结合镜片系列

Question

Monocle是一个很棒的库(而不是唯一一个)实现镜头模式的库,如果我们必须在巨大的嵌套对象中更改一个字段,这是很棒的.例如http://julien-truffaut.github.io/Monocle/

case class Street(number: Int, name: String)
case class Address(city: String, street: Street)
case class Company(name: String, address: Address)
case class Employee(name: String, company: Company)

以下样板

employee.copy(
  company = employee.company.copy(
    address = employee.company.address.copy(
      street = employee.company.address.street.copy(
        name = employee.company.address.street.name.capitalize // luckily capitalize exists
      )
    )
  )
)

可以轻松更换

import monocle.macros.syntax.lens._

employee
  .lens(_.company.address.street.name)
  .composeOptional(headOption)
  .modify(_.toUpper)

哪个好.据我了解,宏魔术将所有内容完全转换为与上面相同的代码.

但是,如果我想结合几个动作怎么办？如果我想通过一次通话同时更改街道名称,地址城市和公司名称怎么办？如下:

employee.copy(
  company = employee.company.copy(
    address = employee.company.address.copy(
      street = employee.company.address.street.copy(
        name = employee.company.address.street.name.capitalize // luckily capitalize exists
      ),
      city = employee.company.address.city.capitalize
    ),
    name = employee.company.name.capitalize
  )
)

如果我只是在这里重复使用镜头,我会有以下代码:

employee
  .lens(_.company.address.street.name).composeOptional(headOption).modify(_.toUpper)
  .lens(_.company.address.city).composeOptional(headOption).modify(_.toUpper)
  .lens(_.company.name).composeOptional(headOption).modify(_.toUpper)

哪个最终将转换为三个 employee.copy(...).copy(...).copy(...)调用,而不仅仅是一个 employee.copy(...).如何让它变得更好？

此外,应用一系列操作真的很棒.像对的序列一样,Seq[(Lens[Employee, String], String => String)]其中第一个元素是指向正确字段的镜头,第二个元素是修改它的函数.它将有助于从外部构建这样的操作序列.对于上面的例子:

val operations = Seq(
  GenLens[Employee](_.company.address.street.name) -> {s: String => s.capitalize},
  GenLens[Employee](_.company.address.city) -> {s: String => s.capitalize},
  GenLens[Employee](_.company.name) -> {s: String => s.capitalize}
)

或类似的东西......

Answer 1

据我了解,宏魔术将所有内容完全转换为与上面相同的代码.

它没有.

这个简单的代码:

employee.lens(_.name)
  .modify(_.capitalize)

变得像怪物那样的东西*:

monocle.syntax.ApplyLens(employee,
    new monocle.PLens[Employee, Employee, String, String] {
      def get(e: Employee): String = e.name;
      def set(s: String): Employee => Employee = _.copy(s);
      def modify(f: String => String): Employee => Employee = e => e.copy(f(e.name))
    }
}).modify(_.capitalize)

这与简单相去甚远

employee.copy(name = employee.name.capitalize)

并包括三个冗余对象(匿名镜头类,ApplyLens用于语法糖和lambda返回modify).我们通过capitalize直接使用而不是用作曲来跳过更多headOption.

所以不,没有免费的晚餐.然而,大多数情况下,它足够好,没有人关心额外的镜头对象和中间结果.

多个操作

你可以建立从多个镜头遍历(集光透镜),如果它们的类型一致(这里是Employee来String)

val capitalizeAllFields = Traversal.applyN(
  GenLens[Employee](_.name),
  GenLens[Employee](_.company.address.street.name),
  GenLens[Employee](_.company.address.city),
  GenLens[Employee](_.company.name)
).modify(_.capitalize)

这仍然会copy多次调用.为了提高效率,您可以使用Traversal.apply4et al.品种,这将要求你copy手动编写(我现在懒得做).

最后,如果要将各种转换应用于不同类型的字段,则应该使用modify并set返回类型函数的事实Employee => Employee.对于你的例子,那将是:

val operations = Seq(
  GenLens[Employee](_.company.address.street.name).modify(_.capitalize),
  GenLens[Employee](_.company.address.street.number).modify(_ + 42),
  GenLens[Employee](_.company.name).set("No Company Inc.")
)

val modifyAll = Function.chain(operations)

// does all above operations of course, with two extra copy calls
modifyAll(employee) 

* - 这是desugarAmmonite-REPL的简化输出.我跳过了modifyF,顺便说一下