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scala - cats.data.EitherT with traverse

问题描述

I have a question about using Traverse together with EitherT. Let's say we have a code like this:

def validate(s: String): EitherT[Future, NumberFormatException, Int] = {
  EitherT(Future.successful(try { Right(s.toInt) } catch { case e: 
  NumberFormatException => Left(e)}))
}

List("1", "2").traverse(validate)

Unfortunately this code does not compile, because we are missing something: 

error: could not find implicit value for evidence parameter of type cats.Applicative[G] List("1", "2").traverse(validate)

I tried to look this up and found for example this answer: Switching between EitherT and Validation to accumulate error or traverse or Validation versus disjunction

So it seems some solution could exists. But the problem is that both of them are using traverseU, which is no longer an option in scala 2.12. So how can this be done please?

EDIT This is the code including imports:

import cats.data.EitherT
import cats.syntax.traverse._
import cats.instances.list._
import cats.instances.future._
import scala.concurrent.ExecutionContext.global

import scala.concurrent.Future

def validate(s: String): EitherT[Future, NumberFormatException, Int] = {
  EitherT(Future.successful(try { Right(s.toInt) } catch { case e:
    NumberFormatException => Left(e)}))
}

List("1", "2").traverse(validate)

标签： scalascala-cats

解决方案

通常猫会在不同的导入下打包特定的证据隐含。在这种情况下，您需要 和 的List证明Future。

  import cats.data.EitherT
  import cats.syntax.traverse._
  import cats.instances.list._
  import cats.instances.future._

  import scala.concurrent.Future

  def validate(s: String): EitherT[Future, NumberFormatException, Int] = {
    EitherT(Future.successful(try { Right(s.toInt) } catch { case e: NumberFormatException => Left(e) }))
  }

  List("1", "2").traverse(validate)

这段代码对我来说是用猫 1.2.0 编译的。

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