将整型变压器提升到单台变压器

Question

我有这3台单台变压器

type T[A] = OptionT[Future, A]
type E[A] = EitherT[Future, String, A]
type P[A] = OptionT[E, A]

我想要提升相应的完整类型(意思是确切的对应类型)到这些。所以对于T，我想把Future[OptionInt]提升到它里面。对于E，我想要提升未来(EitherString，Int)，对于P，我想要提升(未来[字符串，OptionInt])到它。

我写了这段代码，它会编译。但我需要一个更简洁的方法来实现同样的目标。

val x : T[Int] = OptionT(Future(Option(10)))
val y : E[Int] = EitherT(Future(Right(10).asInstanceOf[Either[String, Int]]))
val z : P[Int] = OptionT(EitherT(Future(Right(Option(10)).asInstanceOf[Either[String, Option[Int]]])))

我使用的是Cats 1.1.0和Scala2.12.3。

asInstanceOf的事情很烦人。但如果我把最后一行改为

val z : P[Int] = OptionT(EitherT(Future(Right(Option(10)))))

我得到了这个编译器错误

[info] Compiling 1 Scala source to 
[error] /Users//code/dallasscalacats/src/main/scala/com//Transformers.scala:32: no type parameters for method apply: (value: F[Either[A,B]])cats.data.EitherT[F,A,B] in object EitherT exist so that it can be applied to arguments (scala.concurrent.Future[scala.util.Right[Nothing,Option[Int]]])
[error]  --- because ---
[error] argument expression's type is not compatible with formal parameter type;
[error]  found   : scala.concurrent.Future[scala.util.Right[Nothing,Option[Int]]]
[error]  required: ?F[Either[?A,?B]]
[error]     val z : P[Int] = OptionT(EitherT(Future(Right(Option(10)))))
[error]                              ^
[error] /Users//code/dallasscalacats/src/main/scala/com//Transformers.scala:32: type mismatch;
[error]  found   : scala.concurrent.Future[scala.util.Right[Nothing,Option[Int]]]
[error]  required: F[Either[A,B]]
[error]     val z : P[Int] = OptionT(EitherT(Future(Right(Option(10)))))
[error]                                            ^
[error] /Users//code/dallasscalacats/src/main/scala/com//Transformers.scala:32: type mismatch;
[error]  found   : cats.data.EitherT[F,A,B]
[error]  required: com.abhi.Transformers.E[Option[Int]]
[error]     (which expands to)  cats.data.EitherT[scala.concurrent.Future,String,Option[Int]]
[error]     val z : P[Int] = OptionT(EitherT(Future(Right(Option(10)))))
[error]                                     ^
[error] three errors found
[error] (compile:compileIncremental) Compilation failed
[error] Total time: 0 s, completed Jul 11, 2018 9:46:21 PM
>

Answer 1

尝试为右提供类型对齐：

val z : P[Int] = OptionT(EitherT(Future(Right[String,Option[Int]](Option(10)))))

如果没有类型参数，那么当您执行Right(1)时，scala会推断出Either[Nothing,Int]。