具有不同属性名称的Scala通用类型层次结构 [英] scala generic type hierarchy with different attribute name

问题描述

我使用 https://pureconfig.github.io/来加载配置值.例如，对于数据库中的每个表，我都存储(db: String, table: String).但是，我需要表示特定的表.因此，每个人都有各自的特质.即:

I use https://pureconfig.github.io/ to load configuration values. For example for each table in a database, I store (db: String, table: String).However, I need to denote specific tables. Therefore, each one has a separate trait. I.e.:

trait Thing
trait ThingWithStuff extends Thing {
    def value:String
}

trait FooThing extends Thing{
    def fooThing: ThingWithStuff
}

trait BarThing extends Thing{
    def barThing: ThingWithStuff
}

它们都具有不同的属性名称，且具有相同的类型，而属性名称又包含db和table.使用某些方法处理这些文件时:

They all have a different attribute name with the same type which in return holds i.e. db and table. When processing these with some methods:

def myMethodFoo(thing:FooThing)= println(thing.fooThing)
def myMethodBar(thing:BarThing)= println(thing.barThing)

它导致代码重复.试图使用泛型修复这些问题，我无法编写类似这样的函数:

it leads to code duplication. Trying to fix these using generics I am not able to write a function like:

def myMethod[T<: Thing] = println(thing.thing)

，因为属性名称会有所不同. 有没有解决的聪明方法? 注意:

as the attribute name would be different. Is there a smart way around it? Note:

table-first {
db = "a"
table = "b"
}
table-second {
db = "foo"
table = "baz"
}

不能在前面具有相同的标识符，否则它将覆盖每个值以仅保留该标识符的最后一项的值.因此，我求助于使用不同的属性名称(table-first, table-second或专门用于示例:fooThing, barThing)

cannot have the same identifier up front as otherwise it would overwrite each value to hold only the value of the last item for this identifier. Therefore, I resorted to use different attribute names (table-first, table-second or specifically for the example: fooThing, barThing)

如何解决此问题以防止代码重复?

How can I fix this issue to prevent code duplication?

推荐答案

以下是对FooThing和BarThing使用类型类的解决方案:

Here is a solution using type classes for FooThing and BarThing:

trait Thing

trait ThingWithStuff {
    def value: String
}

trait FooThing extends Thing {
    def fooThing: ThingWithStuff
}

trait BarThing extends Thing {
    def barThing: ThingWithStuff
}

// Define implicits:

trait ThingEx[SomeThing <: Thing] {
  def extract(thing: SomeThing): ThingWithStuff
}

implicit val fooThingEx = new ThingEx[FooThing]{
  def extract(thing: FooThing): ThingWithStuff = thing.fooThing
}

implicit val barThingEx = new ThingEx[BarThing]{
  def extract(thing: BarThing): ThingWithStuff = thing.barThing
}

// Define the method:

def myMethod[SomeThing <: Thing](thing: SomeThing)(implicit thingEx: ThingEx[SomeThing]) =
  println(thingEx.extract(thing).value)

// Try it out:

val foo = new FooThing {
  def fooThing = new ThingWithStuff {
    def value = "I am a FooThing!"
  }
}


val bar = new BarThing {
  def barThing = new ThingWithStuff {
    def value = "I am a BarThing!"
  }
}

myMethod(foo)

myMethod(bar)

结果:

I am a FooThing!
I am a BarThing!

尝试一下！

基本上，我们在没有任何差异的情况下创建"多态性-两个隐式ThingEx允许您将fooThing和barThing绑定在一起.您只需定义一次此绑定-然后就可以在任何地方使用它.

Basically, we "create" polymorphism where there isn't any - the two implicit ThingEx allow you to bind fooThing and barThing together. You only have to define this bind once - and then you can use it everywhere.

如果您是 ad-hoc-polymorphism 和 type类的新手，则可以开始

If ad-hoc-polymorphism and type classes are new to you, you can start here for example.

我希望这会有所帮助！

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