Scala：for-comprehension with guard inside reader

Question

这是代码示例：

  type FailFast[A] = Either[List[String], A]
  import cats.instances.either._
  def f1:ReaderT[FailFast, Map[String,String], Boolean] = ReaderT(_ => Right(true))
  def f2:ReaderT[FailFast, Map[String,String], Boolean] = ReaderT(_ => Right(true))

  def fc:ReaderT[FailFast, Map[String,String], Boolean] =
    for {
      b1 <- f1
      if (b1)
      b2 <- f2
    } yield b2

错误是：

Error:(17, 13) value withFilter is not a member of cats.data.ReaderT[TestQ.this.FailFast,Map[String,String],Boolean] b1 <- f1

如何将 f1 与 f2 合成。仅当 f1 returns Right(true) 时才必须应用 f2。我通过以下方式解决了它：

  def fc2:ReaderT[FailFast, Map[String,String], Boolean] =
    f1.flatMap( b1 => {
      if (b1)
        f2
      else ReaderT(_ => Right(true))
    })

但我希望有更优雅的解决方案。

Answer 1

巨大的ReaderT[FailFast, Map[String, String], Boolean]类型很烦人。我用 ConfFF-shortcut ("map-configured fail-fast") 替换了它；您可能可以找到更好的名称。
如果需要，您仍然可以使用 for-comprehension 语法。
无需每次都写出所有_ =>和Right(...)，只需使用applicative中适当的pure即可。

因此，您的 fc2 变为：

  def fc3: ConfFF[Boolean] =
    for {
      b1 <- f1
      b2 <- if (b1) f2 else true.pure[ConfFF]
    } yield b2

完整代码：

import scala.util.{Either, Left, Right}
import cats.instances.either._
import cats.data.ReaderT
import cats.syntax.applicative._

object ReaderTEitherListExample {

  type FailFast[A] = Either[List[String], A]
  /** Shortcut "configured fail-fast" */
  type ConfFF[A] = ReaderT[FailFast, Map[String, String], A]

  def f1: ConfFF[Boolean] = ReaderT(_ => Right(true))
  def f2: ConfFF[Boolean] = ReaderT(_ => Right(true))

  def fc3: ConfFF[Boolean] =
    for {
      b1 <- f1
      b2 <- if (b1) f2 else true.pure[ConfFF]
    } yield b2
}

Scala：for-comprehension with guard inside reader

Scala: for-comprehension with guard inside reader

scala

composition

scala-cats

for-comprehension

reader-monad