使用shapeless scala合并两个不同case类的字段 [英] Using shapeless scala to merge the fields of two different case classes

问题描述

我想将两个不同案例类的字段合并为一个案例类.

I want to merge the fields of two different case classes into a single case class.

例如，如果我有以下案例类:

For example, if I have the following case classes:

case class Test(name:String, questions:List[Question], date:DateTime)

case class Answer(answers:List[Answers])

我想要一种简洁的无形方式将两者合并为:

I want a concise shapeless way to merge both into:

TestWithAnswers(name:String, questions:List[Question], date:DateTime, answers:List[Answer]). 

那里有一个很好的无形答案吗?

Is there a nice shapeless answer out there?

推荐答案

你可以使用 shapeless Generic 来做到这一点.

You can use shapeless Generic to do this.

val t: Test = ???
val a: Answer = ???
val gt = Generic[Test]
val ga = Generic[Answer]
val gta = Generic[TestWithAnswers]

val ta = gta.from(gt.to(t) ++ ga.to(a))

或者好吧，如果你有单线的东西

or well, if you have a thing for one-liners

val ta = Generic[TestWithAnswers].from(Generic[Test].to(t) ++ Generic[Answer].to(a))

提供的t是Test的实例，a是Answer的实例，ta 将是 TestWithAnswers 的一个实例.

Provided t is an instance of Test and a is an instance of Answer, ta will be an instance of TestWithAnswers.

快速解释:Generic 可以将 case 类转换为通用的 HList 表示.因此，根据您的类的结构，您可以简单地将 test 和 answer 都转换为一个 hlist，将这两个列表连接成一个列表，然后从中构建一个 TestWithAnswers 实例.

A quick explanation: Generic can convert a case class to and from a generic HList representation. So, given the structure of your classes, you can simply convert both test and answer to an hlist, concatenate the two lists into a single one and build a TestWithAnswers instance from it.

这是一个更简单的示例，您可以复制粘贴并在 REPL 中尝试

Here's a simpler example, which you can copy-paste and try in a REPL

import shapeless._
case class A(a: Int)
case class B(a: String)
case class AB(a: Int, b: String)
Generic[AB].from(Generic[A].to(A(42)) ++ Generic[B].to(B("foo")))
// AB(42, "foo")

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