MayBe 函子

我们在编程过程中可能会遇到很多错误，需要对这些错误做相应的处理
MayBe 函子的作用就是可以对外不的空值情况做处理（控制副作用在允许的范围内）

 class MayBe {
   static of (value) {
     return new MayBe(value)
   }
   constructor (value) {
     this._value = value
   }
   map(fn) {
     return this.isNothing() ? MayBe.of(null) : MayBe.of(fn(this._value))
   }
   isNothing() {
     return this._value === null || this._value === undefined
   }
 }

 let r = MayBe.of('hello world').map(x => x.toUpperCase())
 let r = MayBe.of('hello world').map(x => x.toUpperCase()).map(x => null).map(x => x.split('  '))
 console.log(r)

MayBe函子的问题在于不知道是哪次调用出现了问题。Either函子解决这个问题=》下篇