猫自由一元基于代数组成

Question

>假设我有以下代数来处理文件系统：

sealed trait Fs[A]
case class Ls(path: String) extends Fs[Seq[String]]
case class Cp(from: String, to: String) extends Fs[Unit]
def ls(path: String) = Free.liftF(Ls(path))
def cp(from: String, to: String) = Free.liftF(Cp(from, to))

以及以下代数解释器：

def fsInterpreter = new (Fs ~> IO) {
  def apply[A](fa: Fs[A]) = fa match {
    case Ls(path) => IO(Seq(path))
    case Cp(from, to) => IO(())
  }
}

现在假设我想构建另一个使用第一个代数的代数。例如：

sealed trait PathOps[A]
case class SourcePath(template: String) extends PathOps[String]
def sourcePath(template: String) = Free.liftF(SourcePath(template))

接下来我想为PathOps ~> IO写一个解释器，它会做这样的事情：

for {
  paths <- ls(template)
} yield paths.head

换句话说，我的PathOps解释器应该调用Fs代数。

我该怎么做？

Answer 1

我假设你想写两个解释器PathOps ~> Free[Fs, ?]并Fs ~> IO，然后将它们组合成一个解释器PathOps ~> IO。

下面是一个可编译的示例。以下是我用于此示例的所有导入：

import cats.~>
import cats.free.Free
import cats.free.Free.liftF

以下是IO和代数的模拟实现：

// just for this example
type IO[X] = X 
object IO {
  def apply[A](a: A): IO[A] = a
}
sealed trait Fs[A]
case class Ls(path: String) extends Fs[Seq[String]]
case class Cp(from: String, to: String) extends Fs[Unit]
type FreeFs[A] = Free[Fs, A]
def ls(path: String) = Free.liftF(Ls(path))
def cp(from: String, to: String) = Free.liftF(Cp(from, to))

这是从您的代码Fs ~> IO复制的解释器：

def fsToIoInterpreter = new (Fs ~> IO) {
  def apply[A](fa: Fs[A]) = fa match {
    case Ls(path) => IO(Seq(path))
    case Cp(from, to) => IO(())
  }
}
sealed trait PathOps[A]
case class SourcePath(template: String) extends PathOps[String]
def sourcePath(template: String) = Free.liftF(SourcePath(template))

这是你for理解转换为PathOps ~> Free[Fs, ?]解释器：

val pathToFsInterpreter = new (PathOps ~> FreeFs) {
  def apply[A](p: PathOps[A]): FreeFs[A] = p match {
    case SourcePath(template) => {
      for {
        paths <- ls(template)
      } yield paths.head
    }
  }
}

现在您可以使用 Free.foldMap 将Fs ~> IO提升到Free[Fs, ?] ~> IO中，并使用 andThen 与 PathOps ~> Free[Fs, ?] 解释器进行组合：

val pathToIo: PathOps ~> IO = 
  pathToFsInterpreter andThen 
  Free.foldMap(fsToIoInterpreter)

这为您提供了一个来自PathOps ~> IO的解释器，该解释器由两个可以单独测试的独立层组成。