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                <h2 id="header"></h2>

<label for="someinput">Title: </label>
<input id="someinput" type="text">




                // Based on mithril-stream

function Stream(state) {
  let listeners = [];

  let stream = function(value) {
    if(arguments.length > 0) {
      state = value;
      listeners.forEach(fn => fn(value));

    return state;
  } = function(fn) {
    // Create new instance with transformed state.
    // This will execute the callback when calling `map`
    // this might not be what you want if you use a 
    // function that has side effects. Just beware.
    let target = Stream(fn(state));

    // Transform the value and update stream
    const listener = value => target(fn(value));

    // Update the source listeners

    return target;

  return stream;

function slug_str(value) {
  return value.toLowerCase().replace(/\W/g, "-")

function update_header(text) {
  header.textContent = text;

const title = Stream('Initial title');
const slug =;

// `update_header` returns undefined
// so the new stream is useless;

// Input initial state
someinput.value = title();

// Bind the input to header
someinput.addEventListener('keyup', e => title(