堆叠M，Either和Writer(Stacking M, Either and Writer)

堆叠M，Either和Writer(Stacking M, Either and Writer)

我目前正在使用EitherT堆叠Futures和Eithers：

type ErrorOr[A] = Either[Error, A] def getAge: Future[ErrorOr[Int]] = ??? def getDob(age: Int): ErrorOr[LocalDate] = ??? for { age <- EitherT(getAge) dob <- EitherT.fromEither[Future](getDob(age)) } yield dob

我现在想介绍一下Writer monad ie

type MyWriter[A] = Writer[Vector[String], ErrorOr[A]] def getAge: Future[MyWriter[Int]] = ??? def getDob(age: Int): MyWriter[LocalDate] = ???

我的问题是，对getAge和getDob调用进行排序的最佳方法是什么？ 我知道monad可以堆叠，即Future -> Writer -> Either在这种情况下我可以继续使用EitherT吗？ 如果是这样的话？

I'm currently stacking Futures and Eithers using EitherT:

type ErrorOr[A] = Either[Error, A] def getAge: Future[ErrorOr[Int]] = ??? def getDob(age: Int): ErrorOr[LocalDate] = ??? for { age <- EitherT(getAge) dob <- EitherT.fromEither[Future](getDob(age)) } yield dob

I would now like to introduce the Writer monad i.e.

type MyWriter[A] = Writer[Vector[String], ErrorOr[A]] def getAge: Future[MyWriter[Int]] = ??? def getDob(age: Int): MyWriter[LocalDate] = ???

My question is, what is the best way to sequence the getAge and getDob calls? I know monads can be stacked i.e. Future -> Writer -> Either but can I continue to use EitherT in this scenario? if so how?

最满意答案

是的，你可以继续使用这样的WriterT monad变换器：

type FutureErrorOr[A] = EitherT[Future, Error, A] type MyStack[A] = WriterT[FutureErrorOr, Vector[String], A]

如果你解压缩这种类型，它类似于Future[Either[Error, Writer[Vector[String], A]]

现在棘手的部分是将函数提升到这个基本monad中，所以这里有一些例子：

def getAge: FutureErrorOr[Int] = ??? def getDob(age: Int): ErrorOr[LocalDate] = ??? for { age <- WriterT.liftF(getAge) dob <- WriterT.liftF(EitherT.fromEither(getDob(age))) } yield dob

为了使这更容易，你可以看看cats-mtl。

Yeah, you can continue using both using the WriterT monad transformers like this:

type FutureErrorOr[A] = EitherT[Future, Error, A] type MyStack[A] = WriterT[FutureErrorOr, Vector[String], A]

If you unpack this type, it's analogous to Future[Either[Error, Writer[Vector[String], A]]

Now the tricky part is lifting your functions into this base monad, so here are some examples:

def getAge: FutureErrorOr[Int] = ??? def getDob(age: Int): ErrorOr[LocalDate] = ??? for { age <- WriterT.liftF(getAge) dob <- WriterT.liftF(EitherT.fromEither(getDob(age))) } yield dob

To make this easier you can have a look at cats-mtl.