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  <h3>Mori Painter</h3>
<div id='container'>
  <canvas id='canvas'></canvas>
  <button id='undo'>↶</button>


                body {
  font-family: monospace;  

canvas {
  position: relative;

  width: 200px;
  height: 200px;
  border: solid 5px #000;  


                const {
  list, vector, peek, pop, conj, map, assoc, zipmap,
  range, repeat, each, intoArray, toJs, isEmpty,
} = mori;

const log = (...args) => {

// the dimensions of the canvas
const [height, width] = [20, 20];
// the size of each canvas pixel
const pixelSize = 10;

// converts an integer to a 2d coordinate
const to2D = (i) => vector(
  i % width,
  Math.floor(i / width)

// sequence of coordinates in the canvas
const coords = map(to2D, range(height * width));

// lazy sequence of colors
const colors = repeat('#fff');

// colors stored against coordinates
const pixels = zipmap(coords, colors);

// update pixels to reflect drawing at x, y
const draw = (x, y, pixels, color='#000') => {
  const coord = vector(x, y); 
  return assoc(pixels, coord, color);

// paint the values in pixels onto the ctx
const paint = (ctx, pixels) => {
  const px = pixelSize;
  each(pixels, p => {
    const [coord, color] = intoArray(p);
    const [x, y] = intoArray(coord);
    ctx.fillStyle = color;
    ctx.fillRect(x * px, y * px, px, px);

const canvas = document.getElementById('canvas');
const context = canvas.getContext('2d');
const undo = document.getElementById('undo');

canvas.width = width * pixelSize;
canvas.height = height * pixelSize;
paint(context, pixels);

// keep track of current frames with rebindable variable
let frames = list(pixels);

canvas.addEventListener('click', e => {
  const x = Math.floor(e.layerX / pixelSize);
  const y = Math.floor(e.layerY / pixelSize);
  const pixels = draw(x, y, peek(frames));
  frames = conj(frames, pixels);
  paint(context, pixels)

undo.addEventListener('click', e => {
  if(count(frames) > 1) {
    frames = pop(frames); 
    paint(context, peek(frames));