无法找到或构造类型:Option[a]的Read实例

Question

为什么doobie不能将查询转换为选项[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]]

我得到一个错误

Cannot find or construct a Read instance for type:
Option[A]

我错过了什么?

Answer 1

如果我正确还原了你的代码片段它就像是

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.

如果类型A有类型Get的实例，则类型Option[A]有类型Read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

所以试着用doobie.Get修改CRUDAbs类的定义

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

---

实际上，让我们回到Read上下文绑定。结果是《杜比之书》第12章类型类Get用于非可选(不可空)单变量(单列)类型，而类型类Put也用于可选(可空)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)

也就是我们定义

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

---

或者不定义隐式的fromReadOption，而是用两个隐式形参替换上下文绑定

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]])

或同类投影仪语法

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

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