Scala 部分应用类型构造函数推断 [英] Scala partially applied type constructor inference

问题描述

我正在使用 scala-2.8.1 和 scalaz-5.0.谁能解释一下为什么 PartialApply1Of2 可以在一种情况下被推断出来，而在另一种情况下不能?

I'm using scala-2.8.1 and scalaz-5.0. Can anyone explain exactly why a PartialApply1Of2 can be inferrred in the one case but not in the other?

scala> 1.success[String] <|*|> "Bah".fail[Int]
res1: scalaz.Validation[String,(Int, Int)] = Failure(Bah)

即使(之前已经问过！)方法<|*|> 在 MA 上，它有一个类型参数，而不是两个(如 Validation 有).我无法在 IDEA REPL 中使用 unicode，所以这里是:

That worked even though (as has been asked before!) the method <|*|> is on MA which has one type parameter, not two (as Validation has). I cannot get unicode working in my IDEA REPL, so here goes:

object testy {
  def main(args: Array[String]) {
    import scalaz._
    import Scalaz._
    val ps = NonEmptyList(1.success[String], "Bah".fail[Int])

    val res = ps.∘∘((_ : Int) % 2 == 0) //DOES NOT COMPILE
    println(res)
  }
}

我可以为呼叫提供特定类型，一切都很好.为什么 scalac 不能推断出这个?

I can provide a specific type to the call and all is good. Why can scalac not infer this?

ps.∘∘[PartialApply1Of2[Validation, String]#Apply, Int, Boolean]((_ : Int) % 2 == 0)

在我的 REPL 中，这实际上会导致 scalac 错误，而不是合理的错误消息

In my REPL, this actually causes a scalac error, rather than a sensible error message

推荐答案

在第一种情况下，隐式视图 ValidationMA 是推断出来的，类型参数 Int 是推断:

In the first case, the implicit view ValidationMA is inferred, and the the type argument Int is inferred:

Scalaz.ValidationMA(1.success[String]).<|*|>[Int]("Bah".fail[Int])

在第二种情况下，∘∘ 方法的类型参数无法推断，直到 #2712 已解决.

In the second case, the type argument to the method ∘∘ cannot be inferred, until #2712 is tackled.

我怀疑您遇到的 scalac 内部错误与 #2741 有关/#4079.如果是这样，您可以使用 Type Lambda 重写它以解决该错误.

I suspect that the scalac internal error you encountered is related to #2741 / #4079. If so, you can rewrite it with a Type Lambda to workaround the error.

ps.∘∘[({type X[a]=Validation[String, a]})#X, Int, Boolean]((_ : Int) % 2 == 0)

我建议在所有情况下都使用此语法而不是 PartialApplyNofM，因为我发现它更具可读性.使用最新版本的 IntelliJ，您甚至可以启用代码折叠(设置、代码样式、Scala、折叠、Lambas 类型)，以 隐藏一些语法混乱.

I recommend using this syntax instead of PartialApplyNofM in all cases, as I find it more readable. With a recent build of IntelliJ, you can even enable a code folding (Settings, Code Style, Scala, Folding, Type Lambas), to hide some syntactic clutter.

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