scala - 遵循 fs2 文档中的示例时类型不匹配

Question

我正在尝试使用 FS2 (0.10.0) 构建应用程序。我从文档中获取了这个例子：

import fs2._
// import fs2._

import fs2.async
// import fs2.async

import scala.concurrent.ExecutionContext
// import scala.concurrent.ExecutionContext

import cats.effect.{ Effect, IO }
// import cats.effect.{Effect, IO}

type Row = List[String]
// defined type alias Row

trait CSVHandle {
  def withRows(cb: Either[Throwable,Row] => Unit): Unit
}
// defined trait CSVHandle

def rows[F[_]](h: CSVHandle)(implicit F: Effect[F], ec: ExecutionContext): Stream[F,Row] =
  for {
    q <- Stream.eval(async.unboundedQueue[F,Either[Throwable,Row]])
    _ <- Stream.suspend { h.withRows { e => async.unsafeRunAsync(q.enqueue1(e))(_ => IO.unit) }; Stream.emit(()) }
    row <- q.dequeue.rethrow
  } yield row
// rows: [F[_]](h: CSVHandle)(implicit F: cats.effect.Effect[F], implicit ec: scala.concurrent.ExecutionContext)fs2.Stream[F,Row]

但它在编译时失败：

type mismatch;
[error]  found   : fs2.Stream[F,Row]
[error]  required: fs2.Stream[fs2.Pure,?]
[error] Expanded types:
[error] found   : fs2.Stream[F,List[String]]
[error] required: fs2.Stream[fs2.Pure,?]"
[error]       row <- q.dequeue.rethrow

恐怕我被卡住了，我不明白为什么会这样。任何的想法？

Answer 1

根据这个 Gitter 对话，示例中有一个错误。改用：

 def rows[F[_]](h: CSVHandle)(implicit F: Effect[F], ec: ExecutionContext): Stream[F,Row] =
   for {
     q <- Stream.eval(async.unboundedQueue[F,Either[Throwable,Row]])
     _ <-  Stream.eval { F.delay(h.withRows(e => async.unsafeRunAsync(q.enqueue1(e))(_ => IO.unit))) }
     row <- q.dequeue.rethrow
   } yield row