5.1-函数式组合的精华.md

Code

• dispatch函数接收多个函数，依次执行直到返回一个非undefined的值。
• 返回高阶函数，接收业务函数真正的参数。

var dispatch = (...args) => {
	let funs = _.toArray(args),
	    size = funs.length;

	return (...params) => {
		let ret = undefined,
			funIndex,
			fun;

		for (funIndex = 0; funIndex < size; funIndex++) {
			fun = funs[funIndex];
			ret = fun.apply(null, params);

			if (ret != null) return ret;
		}

		ret;
	};
};

var stringReverse = (s) => {
	return !_.isString(s) ? undefined : s.split('').reverse().join();
};

var rev = dispatch(stringReverse);
rev('abscd');

用处

• 作为代理函数，负责找到适合的函数执行
• 可以替换switch case。
• 函数的组合，可以体现出更强大的威力。

 var notify = changeView = shutdown = _.identity;

 var performCommandHardCoded = (command) => {
 	let result;
 	 	switch(command.type) {
 		case 'notify':
 		  result = notify(command.message);
 		  break;
 		case 'join' :
 		  result = changeView(command.target);
 		  break;
 		default : 
 		  alert(command.type);
 	}
 	return result;
 };

 performCommandHardCoded({type : 'notify', message: 'hi'});
 performCommandHardCoded({type : 'join', target : 'target'});

 var isa = (type, action) => {
 	return (obj) => {
 		if (obj.type === type) {
 			return action(obj)
 		}
 	}
 };

var performCommand = dispatch(isa('notify', (obj) => obj.message), 
	                          isa('join', (obj) => obj.target),
	                          (obj) => alert(obj.type));

performCommand({type:'notify', message : 'notify'});
performCommand({type:'join', target : 'target'});

函数组合

组合的方式，往往函数的参数，是该函数返回的结果。

var performAdminCommand = dispatch(isa('kill', (obj) => obj.hostname), performCommand);
performAdminCommand({type: 'kill', hostname: 'foo.com'});
performAdminCommand({type:'join', target : 'join target'});