What is the point of this monadic law?

问题

I am reading an article about "monadic laws". The first law the article mentions is:

m map f ≡ m flatMap {x => unit(f(x))}

For Scala Option it means:

option map f ≡ option flatMap {x => Option(f(x))}

Now I wonder what the law point is. Why is the law important ? What if Scala Option does not obey this law ?

回答1:

If it does not obey the monad laws it's not a monad. That's actually why the unit of Option is Some.apply and not Option.apply. Just look at this case:

scala> val f = (x: Int) => null

scala> (option map f) == (option flatMap {x => Option(f(x))})
res4: Boolean = false

The particular law here just says, that map is basically a composition of flatMap and unit

来源：https://stackoverflow.com/questions/20125797/what-is-the-point-of-this-monadic-law