observable.js

var observable = require("observable")

function input(elem) {
  var o = observable()

  o(elem.value)

  elem.addEventListener("change", function () { o(elem.value) })

  return o
}

var i = input(document.getElementById('some-input')

i('actually setting value. naughty naughty')

signal.js

var signal = require("observable/signal")

function input(elem) {
  // signal takes callback in which you have lexical access
  // to the value setting function
  // It also takes initial value for convenience
  return signal(function (setValue) {
    elem.addEventListener("change", function () { setValue(elem.value) })    
  }, elem.value)
}

var i = input(document.getElementById('some-input')

// THIS NOW THROWS EXCEPTION
i('actually setting value. naughty naughty')