如何 flatMap cats Applicatives

Question

我已经开始使用 Cats 学习函数式编程，并且坚持使用 flatMapping（合并）应用程序 F[List]。

在纯 Scala 中非常简单的是像这样的列表的平面映射列表：

val animals = List("Dog", "Cat", "Bird")
def getBreads(animal: String): List[String] = ...

val allAnimalsBreads = animals.flatMap(animal => getBread(animal)) // this will be just List[String]

如果所有内容都用 applicative 包装，我如何做同样的事情？:

val animals = List("Dog", "Cat", "Bird").pure[F]
def getBreads(animal: String): F[List[String]] = ...

val allAnimalsBreads = ? // this should be F[List[String]]

Answer 1

Applicative provides ap and pure, but does not guarantee to provide flatMap, which is provided by Monad:

Monad extends the Applicative type class with a new function flatten.

如果 F 是一个 monad，那么至少在 scalaz 中我们可以使用 ListT，例如，

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

val animals: Option[List[String]] = Some(List("Dog", "Cat", "Bird"))
def getBreeds(animal: String): Option[List[String]] = ???

(for {
  animal <- listT(animals)
  breed <- listT(getBreeds(animal))
} yield breed).run

不过cats好像没有提供ListT:

A naive implementation of ListT suffers from associativity issues; ... It’s possible to create a ListT that doesn’t have these issues, but it tends to be pretty inefficient. For many use-cases, Nested can be used to achieve the desired results.

这是您不应该使用的疯狂解决方案的尝试。考虑 Validated，它只有 Applicative 个实例。让我们提供一个 Monad 实例，即使 Validated is not a Monad:

implicit def validatedMonad[E]: Monad[Validated[E, *]] =
  new Monad[Validated[E, *]] {
    def flatMap[A, B](fa: Validated[E, A])(f: A => Validated[E, B]): Validated[E, B] =
      fa match {
        case Valid(a) => f(a)
        case i @ Invalid(_) => i
      }

    def pure[A](x: A): Validated[E, A] = Valid(x)

    def tailRecM[A, B](a: A)(f: A => Validated[E, Either[A, B]]) = ???
  }

validatedMonad 的实现取自 scala-exercises.org/cats/validated。

接下来让我们通过 shims 互操作层

让 scalaz 的 listT 在猫中可用

libraryDependencies += "com.codecommit" %% "shims" % "2.1.0"

把它们放在一起，我们有

import cats._
import cats.Monad
import cats.data.Validated.{Invalid, Valid}
import cats.data.{Nested, OptionT, Validated, ValidatedNec}
import cats.implicits._
import scalaz.ListT._
import shims._

implicit def validatedMonad[E]: Monad[Validated[E, *]] =
  new Monad[Validated[E, *]] {
    def flatMap[A, B](fa: Validated[E, A])(f: A => Validated[E, B]): Validated[E, B] =
      fa match {
        case Valid(a) => f(a)
        case i @ Invalid(_) => i
      }

    def pure[A](x: A): Validated[E, A] = Valid(x)

    def tailRecM[A, B](a: A)(f: A => Validated[E, Either[A, B]]) = ???
  }

val animals: Validated[String, List[String]] = List("Dog", "Cat", "Bird").valid
def getBreeds(animal: String): Validated[String, List[String]] = ???

(for {
  animal <- listT(animals)
  breed <- listT(getBreeds(animal))
} yield breed).run

注意"solution"违反一元法则，不通用，容易造成混淆，请勿使用。

Answer 2

对于Applicative这是不可能的。对于Monad，如果你不想使用非常规变压器ListT，你可以

import cats.syntax.traverse._
import cats.syntax.applicative._
import cats.syntax.functor._
import cats.syntax.flatMap._
import cats.instances.list._

val allAnimalsBreads: F[List[String]] =
  animals.map(_.map(getBreads)) // F[List[F[List[String]]]]
    .map(_.sequence) // F[F[List[List[String]]]]
    .flatten // F[List[List[String]]]
    .map(_.flatten) // F[List[String]]

如何 flatMap cats Applicatives

How to flatMap cats Applicatives

functional-programming

scala

applicative

flatmap

scala-cats