dewey92

declare function convertCtoF(degree: Celcius): Fahrenheit;

declare const fiveC: Celcius;
declare const fiveF: Fahrenheit;

convertCtoF(fiveF); // error :)
// Argument of type 'Brand<"Fahrenheit", number>' is not assignable to parameter of type 'Brand<"Celcius", number>'.
//  Type 'Brand<"Fahrenheit", number>' is not assignable to type '{ _brand: "Celcius"; }'.
//    Types of property '_brand' are incompatible.
//      Type '"Fahrenheit"' is not assignable to type '"Celcius"'.

dewey92

type Brand<Name, Type> = Type & { _brand: Name };

type Celcius = Brand<'Celcius', number>;
type Fahrenheit = Brand<'Fahrenheit', number>;

dewey92

class A {}
class B {}

declare function onlyAcceptA(a: A): any;
declare const b: B;

onlyAcceptA(b); // typechecked :(

dewey92

class Temp<A> {
    private readonly __unit__: A;
    constructor(degree: number) { }
}

enum Unit { Celcius, Fahrenheit }

const isBoiling = (temp: Temp<Unit.Celcius>) => temp.degree >= 100;

const c100 = new Temp<Unit.Celcius>(100);

dewey92

class FormData<T> {
    private readonly __data__: T;
    constructor(value: any) { }
}

interface Unvalidated { __unval__: never }
interface Validated { __val__: never }
  
declare function validate(data: FormData<Unvalidated>): FormData<Validated>;
declare function sendToBackend(payload: FormData<Validated>): Promise<any>

dewey92

class Temp<A> {
    private readonly __unit__: A; // structural diff used here
    constructor(degree: number) { }
}

interface Celcius { __celcius__: never } // structural diff
interface Fahrenheit { __fahrenheit__: never } // structural diff

const isBoiling = (temp: Temp<Celcius>) => temp.degree >= 100;

dewey92

class Temp<A> {
    constructor(degree: number) { }
}

interface Celcius {}
interface Fahrenheit {}

declare function isBoiling(temp: Temp<Celcius>): boolean;

dewey92

const accRepo: AccountRepo = {
  find: no => ({ name: 'Bambang', email: 'tampan@tampan.com' })
};

declare function debit(no: string, amount: number): () => Reader<AccountRepo, Account>;
declare function credit(no: string, amount: number): () => Reader<AccountRepo, Account>;
declare function balance(no: string): () => Reader<AccountRepo, number>;
  
const transaksiBambangF = (no: string) => composeK(
  balance(no),    // <- Cleaner

dewey92

export const AsyncState = {  // <- HERE
  initial: 'initial',
  fetching: 'fetching',
  success: 'success',
  error: 'error'
}

const initialState = {
  asyncState: AsyncState.initial,
};

dewey92

const initialState = { ... };

export default (state = initialState, action) => {
  switch(action.type) {
    case 'GET_XXX_REQUEST':
      return {
        ...state,
        isLoading: true, isLoaded: false, error: null  // <- Here
      }
    case 'GET_XXX_SUCCESS':