Question

我有以下函数，它做递归：

  @tailrec
  private def pool[F[_]: Monad, A]
  : Consumer[String, String] => (Vector[KkConsumerRecord] => F[A]) => IO[Unit]
  = consumer => cb => {
    val records: ConsumerRecords[String, String] = consumer.poll(Long.MaxValue)
    val converted = records.iterator().asScala.map(rec => {
      KkConsumerRecord(rec.key(), rec.value(), rec.offset(), rec.partition(), rec.topic())
    })

    val vec = converted.foldLeft(Vector.empty[KkConsumerRecord]) { (b, a) =>
      a +: b
    }
    cb(vec)
    pool(consumer)(cb)
  }

编译器抱怨：

[error] /home/developer/Desktop/microservices/bary/kafka-api/src/main/scala/io/khinkali/Consumer/KkConsumer.scala:57:10: type mismatch;
[error]  found   : org.apache.kafka.clients.consumer.Consumer[String,String]
[error]  required: cats.Monad[?]
[error]     pool(consumer)(cb)
[error]          ^
[error] two errors found

我做错了什么？

Answer 1

以下代码编译：

OK

import cats.Monad import cats.effect.IO import org.apache.kafka.clients.consumer.{Consumer, ConsumerRecords} import scala.collection.JavaConverters._ import scala.annotation.tailrec object App { case class KkConsumerRecord(key: String, value: String, offset: Long, partition: Int, topic: String) // @tailrec private def pool[F[_]: Monad, A] : Consumer[String, String] => (Vector[KkConsumerRecord] => F[A]) => IO[Unit] = consumer => cb => { val records: ConsumerRecords[String, String] = consumer.poll(Long.MaxValue) val converted = records.iterator().asScala.map(rec => { KkConsumerRecord(rec.key(), rec.value(), rec.offset(), rec.partition(), rec.topic()) }) val vec = converted.foldLeft(Vector.empty[KkConsumerRecord]) { (b, a) => a +: b } cb(vec) pool.apply(consumer)(cb) } }表示def pool[F[_]: Monad, A]，因此编译器将def pool[F[_], A](implicit monad: Monad[F])误认为隐式参数。

consumer注释被移除，因为tailrec不是tail recursive（最后一个操作是构建lambda，我猜它叫tail recursion modulo cons）。

如果你想让它尾递归，你可以把它重写为

pool

类型不匹配cats.Monad [？]？

1 个答案: