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我花了一段时间来研究这个，但仍然无法让类型系统同意我的观点，即这种抽象实际上是一个

ObjectMapper => A => Either[Throwable, B]

我目前的类型看起来像

import cats._
import cats.data.{Reader, Kleisli, EitherT}
import cats.implicits._
import com.fasterxml.jackson.databind.{ObjectMapper, JsonNode}

type JsonParser[A, B] = Kleisli[EitherT[Reader[A, ?], Throwable, ?], ObjectMapper, B]

为了测试实例化这种类型，我编写了一个非常简单的函数，它接受一个 jsonString并返回一个Either[Throwable, JsonNode]

val makeJsonNode: JsonParser[String, JsonNode] =
  Kleisli(mapper => EitherT(Reader(json => tryToEither(Try(mapper.readTree(json))))))

我收到此错误消息：

[info] Compiling 6 Scala sources to .../scala-2.11/classes...
[error] Test.scala:140: missing parameter type
[error]     Kleisli(mapper => EitherT(Reader(json => tryToEither(Try(mapper.readTree(json))))))
[error]             ^
[error] Test.scala:140: missing parameter type
[error]     Kleisli(mapper => EitherT(Reader(json => tryToEither(Try(mapper.readTree(json))))))
[error]                                      ^
[error] Test.scala:140: 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 (cats.data.Reader[Any,Nothing])
[error]  --- because ---
[error] argument expression's type is not compatible with formal parameter type;
[error]  found   : cats.data.Reader[Any,Nothing]
[error]     (which expands to)  cats.data.Kleisli[cats.Id,Any,Nothing]
[error]  required: ?F[Either[?A,?B]]
[error]     Kleisli(mapper => EitherT(Reader(json => tryToEither(Try(mapper.readTree(json))))))
[error]                       ^
[error] Test.scala:140: type mismatch;
[error]  found   : cats.data.Reader[Any,Nothing]
[error]     (which expands to)  cats.data.Kleisli[cats.Id,Any,Nothing]
[error]  required: F[Either[A,B]]
[error]     Kleisli(mapper => EitherT(Reader(json => tryToEither(Try(mapper.readTree(json))))))
[error]                                     ^
[error] Test.scala:140: no type parameters for method apply: (run: A => F[B])cats.data.Kleisli[F,A,B] in object Kleisli exist so that it can be applied to arguments (<error> => cats.data.EitherT[F,A,B])
[error]  --- because ---
[error] argument expression's type is not compatible with formal parameter type;
[error]  found   : <error> => cats.data.EitherT[F,A,B]
[error]  required: ?A => ?F[?B]
[error]     Kleisli(mapper => EitherT(Reader(json => tryToEither(Try(mapper.readTree(json))))))
[error]     ^
[error] Test.scala:140: type mismatch;
[error]  found   : cats.data.Kleisli[F,A,B]
[error]  required: Test.this.Parser[String,com.fasterxml.jackson.databind.JsonNode]
[error]     (which expands to)  cats.data.Kleisli[[γ$1$]cats.data.EitherT[[β$0$]cats.data.Kleisli[[A]A,String,β$0$],Throwable,γ$1$],com.fasterxml.jackson.databind.ObjectMapper,com.fasterxml.jackson.databind.JsonNode]
[error]     Kleisli(mapper => EitherT(Reader(json => tryToEither(Try(mapper.readTree(json))))))
[error]            ^
[error] 6 errors found
[error] (compile:compileIncremental) Compilation failed
[error] Total time: 2 s, completed Jun 16, 2017 1:28:17 PM

我意识到这种类型有点麻烦，并且肯定可以改进（并且可能是矫枉过正），但我现在正在深入研究这个，以了解更多关于 Cats 和 Scala 类型系统的信息，以便我自己的个人成长。Scala 试图告诉我什么是错误的，在这里，我应该如何解决这个问题？

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1 回答 1

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基本问题是缺少参数类型，请尝试

Kleisli((mapper: ObjectMapper) => EitherT(Reader((json: String) => tryToEither(Try(mapper.readTree(json))))))

当存在提供此信息的预期类型时，您只能在不指定参数类型的情况下使用 lambda，在这种情况下makeJsonNode: JsonParser[String, JsonNode]似乎还不够。

于 2017-06-17T09:40:08.250 回答