无法找到或构造类型的读取实例：选项 [A]

Question

为什么 doobie 不能将查询转换为 Option[A]？

abstract class CRUDAbs[A: Read](val tableName: String) extends TransactSQL {
  def table: Fragment = Fragment.const(s"$tableName")
  def columnsList: Array[String] = {
    val cls = classTag[A].runtimeClass
    cls.getDeclaredFields.map(_.getName).map(snakeCase)
  }
  def columns: Fragment = Fragment.const(columnsList.mkString(","))
  def find(id: Int): doobie.Query0[Option[A]] =
    (sql"select " ++ columns ++ sql" from " ++ table ++ sql" where " ++ Fragment.const(
      s"${columnsList.head}"
    ) ++ sql" = $id").query[Option[A]]

我收到一个错误

Cannot find or construct a Read instance for type:

  Option[A]

我错过了什么？

Answer 1

如果我正确恢复了您的代码片段，就像

import doobie.Read
import doobie.implicits.toSqlInterpolator
import doobie.util.fragment.Fragment

import scala.reflect.{ClassTag, classTag}

object App {
  val snakeCase = ???

  abstract class CRUDAbs[A: Read: ClassTag](val tableName: String) /*extends TransactSQL*/ {
    def table: Fragment = Fragment.const(s"$tableName")

    def columnsList: Array[String] = {
      val cls = classTag[A].runtimeClass
      cls.getDeclaredFields.map(_.getName).map(snakeCase)
    }

    def columns: Fragment = Fragment.const(columnsList.mkString(","))

    def find(id: Int): doobie.Query0[Option[A]] =
      (sql"select " ++ columns ++ sql" from " ++ table ++ sql" where " ++ Fragment.const(
        s"${columnsList.head}"
      ) ++ sql" = $id").query[Option[A]]
  }
}

整个编译错误是

Cannot find or construct a Read instance for type:

  Option[A]

This can happen for a few reasons, but the most common case is that a data
member somewhere within this type doesn't have a Get instance in scope. Here are
some debugging hints:

- For Option types, ensure that a Read instance is in scope for the non-Option
  version.
- For types you expect to map to a single column ensure that a Get instance is
  in scope.
- For case classes, HLists, and shapeless records ensure that each element
  has a Read instance in scope.
- Lather, rinse, repeat, recursively until you find the problematic bit.

You can check that an instance exists for Read in the REPL or in your code:

  scala> Read[Foo]

and similarly with Get:

  scala> Get[Foo]

And find the missing instance and construct it as needed. Refer to Chapter 12
of the book of doobie for more information.

      ) ++ sql" = $id").query[Option[A]]

请注意For Option types, ensure that a Read instance is in scope for the non-Option version.

Option[A] 类型有一个类型类Read 的实例，前提是A 类型有一个类型类Get 的实例

implicit def fromGetOption[A](implicit ev: Get[A]): Read[Option[A]] =
  new Read(List((ev, Nullable)), ev.unsafeGetNullable)

https://github.com/tpolecat/doobie/blob/main/modules/core/src/main/scala/doobie/util/read.scala#L76-L77

所以尝试使用doobie.Get修改类CRUDAbs的定义

abstract class CRUDAbs[A: Get: ClassTag](val tableName: String)

---

其实，让我们回到Read 上下文绑定。 turns out（上面的编译错误中提到的“doobie 书第 12 章”）类型类 Get 用于非可选（不可为空）单变量（单列）类型，而类型Put 类也适用于可选（可为空）或多变量（向量）类型。让我们定义一个类似于Read.fromGetOption的隐式

implicit def fromGetOption[A](implicit ev: Get[A]): Read[Option[A]] =
  new Read(List((ev, Nullable)), ev.unsafeGetNullable)

即让我们定义

implicit def fromReadOption[A: Read]: Read[Option[A]] = Read[A].map(Some(_)) // Read[A].map(Option(_))

现在下面的代码编译

import doobie.Read
import doobie.implicits.toSqlInterpolator
import doobie.util.fragment.Fragment
import scala.reflect.{ClassTag, classTag}

object App {
  val snakeCase = ???

  implicit def fromReadOption[A: Read]: Read[Option[A]] = Read[A].map(Some(_))

  abstract class CRUDAbs[A: Read: ClassTag](val tableName: String) /*extends TransactSQL*/ {
    def table: Fragment = Fragment.const(s"$tableName")

    def columnsList: Array[String] = {
      val cls = classTag[A].runtimeClass
      cls.getDeclaredFields.map(_.getName).map(snakeCase)
    }

    def columns: Fragment = Fragment.const(columnsList.mkString(","))

    def find(id: Int): doobie.Query0[Option[A]] =
      (sql"select " ++ columns ++ sql" from " ++ table ++ sql" where " ++ Fragment.const(
        s"${columnsList.head}"
      ) ++ sql" = $id").query[Option[A]]
  }

  case class Chain0(i: Int)
  case class Chain(i: Int, s: String)
  case class Chain1(i: Int, s: Option[String])
  class CRUDChain0 extends CRUDAbs[Chain0]("chain") // compiles
  class CRUDChain extends CRUDAbs[Chain]("chain") // compiles
  class CRUDChain1 extends CRUDAbs[Chain1]("chain") // compiles
}

https://scastie.scala-lang.org/DmytroMitin/2kjpdtvaSVqxK6IhnmVEVQ

---

或者不定义隐式fromReadOption而是用两个隐式参数替换绑定的上下文

abstract class CRUDAbs[A: ClassTag](val tableName: String)(implicit r: Read[A], optR: Read[Option[A]])

或

abstract class CRUDAbs[A: Read : ClassTag](val tableName: String)(implicit optR: Read[Option[A]])

或kind-projector 语法

abstract class CRUDAbs[A: Read : λ[X => Read[Option[X]]] : ClassTag](val tableName: String)

https://scastie.scala-lang.org/DmytroMitin/2kjpdtvaSVqxK6IhnmVEVQ/1