dewey92

interface Reader<D, A> {
  (deps: D): A
}

const fmap = <D, A, B>(f: (v: A) => B, readerA: Reader<D, A>): Reader<D, B> =>
  deps => f(readerA(deps))

const bind = <D, A, B>(f: (v: A) => Reader<D, B>, readerA: Reader<D, A>): Reader<D, B> =>
  deps => f(readerA(deps))(deps)

dewey92

interface IReader<D, A> {
  map<B>(f: (value: A) => B): IReader<D, B>,
  flatMap<B>(f: (value: A) => IReader<D, B>): IReader<D, B>,
  run(deps: D): A
}

const Reader = <D, A>(fn: (deps: D) => A): IReader<D, A> => {
  const run = (deps: D) => fn(deps);
  return {
    map: f => Reader((deps: D) => f(run(deps))),

dewey92

interface User {
  name: string
}

const generateId = () => (Math.random() * 10).toString();
const depsTest = {
  env: 'testing',
  db: {
    selectAll: (tableName: string) => [],
    select: (x: { id: string, name: string }) => x, // do nothing

dewey92

interface Reader<D, A> {
  (deps: D): A
}

// Hindley-Milner style:
// Reader d a = d -> a

dewey92

interface Reader<D, A> {
  (deps: D): A
}
// Hindley-Milner style:
// Reader d a = d -> a


const fmap = <D, A, B>(f: (v: A) => B, readerA: Reader<D, A>): Reader<D, B> =>
  deps => f(readerA(deps))
// ES6:

dewey92

fmap<A, B>(f: (v: A) => B, functorA: Functor<A>): Functor<B>
// for Reader, it should be
fmap<D, A, B>(f: (v: A) => B, readerA: Reader<D, A>): Reader<D, B>
  
// Hindley-Milner style
// fmap :: Functor f => (a -> b) -> f a -> f b

// fmap :: (a -> b) -> Reader d a -> Reader d b
// fmap :: (a -> b) -> (d -> a)   -> (d -> b)

dewey92

bind<A, B>(f: (v: A) => Functor<B>, functorA: Functor<A>): Functor<B>
// for Reader, it should be
bind<D, A, B>(f: (v: A) => Reader<D, B>, readerA: Reader<D, A>): Reader<D, B>

// Hindley-Milner style
// bind :: (a -> f b) -> f a -> f b
// bind :: (a -> Reader d b) -> Reader d a -> Reader d b

// karena (Reader d b) adalah (d -> b), maka
// bind :: (a -> d -> b) -> (d -> a) -> (d -> b)

dewey92

interface Reader<D, A> {
  (deps: D): A
}
// Hindley-Milner style:
// Reader d a = d -> a

const bind = <D, A, B>(f: (v: A) => Reader<D, B>, readerA: Reader<D, A>): Reader<D, B> =>
  deps => f(readerA(deps))(deps)
// ES6:
// const bind = (f, reader) => deps => f(reader(deps))(deps)

dewey92

interface Account {
  name: string,
  email: string,
}

interface AccountRepo {
  find: (no: string) => Account
}

const accRepo: AccountRepo = {

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),