map.js

function Map(source, f) {
  this.f = f
  this.source = source
}
MapInput.prototype = Object.create(InputPort.prototype)
MapInput.prototype.take = function() {
  var take = this.source.take()
  var f = this.f
  return Object.create(take, {
    valueOf: function() {
      return f(take.valueOf())
    }
  })
}

function map(source, f) {
  return new Map(source, f)
}