List[EitherT[Future, String, CustomObj]] => EitherT[Future, String, List[CustomObj]]

Question

如何将sequence 函数用于带有自定义类 CustomObj 的 List[EitherT[Future, String, CustomObj]] ？我想要这样的东西：

import scala.language.postfixOps
import cats.instances.list._
import cats.syntax.traverse._
import cats.data.EitherT
import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global

case class CustomObj(int: Int)

private val fst: EitherT[Future, String, CustomObj] =
  EitherT.pure[Future, String](CustomObj(1))
private val snd: EitherT[Future, String, CustomObj] =
  EitherT.pure[Future, String](CustomObj(2))

val source: List[EitherT[Future, String, CustomObj]] = fst :: snd :: Nil

val result: EitherT[Future, String, List[CustomObj]] = source.sequence

import scala.concurrent.duration._
val res = scala.concurrent.Await.result(result.value, 1 second)
println(res) // Right(List(CustomObj(1), CustomObj(2)))

每次编译时我都会得到

 error: Cannot prove that EitherT[Future,String,CustomObj] <:< G[A].

什么意思 <: g>

Answer 1

这是编辑版，代码@https://scastie.scala-lang.org/Yaneeve/Jro89ZHwS3G23Aveanxc5A：

import scala.language.postfixOps
import cats.implicits._
import cats.data.EitherT
import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global

case class CustomObj(int: Int)

private val fst: EitherT[Future, String, CustomObj] =
  EitherT.pure[Future, String](CustomObj(1))
private val snd: EitherT[Future, String, CustomObj] =
  EitherT.pure[Future, String](CustomObj(2))

val source: List[EitherT[Future, String, CustomObj]] = fst :: snd :: Nil

val result: EitherT[Future, String, List[CustomObj]] = source.sequence

import scala.concurrent.duration._
val res = scala.concurrent.Await.result(result.value, 1 second)
println(res) // Right(List(CustomObj(1), CustomObj(2)))

最初我写道：

请注意它没有编译并产生奇怪的编译消息 直到我添加了import scala.concurrent.ExecutionContext.Implicits.global

确实如此，但是你的麻烦不在于那里。当我再次去重现时，我得到了你指定的错误。缺少的是 SBT/scalac 标志：scalacOptions += "-Ypartial-unification"