.Println("QUACK!")
}

func (p Pekin) Quack() {
  fmt.Println("quack.")
}

func main() {
  ducks := []Duck{Mallard{}, Pekin{}}
  for _, duck := range ducks {
    duck.Quack()
  }
}

And here’s the cool part about how Go uses interfaces: take a look at the definitions for Mallard and Pekin again. Neither one calls itself a duck. That happens automatically, by the fact that they implement Quack(), and that’s all the Duck interface asks for. They Quack() like Ducks, and therefore, they’re Ducks. This differs from the Java implementation in which each class had to explicitly call out the fact that they can be treated as Ducks. This, to me, seem like what Go has is truer duck-typing than Ruby. Why? Take another look at the Ruby implementation. What’s missing from it? Any mention anywhere of the name duck. Ruby doesn’t give a name to the grouping, only the behaviour. Go has duck-typing, Ruby has quack-typing.

Lisa’s Notes

Duck Typing