postcss-cli with yield-yield

index.js

var sync = require('yield-yield');

var processCSS = sync(function* (processor, input, output) {

    var result = yield fs.readFile(input, 'utf8', yield);
    var css = result[1];

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

    var content;
    if (typeof result.then === 'function') {
      content = yield result;
    } else{
      yield process.nextTick(yield);
      content = result;
    }
    
    if (typeof content.warnings === 'function') {
        content.warnings().forEach(console.error);
    }
    
    yield fs.writeFile(output, content, yield);
});

sync.run(function* () {
    for (var i = 0; i < argv._.length; i++) {
        var output = argv.output;
        if(!output) {
            output = path.join(argv.dir, path.basename(input));
        }
        
        var res = yield processCSS(processor, input, output);
       
        if (!res[0]) { continue; }
        
        if (err.message && typeof err.showSourceCode === 'function') {
          console.error(err.message, err.showSourceCode());
        } else {
          console.error(err);
        }
        
        process.exit(1);
        
    }
});