问题
There's an example of a Scalaz map lens here: Dan Burton calls it containsKey
, and it's inspired by the Edward Kmett talk. There is also something called mapVPLens
in Scalaz 7 which is useful for modifying values in a map.
My question is: if I have a lens for modifying type V
, and a lens for a Map[K,V]
, how can I compose them? I've been searching for a while for a good simple example, but there's still a dearth of examples in Scalaz.
I'm interested in both Scalaz 6 and Scalaz 7 solutions.
回答1:
If the lens you're trying to compose with the map lens is a partial lens, you can just use compose
:
import scalaz._, Scalaz._, PLens._
def headFoo[A] = listHeadPLens[A] compose mapVPLens("foo")
And then:
scala> headFoo.get(Map("foo" -> List(42)))
res0: Option[Int] = Some(42)
scala> headFoo.get(Map("foo" -> Nil))
res1: Option[Nothing] = None
scala> headFoo.get(Map("bar" -> List(13)))
res2: Option[Int] = None
Note that this is Scalaz 7.
If the lens you want to compose isn't partial, you can make it so with ~
:
scala> def firstFoo[A, B] = ~Lens.firstLens[A, B] compose mapVPLens("foo")
firstFoo: [A, B]=> scalaz.PLensFamily[Map[String,(A, B)],Map[String,(A, B)],A,A]
scala> firstFoo.get(Map("foo" -> (42, 'a)))
res6: Option[Int] = Some(42)
There's also a .partial
method if you don't like the unary operator.
来源:https://stackoverflow.com/questions/18835140/scalaz-how-to-compose-a-map-lens-with-a-value-lens