在将State与Either集成(幻灯片88页)中,给定
以下是演示文稿中代码的可运行示例,经过调整后可以在2.11.8与Scalaz 7.2.8上工作。是否有一种方法能够清晰地在现有行为之上添加新的单子变换器,简化重构?在Scalaz中堆叠StateT适用于堆叠,但不处理由
State
层在Either
下的模式,是否有推荐的方法来添加另一种类型的状态,例如通过Writer
记录日志?似乎新状态必须位于现有State
和Either
之间,以利用flatMap
中Either
的快速失败行为。以下是演示文稿中代码的可运行示例,经过调整后可以在2.11.8与Scalaz 7.2.8上工作。是否有一种方法能够清晰地在现有行为之上添加新的单子变换器,简化重构?在Scalaz中堆叠StateT适用于堆叠,但不处理由
Either
的快速失败flatMap
行为所创建的排序问题。// Based on slide 88+ in https://speakerdeck.com/mpilquist/scalaz-state-monad
// Adjusted for Scala 2.11 (invariant A), Scalaz 7.2 (Pointed->Applicative) and Throwable on lhs of Either
object IntegratingStateAndEither {
import scalaz._
import scalaz.Scalaz._
import EitherT._
import scalaz.StateT.stateMonad
type QueryStateS[A] = State[QueryState, A]
type ET[F[_], A] = EitherT[F, Throwable, A]
type QueryStateES[A] = ET[QueryStateS, A]
object QueryStateES {
def apply[A](s: QueryStateS[Throwable \/ A]): QueryStateES[A] = EitherT(s)
def liftE[A](e: Throwable \/ A): QueryStateES[A] = apply(Applicative[QueryStateS].point(e))
def liftS[A](s: QueryStateS[A]): QueryStateES[A] = MonadTrans[ET].liftM(s)
}
def runQuery(s: String, m: Model): QueryStateES[QueryResult] = for {
query <- parseQuery(s)
res <- performQuery(query, m)
} yield res
def parseQuery(s: String): QueryStateES[StatsQuery] =
QueryStateES.liftE(new Exception("TODO parse").left)
def performQuery(q: StatsQuery, m: Model): QueryStateES[QueryResult] =
QueryStateES.liftE(new Exception("TODO perform").left)
// Just examples that do nothing
case class Model()
case class StatsQuery()
case class QueryResult()
case class QueryState()
def test = runQuery("a + b", Model()).run.run(QueryState())
}
back(3)
就是一个例子。 - Sim