Index

Index is a typeclass that provides an AffineTraversal, which allows reading, writing or deleting the value associated with a key in a Map-like containers.

/**
  * @tparam S source of an AffineTraversal
  * @tparam I index an AffineTraversal
  * @tparam A target of an AffineTraversal
  */
trait Index[S, I, A] {
  def ix(i: I):  AffineTraversal[S, A]
}

It defines one method ix, which given an index of I returns an AffineTraversal.

Constructing an Index

Index[S, I, A] is constructed using the index function.
For a given Index[S, I, A] it takes two functions as arguments, viewOrModify: I => S => Either[T, A] which is a matching function that produces an Either[T, A] given an index of I and a structure of S, and set: I => S => A => S, a function which takes an index of I, a structure S and a focus A and returns a new structure of S.

object Index {
  def index[S, I, A](viewOrModify: I => S => Either[S, A])(set: I => S => A => S): Index[S, I, A]
}

A more concise version would be using the fromPreview method

object Index {
  def fromPreview[S, I, A](preview: I => S => Option[A])(set: I => S => A => S): Index[S, I, A]
}

An even more concise version would be using the fromPartial method

object Index {
  def fromPartial[S, I, A](preview: I => PartialFunction[S, A])(set: I => S => A => S): Index[S, I, A]
}

Most of the time we will be using the predefined implicit instances of Index, for example an implicit instance of Map. We can use the apply method to summon an instance of Map.

val indexMap = Index[Map[String, List[String]], String, List[String]]

// import proptics.Index

import proptics.instances.index._
import proptics.typeclass.Index // summoner of the instance
// import proptics.instances.index._

val seriesMap: Map[String, List[String]] = Map[String, List[String]](
  "tt0903747" -> List("True Detective", "Fargo", "Dexter"),
  "tt2356777" -> List("Breaking Bad", "Fargo", "Dexter"),
  "tt2802850" -> List("Breaking Bad", "True Detective", "Dexter"),
  "tt0773262" -> List("Breaking Bad", "True Detective", "Fargo")
)
// seriesMap: Map[String,List[String]] = 
//   Map(tt0903747 -> List(True Detective, Fargo, Dexter), 
//       tt2356777 -> List(Breaking Bad, Fargo, Dexter), 
//       tt2802850 -> List(Breaking Bad, True Detective, Dexter), 
//       tt0773262 -> List(Breaking Bad, True Detective, Fargo))

// summon of the instance
val indexMap: Index[Map[String, List[String]], String, List[String]] =
  Index[Map[String, List[String]], String, List[String]]  
// proptics.Index[Map[String,List[String]],String,List[String]] = 
//   proptics.instances.IndexInstances$$anon$8@46398364

atMap.at("tt0903747").set(List())(seriesMap)
// res0: Option[List[String]] = Some(List(True Detective, Fargo, Dexter))