结合`OptionT`和`EitherT`来处理`Future [[Error,Option [T]]]`

Question

我想使用Cats EitherT并OptionT处理该类型Future[Either[Error, Option[T]].假设有以下方法:

def findTeacher(id: Int): Future[Either[String, Option[Teacher]]]
def findSchool(teacher: Teacher): Future[Either[String, Option[School]]]

现在,如果我想随后在for-comprehension中调用它们,我可以使用EitherT并OptionT喜欢这样:

def getSchoolByTeacherId(id: Int): Future[Either[String, Option[School]]] = {
  val result = for {
    maybeTeacher <- EitherT(findTeacher(id))
    schoolF = maybeTeacher.map(findSchool).getOrElse(Future.successful(Right(None)))
    school <- EitherT(schoolF)
  } yield {
    school
  }

  result.value
}

我不知道是否有可能通过合并,使之更简洁,也许OptionT用EitherT？

Answer 1

如果正确地理解你的问题,你想建立的组合单子变压器EitherT和OptionT.使用Cats,您可以尝试这样的事情:

type FutureEither[X] = EitherT[Future, String, X]
type OResult[X] = OptionT[FutureEither, X]

object OResult {

  implicit def apply[A](value: Future[Either[String, Option[A]]]): OResult[A] = OptionT[FutureEither, A](EitherT(value))

  implicit class OResultOps[A](val value: OResult[A]) extends AnyVal {
    @inline
    def directValue: Future[Either[String, Option[A]]] = value.value.value
  }

}

然后你可以重写你getSchoolByTeacherId的

import OResult._
def getSchoolByTeacherId(id: Int): Future[Either[String, Option[School]]] = {
  val result = for {
    teacher <- OResult(findTeacher(id))
    school <- findSchool(teacher)
  } yield school

  result.directValue
}

不幸的是,即使OResult.apply是implicit你仍然有它在你换理解的第一行明确写入但这允许跳过它的进一步线.