How to represent a partial update on case classe in Scala ?

Question

I have the following case class :

case class PropositionContent(title:String,content:String)

And I would like to represent a partial modification of it as Data.

One way would be to create the case class :

case class PartialPropositionContent(title:Option[String],content:Option[String)

and then some methods :

object PropositionContent {

   def append( pc  : PropositionContent
             , ppc : PartialPropositionContent) =
   PropositionContent ( ppc.title.getOrElse(pc.title)
                      , ppc.content.getOrElse(pc.content) )

   def append( ppc  : PartialPropositionContent
             , ppc2 : PartialPropositionContent ):  PartialPropositionContent = {...}

}

But it's a bit boilerplaty !

I think a case class PropositionContent[M[_]](title:M[String],content:M[String]) will not really solve the stuff, and I don't know how to use Shapeless to solve the stuff.

So do you have an idea ?

Answer 1

Just ported Travis's code (which I don't understand) to Shapeless 2.1:

import shapeless._
import shapeless.ops.hlist._

object orElser extends Poly1 {
  implicit def default[A]: Case[Option[A]] { type Result = Option[A] => Option[A] } = at[Option[A]] {
    oa => (o: Option[A]) => oa orElse o
  }
}

object getOrElser extends Poly1 {
  implicit def default[A]: Case[Option[A]] { type Result = A => A } = at[Option[A]] {
    oa => (a: A) => oa getOrElse a
  }
}

import UnaryTCConstraint._

def append[U <: HList: *->*[Option]#λ, F <: HList](u: U, v: U)
                                                  (implicit mapper: Mapper.Aux[orElser.type, U, F],
                                                            zipper: ZipApply.Aux[F, U, U]): U =
  v map orElser zipApply u

def update[T, L <: HList, F <: HList, U <: HList](t: T, u: U)
                                                 (implicit gen: Generic.Aux[T, L],
                                                           mapped: Mapped.Aux[L, Option, U],
                                                           mapper: Mapper.Aux[getOrElser.type, U, F],
                                                           zipper: ZipApply.Aux[F, L, L]) =
  gen from (u map getOrElser zipApply (gen to t))

API is intact.

Answer 2

Here's a relatively boilerplate-free approach using Shapeless. First we define some polymorphic versions of the relevant functions on Option:

import shapeless._

object orElser extends Poly1 {
  implicit def default[A] = at[Option[A]] {
    oa => (o: Option[A]) => oa orElse o
  }
}

object getOrElser extends Poly1 {
  implicit def default[A] = at[Option[A]] {
    oa => (a: A) => oa getOrElse a
  }
}

We'll represent an update as an HList where every element is an Option, and we can write an append method that allows us to append two updates:

import UnaryTCConstraint._

def append[U <: HList: *->*[Option]#λ, F <: HList](u: U, v: U)(implicit
  mapper: MapperAux[orElser.type, U, F],
  zipper: ZipApplyAux[F, U, U]
): U = v.map(orElser).zipApply(u)

And finally we can write our update method itself:

def update[T, L <: HList, F <: HList, U <: HList](t: T, u: U)(implicit
  iso: Iso[T, L],
  mapped: MappedAux[L, Option, U],
  mapper: MapperAux[getOrElser.type, U, F],
  zipper: ZipApplyAux[F, L, L]
) = iso from u.map(getOrElser).zipApply(iso to t)

Now we need just a little bit of boilerplate (in the future this won't be necessary, thanks to inference-driving macros):

implicit def pcIso =
  Iso.hlist(PropositionContent.apply _, PropositionContent.unapply _)

We'll also define an alias for this specific update type (which isn't strictly necessary, but will make the following examples a little more concise):

type PCUpdate = Option[String] :: Option[String] :: HNil

And finally:

scala> val pc = PropositionContent("some title", "some content")
pc: PropositionContent = PropositionContent(some title,some content)

scala> val u1: PCUpdate = Some("another title") :: None :: HNil
u1: PCUpdate = Some(another title) :: None :: HNil

scala> val u2: PCUpdate = Some("newest title") :: Some("new content") :: HNil
u2: PCUpdate = Some(newest title) :: Some(new content) :: HNil

scala> append(u1, u2)
res0: PCUpdate = Some(newest title) :: Some(new content) :: HNil

scala> update(pc, append(u1, u2))
res1: PropositionContent = PropositionContent(newest title,new content)

Which is what we wanted.