使用单个字段派生案例类的解码器实例 [英] Deriving decoder instances of case classes with a single field

问题描述

这个问题与这个密切相关一个，但不完全相同:上下文相同但问题相反.

This question is tightly related to this one, but not identical: it's the same context but the opposite problem.

我正在尝试导出一个 CellDecoder[A]，它本质上是一个 String =>A，对于 A 是具有单个字段的案例类，其类型具有 CellDecoder.

I'm trying to derive a CellDecoder[A], which is essentially a String => A, for As that are case classes with a single field whose type has a CellDecoder.

为了做到这一点，我需要:

In order to do this, I need to require:

• A 可转换为 R，其中 R 是 H :: HNil 的子类型.
• H 拥有 CellDecoder 实例.

• A to be transformable to R, where R is a subtype of H :: HNil.
• H to have CellDecoder instance.

即:

implicit def caseClassCellDecoder[A, R, H](implicit
  gen: Generic.Aux[A, R],
  ev: R <:< (H :: HNil),
  d: CellDecoder[H]
): CellDecoder[A] = ???

我遇到的问题是，一旦我通过相关的 CellDecoder 实例将 String 转换为 H，我我有点卡住了:ev 允许我将 R 变成 H :: HNil，但不是 H ::HNil 转换为 R.没有 R，我不能使用我的 Generic.Aux[A, R] 来获得最终的 A 实例.

The issue I'm having is that, once I've turned a String to an H through the relevant CellDecoder instance, I'm kind of stuck: ev allows me to turn an R into an H :: HNil, but not an H :: HNil into an R. Not having an R, I can't use my Generic.Aux[A, R] to get the final A instance.

事实证明，将我的 H :: HNil 转换为 R 是有效的，但我不确定为什么并且无法说服自己它总是就是这样.

It turns out that casting my H :: HNil into an R works, but I'm not sure why and can't convince myself that it'll always be the case.

我想过要求 R 严格等于 H :: HNil(也就是说，有一个 R =:= HNil)，但随后隐式无法解析.

I thought about requiring R to be strictly equal to H :: HNil (that is, to have a R =:= HNil), but then the implicits fail to resolve.

有没有办法做我想做的事?

Is there a way to do what I'm trying to do?

推荐答案

我确实使用了 =:= 而不是 <:< 但是交换了 =:=code>R 和 H::HNil 并且这似乎有效:

I did use =:= instead of <:< however swapped the sides of R and H::HNil and that seemed to work:

case class CellDecoder[A](decode: String => A)

implicit val stringCellDecoder: CellDecoder[String] = CellDecoder(identity)

implicit def caseClassCellDecoder[A, R, H](implicit
                                           gen: Generic.Aux[A, R],
                                           ev: (H :: HNil) =:= R,
                                           d: CellDecoder[H]
                                            ): CellDecoder[A] = CellDecoder(
  (str: String) => gen.from(ev(d.decode(str)::HNil))
)

所以我可以按如下方式使用它:

So I was able to use it as follows:

case class TestDecoder(test: String)

def doDecode[A: CellDecoder](str: String): A = {
  implicitly[CellDecoder[A]].decode(str)
}

doDecode[TestDecoder]("encoded value")

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