Diff between async and yield-yield versions of postcss-cli

post-css-index.js

function processCSS(processor, input, output, fn) {
  function doProcess(css, fn) {
    function onResult(result) {
      if (typeof result.warnings === 'function') {
        result.warnings().forEach(console.error);
      }
      
      fn(null, result.css);
    }

    var result = processor.process(css, {
      safe: argv.safe,
      from: input,
      to: output
    });
    
    if (typeof result.then === 'function') {
      result.then(onResult).catch(fn);
    } else{
      process.nextTick(onResult.bind(null, result));
    }
  }

  async.waterfall([
    async.apply(readFile, input),
    doProcess,
    async.apply(writeFile, output)
  ], fn);
}

post-css-index.sync.js

var processCSS = require('yield-yield')(function* (processor, input, output) {
  var css = yield readFile(input, yield);

  function onResult(result) {
    if (typeof result.warnings === 'function') {
      result.warnings().forEach(console.error);
    }

    return result.css;
  }

  var result = processor.process(css, {
    safe: argv.safe,
    from: input,
    to: output
  });

  if (typeof result.then === 'function') {
    result = onResult(yield result);
  } else{
    result = onResult(result);
  }

  yield writeFile(output, result, yield);
});