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scalashapeless

Does Shapeless 2.0.0 lens work with Lists/collections?

If I have this:

case class Foo(
    age: Int,
    name: String,
    bar: List[Bar],
    alive: Boolean
    )
case class Bar(
    hey: String,
    you: String
    )

Can I create a lens that can get/set foo.bar(1).you?

Solution

  • Not out of the box, and you really need a partial lens to look things up in a list by index responsibly. Scalaz provides what you need, though, and shapeless-contrib makes interoperability easy:

    import scalaz.PLens._
import shapeless.lens
import shapeless.contrib.scalaz._

val secondBar = (lens[Foo] >> 'bar).asScalaz.partial.andThen(listNthPLens(1))

    And then:

    scala> secondBar.get(fooB)
res0: Option[Bar] = None

scala> secondBar.get(fooA)
res1: Option[Bar] = Some(Bar(foo,bar))

scala> secondBar.set(fooA, Bar("bar", "foo"))
res2: Option[Foo] = Some(Foo(1,,List(Bar(,), Bar(bar,foo)),false))

scala> secondBar.set(fooB, Bar("bar", "foo"))
res3: Option[Foo] = None

    If you don't mind living dangerously, you could also write your own Shapeless lens for looking up locations in a list (with a type like Lens[List[A], A]), but that would be giving up a lot of the value of both Shapeless and lenses.