Functor chaining with null handler

disjunction.js

/**
 * Test for null when chaining functor wrappers
 * If not null, call chained functions on passed value
 * If null occurs, disregard the function and pass along null
 * When folding value, provide error and success handlers
 */

// functor setup

const Call = x =>
({
  map: f => Call(f(x)),
  fold: (f, g) => g(x)
})

const Pass = x =>
({
  map: f => Pass(x),
  fold: (f, g) => f(x)
})

// null check

const check = x => x != null ? Call(x) : Pass(null);

// instance code

let ideas = {
  mersault: "l'ennui est forte",
  estragon: "we haven't any rope"
}

// function with potentially null return value

let idea = person => check(ideas[person])

// functor chains

let getIdea = person => idea(person)
  .map(x => x.toUpperCase())
  .fold(x => "that person isn't here", x => person + ' says ' + x)


getIdea('mersault')
// -> mersault says L'ENNUI EST FORTE

getIdea('joe')
// -> that person isn't here