用于收集参数的 Scala 镜头

Question

使用镜头更新集合中元素的最佳方法是什么？例如:

case class Ingredient(name: String, quantity: Int)
case class Recipe(val ingredients: List[Ingredient])

如果我想使用镜片来创建一个新配方，并改变单一成分的数量，那么最好的方法是什么？

我尝试过的方法是即时创建一个镜头: Lens[List[Ingredient], Ingredient] .不过这感觉有点笨拙:

case class Recipe(val ingredients: List[Ingredient]) {
  import Recipe._
  def changeIngredientQuantity(ingredientName: String, newQuantity: Int) = {
    val lens = ingredientsLens >=> ingredientLens(ingredientName) >=> Ingredient.quantityLens
    lens.set(this, newQuantity)
  }
}

object Recipe {
  val ingredientsLens = Lens.lensu[Recipe, List[Ingredient]](
    (r, i) => r.copy(ingredients = i),
    r => r.ingredients
  )
  def ingredientLens(name: String) = Lens.lensu[List[Ingredient], Ingredient](
    (is, i) => is.map (x => if (x.name == name) i else x),
    is => is.find(i => i.name == name).get
  )
}

case class Ingredient(name: String, quantity: Int)

object Ingredient {
  val quantityLens = Lens.lensu[Ingredient, Int](
    (i, q) => i.copy(quantity = q),
    i => i.quantity
  )
}

Answer 1

您不能在给定索引处的 List[T] 和 T 之间创建 Lens，因为 Lens 要求您聚焦的对象始终存在。但是，在查找 List 或另一个集合的情况下，索引处可能没有元素。

但是，您可以使用 Traversal，这是一种聚焦 0 到多个元素的 Lens。与 Monocle ，您将使用 index 函数创建从列表到给定索引处的元素的遍历:

import monocle.SimpleLens
import monocle.syntax.lens._     // to use |-> and |->> instead of composeLens, composeTraversal
import monocle.functions.Index._ // to use index Traversal

// monocle also provides a macro to simplify lens creation
val ingredientsLens = SimpleLens[Recipe, List[Ingredient]](_.ingredients, (recipe, newIngredients)  => recipe.copy(ingredients = newIngredients))  
val quantityLens    = SimpleLens[Ingredient, Int](_.quantity            , (ingredient, newQuantity) => ingredient.copy(quantity = newQuantity))  

val applePie = Receipe(List(Ingredient("apple", 3), Ingredient("egg", 2), ...))


applePie |-> ingredientsLens |->> index(0)   headOption // Some(Ingredient("apple", 3))
applePie |-> ingredientsLens |->> index(999) headOption // None
applePie |-> ingredientsLens |->> index(0) |->> quantityLens headOption // 3
applePie |-> ingredientsLens |->> index(0) |->> quantityLens set 5 
// Receipe(List(Ingredient("apple", 5), Ingredient("egg", 2), ...))