scala - Cats Future Monad 给出运行时异常

Question

import cats.Monad
import cats.instances.future._
import scala.concurrent._
import scala.concurrent.duration._

import scala.concurrent.ExecutionContext.Implicits.global

val fm = Monad[Future]

Await.result(
  fm.flatMap(fm.pure(1)) {
    x => fm.pure(x+2)
  }, 1.second
)

这给了我一个运行时异常，我不明白为什么。我使用的是 scala 版本 2.12.2 和 cats 版本 cats-core_2.12

java.lang.NoClassDefFoundError: Could not initialize class $line9.$read$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$ at scala.concurrent.Future.$anonfun$flatMap$1(Future.scala:302) at scala.concurrent.impl.Promise.$anonfun$transformWith$1(Promise.scala:37) at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:60) at scala.concurrent.impl.ExecutionContextImpl$AdaptedForkJoinTask.exec(ExecutionContextImpl.scala:140) at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289) at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056) at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692) at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157) java.util.concurrent.TimeoutException: Futures timed out after [1 second] at scala.concurrent.impl.Promise$DefaultPromise.ready(Promise.scala:255) at scala.concurrent.impl.Promise$DefaultPromise.result(Promise.scala:259) at scala.concurrent.Await$.$anonfun$result$1(package.scala:215) at scala.concurrent.BlockContext$DefaultBlockContext$.blockOn(BlockContext.scala:53) at scala.concurrent.Await$.result(package.scala:142) ... 43 elided

Answer 1

这些错误通常意味着您的依赖项中有同一库的两个非二进制兼容版本。尝试运行 sbt evicted 并查明是否有任何 cats-core 版本被逐出。

然后您必须确保每个人都使用相同的版本(可能您只需将您的版本更新到最新可用版本)。