Compile only edited haml file (using grunt-haml2html grunt task)

Gruntfile.js

module.exports = function(grunt) {

  require('load-grunt-tasks')(grunt);
  
  grunt.initConfig({
    
    haml: {
      options: {
        doubleQuoteAttributes : true
      },
      main: {
        files : [
          { expand: true, cwd:'.', src: '**/*.haml', dest: '.', ext : '.html' }
        ]
      },
      watch: {
        files : {}
      }
    },

    watch: {
      haml: {
        files: '**/*.haml',
        tasks: ['haml:watch'],
        options: {
          spawn: false
        }
      }
    }
    
  });

  grunt.event.on('watch', function(action, filepath) {
    if(filepath.indexOf('.haml') === -1) return;
    grunt.log.writeln('filepath = ' + filepath);
    var file = {};
    var destfile = filepath.replace('.haml','.html');
    file[destfile] = filepath;
    grunt.config('haml.watch.files', file);
  });

  grunt.registerTask('default', ['haml:main']);

};