gpt4 book ai didi

list - 用语义上等效的猫功能替换 scalaz ListT

转载 作者:行者123 更新时间:2023-12-02 02:39:58 24 4
gpt4 key购买 nike

猫会做 not提供 ListT monad transformer 那么我们如何重写以下 snippet它使用 scalaz ListT 来理解 cats 中语义等效的片段

import scalaz._
import ListT._
import scalaz.std.option._

val seeds: Option[List[String]] = Some(List("apple", "orange", "tomato"))
def grow(seed: String): Option[List[String]] = Some(List(seed.toUpperCase))
def family(seed: String, plant: String): Option[List[(String, String)]] = Some(List(seed -> plant))

(for {
seed <- listT(seeds)
plant <- listT(grow(seed))
result <- listT(family(seed, plant))
} yield result).run

这是我利用 flatMapflatTraverse 的尝试

import cats.implicits._

seeds
.flatMap {
_.flatTraverse { seed =>
grow(seed)
.flatMap {
_.flatTraverse { plant =>
family(seed, plant)
}
}
}
}

这种重构似乎可以满足类型检查器的要求,但我不确定快乐的编译器是否能确保 100% 的语义等价。

最佳答案

Cats 不提供 ListT,因为它违反了结合性 Monad 定律。参见 Cats FAQassociated proof using scalaz ListT .

您建议的基于 .flatTraverse 的以下 ListT 实现仍然通过了所有 cats-core 法则测试(错误?)。

我没有软件验证方面的经验,但您可能会发现成功的测试足以将这 2 种实现视为等效。

ListT 实现

case class ListT[M[_], A](value: M[List[A]])
implicit def listTMonad[M[_]: Monad] = new Monad[ListT[M, *]] {
override def flatMap[A, B](fa: ListT[M, A])(f: A => ListT[M, B]): ListT[M, B] =
ListT(
Monad[M].flatMap[List[A], List[B]](fa.value)(
list => Traverse[List].flatTraverse[M, A, B](list)(a => f(a).value)
)
)
override def pure[A](a: A): ListT[M, A] = ListT(Monad[M].pure(List(a)))
// unsafe impl, can be ignored for this question
override def tailRecM[A, B](a: A)(f: A => ListT[M, Either[A, B]]): ListT[M, B] =
flatMap(f(a)) {
case Right(b) => pure(b)
case Left(nextA) => tailRecM(nextA)(f)
}
}

sbt

name := "listT_tests"
version := "0.1"
scalaVersion := "2.11.12"

scalacOptions += "-Ypartial-unification"

libraryDependencies ++= Seq(
"org.typelevel" %% "cats-core" % "2.0.0",
"org.scalaz" %% "scalaz-core" % "7.2.30",
"org.scalacheck" %% "scalacheck" % "1.14.1" % "test",
"org.scalatest" %% "scalatest" % "2.2.6" % "test",
"org.typelevel" %% "discipline-scalatest" % "1.0.1",
"org.typelevel" %% "discipline-core" % "1.0.2",
"org.typelevel" %% "cats-laws" % "2.0.0" % Test,
"com.github.alexarchambault" %% "scalacheck-shapeless_1.14" % "1.2.3" % Test
)

addCompilerPlugin("org.typelevel" %% "kind-projector" % "0.11.0" cross CrossVersion.full)

法律测试

class TreeLawTests extends AnyFunSpec with Checkers with FunSpecDiscipline {

implicit def listTEq[M[_], A] = Eq.fromUniversalEquals[ListT[M, A]]
checkAll("ListT Monad Laws", MonadTests[ListT[Option, *]].stackUnsafeMonad[Int, Int, String])
}

法律测试结果

- monad (stack-unsafe).ap consistent with product + map
- monad (stack-unsafe).applicative homomorphism
- monad (stack-unsafe).applicative identity
- monad (stack-unsafe).applicative interchange
- monad (stack-unsafe).applicative map
- monad (stack-unsafe).applicative unit
- monad (stack-unsafe).apply composition
- monad (stack-unsafe).covariant composition
- monad (stack-unsafe).covariant identity
- monad (stack-unsafe).flatMap associativity
- monad (stack-unsafe).flatMap consistent apply
- monad (stack-unsafe).flatMap from tailRecM consistency
- monad (stack-unsafe).invariant composition
- monad (stack-unsafe).invariant identity
- monad (stack-unsafe).map flatMap coherence
- monad (stack-unsafe).map2/map2Eval consistency
- monad (stack-unsafe).map2/product-map consistency
- monad (stack-unsafe).monad left identity
- monad (stack-unsafe).monad right identity
- monad (stack-unsafe).monoidal left identity
- monad (stack-unsafe).monoidal right identity
- monad (stack-unsafe).mproduct consistent flatMap
- monad (stack-unsafe).productL consistent map2
- monad (stack-unsafe).productR consistent map2
- monad (stack-unsafe).semigroupal associativity
- monad (stack-unsafe).tailRecM consistent flatMap

关于list - 用语义上等效的猫功能替换 scalaz ListT,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/60244629/

24 4 0
Copyright 2021 - 2024 cfsdn All Rights Reserved 蜀ICP备2022000587号
广告合作:1813099741@qq.com 6ren.com