我有一个剥香蕉的错误类型层次结构:
sealed trait PeelBananaError
object PeelBananaError {
case object TooRipe extends PeelBananaError
case object NotRipeEnough extends PeelBananaError
}
我在EitherT
中得到了一些结果,我们知道这些结果只能以两种方式之一失败:
val peelBrownBananaResult: EitherT[Future, TooRipe, String] = ...
val peelGreenBananaResult: EitherT[Future, NotRipeEnough, String] = ...
现在我需要从右边收集String
结果,并将它们组合为最终结果:
val combinedResult: EitherT[Future, PeelBananaError, String] = for {
first <- peelBrownBananaResult
second <- peelGreenBananaResult
} yield (first + second)
但尝试这个会给我一个编译错误:
cmd15.sc:2: inferred type arguments [PeelBananaError.NotRipeEnough.type,String] do not conform to method flatMap's type parameter bounds [AA >: PeelBananaError.TooRipe.type,D]
first <- peelBrownBananaResult
^
cmd15.sc:2: type mismatch;
found : String => cats.data.EitherT[scala.concurrent.Future,PeelBananaError.NotRipeEnough.type,String]
required: String => cats.data.EitherT[scala.concurrent.Future,AA,D]
first <- peelBrownBananaResult
^
Compilation Failed
编译器似乎无法推断Left
类型在PeelBananaError
中共享公共继承,因此for compression不会编译。这里有什么变通办法吗?我可以给编译器一些提示,这样我就可以运行它来理解吗?
我尝试过以下方法,但使用.asInstanceOf
似乎很麻烦,结果代码看起来很难看,这真的是唯一的解决方案吗?
val combinedResult: EitherT[Future, PeelBananaError, String] = for {
first <- peelBrownBananaResult.asInstanceOf[EitherT[Future,PeelBananaError,String]]
second <- peelGreenBananaResult.asInstanceOf[EitherT[Future,PeelBananaError,String]]
} yield (first + second)
EitherT
monad transformer在A
或B
类型参数中不是协变的
case class EitherT[F[_], A, B]
不同于CCD_ 9在CCD_ 10和CCD_
sealed abstract class Either[+A, +B]
这就是为什么Either
类型检查到超类型
Either.left(TooRipe): Either[PeelBananaError, String] // ok
但EitherT
不
EitherT.left(Future(TooRipe)): EitherT[Future, PeelBananaError, String] // error
然而,我们可以安全地拓宽monad变压器
for {
first <- peelBrownBananaResult.leftWiden[PeelBananaError]
second <- peelGreenBananaResult.leftWiden[PeelBananaError]
} yield (first + second)
// : EitherT[Future, PeelBananaError, String] = ...
与asInstanceOf
的不安全加宽不同
type T = Either[TooRipe.type, String]
val a = Either.left[PeelBananaError, String](new PeelBananaError {})
val b: Either[TooRipe.type, String] = a.asInstanceOf[T] // run-time error
val c: Either[TooRipe.type, String] = a.widen[T] // compile-time error