Lerning monads via https://drboolean.gitbooks.io/mostly-adequate-guide/content/

monad.js

const
  R = require('ramda'),
  fs = require('fs')

// == Maybe ====================================================================
const
  Maybe = function(x) {
    this.__value = x
  }

Maybe.of = function(x) {
  return new Maybe(x)
}

Maybe.prototype.isNothing = function() {
  return null === this.__value || 'undefined' === typeof this.__value
}

Maybe.prototype.map = function(f) {
  return this.isNothing() ? Maybe.of(null) : Maybe.of(f(this.__value))
}

Maybe.prototype.join = function() {
  return this.isNothing() ? Maybe.of(null) : this.__value
}

Maybe.prototype.chain = function(f) {
  return this.map(f).join()
}
// == Maybe ====================================================================

// == IO =======================================================================
const
  IO = function(f) {
    this.__value = f
  }

IO.of = function(x) {
  return new IO(function() {
    return x
  })
}

IO.prototype.map = function(f) {
  return new IO(R.compose(f, this.__value))
}

IO.prototype.join = function() {
  return this.__value()
}

IO.prototype.chain = function(f) {
  return this.map(f).join()
}
// == IO =======================================================================

const
  readFile = function(filename) {
    return new IO(function() {
      return fs.readFileSync(filename, 'utf-8')
    })
  },

  print = function(x) {
    return new IO(function() {
      console.log('Printing:', x)

      return x
    })
  },

  safeProp = R.curry(function(x, obj) {
    return new Maybe(obj[x])
  }),

  safeHead = safeProp(0),

  join = m => m.join(),

  // chain = R.curry(function(f, m) {
  //   return m.chain(f)
  // })

  cat = R.compose(
    join,
    R.chain(print),
    readFile
  ),

  firstAddressStreet = R.compose(
    R.chain(safeProp('street')),
    R.chain(safeHead),
    safeProp('addresses')
  )

console.log('== IO ==================================')
console.log(
  cat('package.json')
)

console.log('\n== Maybe (basic) =======================')
console.log(
  Maybe.of('Malkovich Malkovich').map(R.match(/a/ig))
)
console.log(
  Maybe.of(null).map(R.match(/a/ig))
)
console.log(
  Maybe.of({ 'name': 'Pissa' }).map(R.prop('age')).map(R.add(10))
)
console.log(
  Maybe.of({ 'name': 'Suda', 'age': 36 }).map(R.prop('age')).map(R.add(10))
)

console.log('\n== Maybe (nested) ======================')
console.log(
  firstAddressStreet({addresses:	[{street:	{name:	'Mulburry',	number:	8402},	postcode:	"WC2N"	}]})
)

console.log('\n========================================')