monad.js

const data = {
  a: {
    b: {
      c: 10
    }
  }
}

const pipe = (fs) => (v) => fs.reduce((acc, f) => f(acc), v)

const tap = (f) => (v) => {
  f(v)
  return v
}

const log = tap((v) => console.log('>>>', v))

const get = (key) => (data) => data[key] ? data[key] : null

console.log(

pipe([
  get('a'),
  get('b'),
  get('c')
])(data)

)

let bind = (v, f) => !v ? null : f(v)

let result = (v) => v

console.log(

bind(get(log('a'))(data), (a) =>
  bind(get(log('b'))(a), (b) =>
    bind(get(log('c'))(b), (c) =>
      result(c))))

)

const sum = (a, b, c) => a + b + c



console.log(

bind(null, (a) =>
  bind(2, (b) =>
    bind(3, (c) =>
      result(sum(a, b, c)))))

)

const maybeM = {
  unit: (v) => [v],
  bind: ([v], f) => !v ? [null] : [f(v)]
}

const writerM = (s = []) => ({
  unit: (v) => [s, v],
  bind: ([s, v], f) => {
    const [w, nv] = f(v)
    return [s.concat(w), nv]
  }
})

const w = (f, m) => (v) => [m, f(v)]

const doM = ({unit, bind}, fs) => (data) => pipe(fs.map((f) => (v) => bind(v, f)))(unit(data))

console.log(
  'doM:',
  doM(maybeM, [get('a'), get('b'), get('c')])(data),
  doM(writerM(), [w(get('a'), 'get a'), w(get('b'), 'get b'), w(get('c'), 'get c')])(data)
)