JsResult-Monad还是适用性？

Question

My understanding of one of the distinctions between Monad and Applicative is that flatMap is available on Monad , but not Applicative .

If that's true, I'm confused by these Scala Play JSON docs :

So what's interesting there is that JsResult[A] is a monadic structure and can be used with classic functions of such structures:

flatMap[X](f: A => JsResult[X]): JsResult[X]

etc

But, then the docs go on to say:

Please note that JsResult[A] is not just Monadic but Applicative because it cumulates errors. This cumulative feature makes JsResult[T] makes it not very good to be used with for comprehension because you'll get only the first error and not all.

Since, as I understand, a for-comprehension is syntactic sugar for flatMap , how can JsResult be both a Applicative and Monad ?

Answer 1

Monad is a subclass of an Applicative . Applicative 's apply is weaker operation than flatMap . Thus apply could be implemented in terms of flatMap .

But , in the JsResult (or actually Reads ) case, it has special implementation which exploits Applicative computation's static form.

Eg the two definitions below behave equivalently with correct JSON, yet Applicative (which uses and ) have better error messages in erroneous cases (eg mentions if both bar and quux are invalid):

val applicativeReads: Reads[Foo] = (
  (__ \ "bar").read[Int] and
  (__ \ "quux").read[String]
)(Foo.apply _)

val monadicReads: Reads[Foo] = for {
  bar <- (__ \ "bar").read[Int]
  quux <- (__ \ "quux").read[String]
} yield Foo(bar, quux)