值 withFilter 不是 Cats IO in 的成员以供理解 [英] Value withFilter is not a member of Cats IO in for comprehension

问题描述

我写了这段代码，它编译得很好

I wrote this code and it compiles fine

for {
  list : List[Int] <- Future(List(1, 2, 3))
} yield list.size 

res7: Future[Int] = Future(Success(3))

但是如果我将此代码转换为

But if I convert this code to

for {
  list : List[Int] <- IO(List(1, 2, 3))
} yield list.size

我收到一个编译时错误

value withFilter is not a member of cats.effect.IO[List[Int]]

如果我删除类型，那么它编译正常

If I remove the type then it compiles fine

for {
  list  <- IO(List(1, 2, 3)) // returns IO[List[Int]]
} yield list.size 
res8: IO[Int] = Map(Delay(<function0>), <function1>, 0)

为什么我不能用 IO 指定类型?

Why can't I specify the type with IO?

我启用了部分统一，所以不可能是这样:)

I have partial unification enabled so it can't be that :)

推荐答案

你的 for-comprehension 被脱糖形成，它使用函数 withFilter 并且因为 IO 没有有那个方法，编译失败.

Your for-comprehension gets desugared to form, which uses function withFilter and because IO doesn't have that method, compilation fails.

幸运的是，有编译器插件 better-monadic-for，解决了这个问题.

Fortunately, there is the compiler plugin better-monadic-for, which solves that problem.

只需在 build.sbt 中添加 addCompilerPlugin("com.olegpy" %% "better-monadic-for" % "0.3.0") ，你应该没事.

Just add addCompilerPlugin("com.olegpy" %% "better-monadic-for" % "0.3.0") in your build.sbt and you should be fine.

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