TypeScript(Structural Type System vs Nominal Type System)

Structural Type System

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구조가 같으면, 같은 타입이다.

interface IPerson {
  name: string;
  age: number;
  speak(): string;
}

type PersonType = {
  name: string;
  age: number;
  speak(): string;
};

let personInterface: IPerson = {} as any;
let personType: PersonType = {} as any;

personInterface = personType;
personType = personInterface;

Nominal Type System

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구조가 같아도 이름이 다르면, 다른 타입이다.

type PersonID = string & { readonly brand: unique symbol };

function PersonID(id: string): PersonID {
  return id as PersonID;
}

function getPersonById(id: PersonID) {}

getPersonById(PersonID('id-aaaaaa'));
getPersonById('id-aaaaaa'); // error TS2345: Argument of type 'string' is not assignable to parameter of type 'PersonID'. Type 'string' is not assignable to type '{ readonly brand: unique symbol; }'.

duck typing

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만약 어떤 새가 오리처럼 걷고, 헤엄치고, 꽥꽥거리는 소리를 낸다면
나는 그 새를 오리라고 부를 것이다.

class Duck:
   def sound(self):
      print u"꽥꽥"

class Dog:
   def sound(self):
      print u"멍멍"

def get_sound(animal):
   animal.sound()

def main():
   bird = Duck()
   dog = Dog()
   get_sound(bird)
   get_sound(dog)