mappables.js

/** Consider the interface

interface Mappable<A> {
  map: (A → B) → Mappable<B>
}

*/


function Identity(value) {
  return {
    map: function(f){ return Identity(f(value)) }
  }
}

function Sequence(xs) {
  return {
    map: function(f){ return Sequence(xs.map(f)) }
  }
}

function Map(o) {
  return {
    map: function(f){ return Map(mapObject(f, o)) }
  }

  function mapObject(f, o){
    return Object.keys(o).reduce(function(r, k){ r[k] = f(o[k]); return r }, {})
  }
}

function Stream(h, t) {
  return {
    map: function(f){
      return Stream(f(h), function(){ return t().map(f) })
    }
  }
}

Stream.Empty = {
  map: function(){ return Stream.Empty }
}

// So we can use all of these "Mappable" interfaces without caring about the object:

function succ(a){ return a + 1 }

var id = Identity(1)
id.map(succ)  // => Identity(2)

var xs = Sequence([1, 2, 3, 4])
xs.map(succ)  // => Sequence([2, 3, 4, 5])

var o = Map({ a: 1, b: 2, c: 3 })
o.map(succ)   // => Map({ a: 2, b: 3, c: 4 })

var s = Stream(1, function(){ return Stream(2, function(){ return Stream.Empty }) })
s.map(succ)   // => Stream(2, lazy(3, lazy(Empty)))