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问题描述
假设我有以下设置:
def foo: Either[Error, A] = ???
def bar: EitherT[Future, Error, B] = ???
case class Baz(a: A, b: B)
如何使用理解来实例化 Baz 类?我试过:
How can I use for comprehension to instantiate the class Baz? I tried with:
val res = for {
a <- foo
b <- bar
} yield Baz(a, b)
但是，结果的类型为 Either[Error, Nothing].我不知道在这种情况下什么是正确的返回类型，但显然我不想要 Nothing...
but, the result has type Either[Error, Nothing]. I don't know what is the right return type in this case, but obviously I don't want Nothing...
将 Either 和 EitherT 结合起来进行理解的正确方法是什么?
What is the right way to combine Either and EitherT in for comprehension?
推荐答案
使用EitherT.fromEither函数从Either创建EitherT>
Use EitherT.fromEither function to create EitherT from Either
import cats.data._
import cats.implicits._
def foo[A]: Either[Error, A] = ???
def bar[B]: EitherT[Future, Error, B] = ???
case class Baz[A, B](a: A, b: B)
def res[A, B] = for {
a <- EitherT.fromEither[Future](foo[A])
b <- bar[B]
} yield Baz(a, b)
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