为只有一个字段的案例类派生类型类实例 [英] Deriving type class instances for case classes with exactly one field

问题描述

我正在研究 CSV 解析库(tabulate).它使用简单的类型类进行编码/解码:例如，编码是通过 CellEncoder(编码单个单元格)和 RowEncoder(编码整行)的实例完成的.

I'm working on a CSV parsing library (tabulate). It uses simple type classes for encoding / decoding: encoding, for example, is done with instances of CellEncoder (to encode a single cell) and RowEncoder (to encode entire rows).

使用 shapeless，我发现自动派生以下类型类实例非常简单:

Using shapeless, I've found it pretty straightforward to automatically derive the following type class instances:

• RowEncoder[A] 如果 A 是一个 case 类，其字段都有一个 CellEncoder.
• RowEncoder[A] 如果 A 是一个 ADT，其替代项都有一个 RowEncoder.
• CellEncoder[A] 如果 A 是一个 ADT，其替代项都有一个 CellEncoder.

• RowEncoder[A] if A is a case class whose fields all have a CellEncoder.
• RowEncoder[A] if A is an ADT whose alternatives all have a RowEncoder.
• CellEncoder[A] if A is an ADT whose alternatives all have a CellEncoder.

问题是，最后一个在现实生活中几乎完全没用:ADT 的替代品几乎总是案例类，我无法为具有以下特征的案例类派生 CellEncoder不止一个领域.

The thing is, this last one turns out to be almost entirely useless in real life situations: an ADT's alternatives are almost always case classes, and I cannot derive a CellEncoder for a case class that has more than one field.

但是，我希望能够为具有类型为 CellEncoder 的单个字段的案例类派生一个 CellEncoder.例如，这将涵盖 Either、scalaz 的 \/、cats 的 Xor...

What I'd like to be able to do, however, is derive a CellEncoder for case classes that have a single field whose type has a CellEncoder. That would cover, for example, Either, scalaz's \/, cats' Xor...

这是我目前所拥有的:

implicit def caseClass1CellEncoder[A, H](implicit gen: Generic.Aux[A, H :: HNil], c: CellEncoder[H]): CellEncoder[A] =
    CellEncoder((a: A) => gen.to(a) match {
      case h :: t => c.encode(h)
    })

显式使用时效果很好:

case class Bar(xs: String)
caseClass1CellEncoder[Bar, String]
res0: tabulate.CellEncoder[Bar] = tabulate.CellEncoder$$anon$2@7941904b

但是我无法让它隐式工作，以下失败:

I can't however get it to work implicitly, the following fails:

implicitly[CellEncoder[Bar]]
>> could not find implicit value for parameter e: tabulate.CellEncoder[Test.this.Bar]

我也尝试了以下方法，但都没有成功:

I've also tried the following, with no more success:

implicit def testEncoder[A, H, R <: H :: HNil](implicit gen: Generic.Aux[A, R], c: CellEncoder[H]): CellEncoder[A] =
      CellEncoder((a: A) => gen.to(a) match {
        case h :: t => c.encode(h)
      })

我错过了什么吗?我正在尝试做的事情有可能吗?

Am I missing something? Is what I'm trying to do even possible?

推荐答案

正确推断 H 有点棘手，但您可以使用 <:< 实例:

It's a little tricky to get the H inferred correctly, but you can do it with a <:< instance:

import shapeless._

case class CellEncoder[A](encode: A => String)

implicit val stringCellEncoder: CellEncoder[String] = CellEncoder(identity)
implicit val intCellEncoder: CellEncoder[Int] = CellEncoder(_.toString)

case class Bar(xs: String)

implicit def caseClass1CellEncoder[A, R, H](implicit
  gen: Generic.Aux[A, R],
  ev: R <:< (H :: HNil),
  c: CellEncoder[H]
): CellEncoder[A] = CellEncoder(
  (a: A) => ev(gen.to(a)) match {
    case h :: t => c.encode(h)
  }
)

(为了完整的工作示例，我编写了一个简单的 CellEncoder.)

(I've made up a simple CellEncoder for the sake of a complete working example.)

这是可行的，因为当编译器在寻找 Generic.Aux[A, R] 实例时可以推断 R，然后可以指导 H 查找 ev 的值时.

This works because R can be inferred when the compiler is looking for an Generic.Aux[A, R] instance, and can then guide the inference of H when looking for a value for ev.

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