Using shapeless scala to merge the fields of two different case classes

Question

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

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])

I want a concise shapeless way to merge both into:

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

Is there a nice shapeless answer out there?

Answer 1

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))

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

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

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

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.

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")