Monads
In the above, flatMap
binds stuff together. All the combinator blocks are closure blocks. The map
block, for example, is accessing variables from its enclosing scopes. And all blocks are returning back a list of strings.
What does a flatMap
do? It maps and then flattens the result. For example, the following is a way to pick up numbers from List[Any]
. Using a map does not fully cut it:
scala> val l = List(1, "this", 2, 4.4, 'c') l: List[Any] = List(1, this, 2, 4.4, c) scala> l map { | case i: Int => Some(i) | case _ => None | } res0: List[Option[Int]] = List(Some(1), None, Some(2), None, None)
We just need the numbers; however, we get them wrapped up in Some
or we get them wrapped up in None
. We have already seen both how we could collect and a partial function for picking up numbers:
scala> l flatMap { | case i: Int => Some(i) | case _ => None | } res1: List[Int] = List(1, 2)
So, flatMap
does both the mapping...