scala - Shapeless: generic lens parameterized by case class or field

Question

Based on:

import shapeless._

case class Content(field: Int)
lens[Content] >> 'field

I am trying to make a lens-creating method, something along:

def makeLens[T <: Product](s: Symbol) = lens[T] >> s

But it seems non-obvious. Is it possible to do?

If not, the end result I'm trying to achieve is a generic method for updating nested Maps with case-class contents, e.g.:

import scalaz._
import Scalaz._
import PLens._
import shapeless._
import shapeless.contrib.scalaz._

def nestedMapLens[R, T <: Product](outerKey: String, innerKey: Int, f: Symbol) =
  ~((lens[T] >> f).asScalaz) compose mapVPLens(innerKey) compose mapVPLens(outerKey)

I cannot get it to work when parameterized by T and f. Are there any other idiomatic boilerplate-free solutions?

Thanks!

Answer 1

The problem with your makeLens is that we want e.g. makeLens[Content]('foo) to fail at compile time, and that's not possible with an ordinary Symbol argument. You need some extra implicit arguments to track the singleton type for the given name and to provide evidence that it's the name of a member of the case class:

import shapeless._, ops.record.{ Selector, Updater }, record.FieldType

class MakeLens[T <: Product] {
  def apply[K, V, R <: HList](s: Witness.Aux[K])(implicit
    gen: LabelledGeneric.Aux[T, R],
    sel: Selector.Aux[R, K, V],
    upd: Updater.Aux[R, FieldType[K, V], R]
  ): Lens[T, V] = lens[T] >> s
}

def makeLens[T <: Product] = new MakeLens[T]

And then:

scala> case class Content(field: Int)
defined class Content

scala> makeLens[Content]('field)
res0: shapeless.Lens[Content,Int] = shapeless.Lens$$anon$6@7d7ec2b0

But makeLens[Content]('foo) won't compile (which is what we want).

You need the same kind of tracking for your nestedMapLens:

import scalaz._, Scalaz._
import shapeless.contrib.scalaz._

case class LensesFor[T <: Product]() {
  def nestedMapLens[K, V, R <: HList](
    outerKey: String,
    innerKey: Int,
    s: Witness.Aux[K]
  )(implicit
    gen: LabelledGeneric.Aux[T, R],
    sel: Selector.Aux[R, K, V],
    upd: Updater.Aux[R, FieldType[K, V], R]
  ): PLens[Map[String, Map[Int, T]], V] =
    (lens[T] >> s).asScalaz.partial.compose(
      PLens.mapVPLens(innerKey)
    ).compose(
      PLens.mapVPLens(outerKey)
    )
}

Note that I'm assuming a build.sbt like this:

scalaVersion := "2.11.2"

libraryDependencies ++= Seq(
  "com.chuusai" %% "shapeless" % "2.0.0",
  "org.typelevel" %% "shapeless-scalaz" % "0.3"
)

Now let's define an example map and some lenses:

val myMap = Map("foo" -> Map(1 -> Content(13)))

val myFoo1Lens = LensesFor[Content].nestedMapLens("foo", 1, 'field)
val myBar2Lens = LensesFor[Content].nestedMapLens("bar", 2, 'field)

And then:

scala> myFoo1Lens.get(myMap)
res4: Option[Int] = Some(13)

scala> myBar2Lens.get(myMap)
res5: Option[Int] = None

This is about as "boilerplate-free" as you're going to get. The messy implicit argument lists are intimidating at first, but you get used to them pretty quickly, and their role in pulling together different bits of evidence about the types you're working with becomes fairly intuitive after a little practice.