scala - 在可变参数中使用惰性求值函数

Question

以下方法有什么问题？

def someMethod(funcs: => Option[String]*) = {
 ...
}

Answer 1

如果添加括号，那实际上在 2.7.7 下“有效”:

scala> def someMethod(funcs: => (Option[String]*)) = funcs
someMethod: (=> Option[String]*)Option[String]*

除了它实际上在运行时不起作用:

scala> someMethod(Some("Fish"),None)
    scala.MatchError: Some(Fish)
at scala.runtime.ScalaRunTime$.boxArray(ScalaRunTime.scala:136)
at .someMethod(<console>:4)
at .<init>(<console>:6)
at .<clinit>(<console>) ...

在 2.8 中，它拒绝让您将 X* 指定为任何函数或按名称参数的输出，即使您可以将其指定为输入(这是 r21230，Beta 后 1):

scala> var f: (Option[Int]*) => Int = _
f: (Option[Int]*) => Int = null

scala> var f: (Option[Int]*) => (Option[Int]*) = _
<console>:1: error: no * parameter type allowed here
       var f: (Option[Int]*) => (Option[Int]*) = _

但是，如果您尝试从方法进行转换，它会起作用:

scala> def m(oi: Option[Int]*) = oi
m: (oi: Option[Int]*)Option[Int]*

scala> var f = (m _)
f: (Option[Int]*) => Option[Int]* = <function1>

scala> f(Some(1),None)
res0: Option[Int]* = WrappedArray(Some(1), None)

所以并不完全一致。

在任何情况下，您都可以通过传入一个数组然后将该数组发送到需要重复参数的东西来实现您想要的:

scala> def aMethod(os: Option[String]*) { os.foreach(println) }
aMethod: (os: Option[String]*)Unit

scala> def someMethod(funcs: => Array[Option[String]]) { aMethod(funcs:_*) }
someMethod: (funcs: => Array[Option[String]])Unit

scala> someMethod(Array(Some("Hello"),Some("there"),None))
Some(Hello)
Some(there)
None

如果你真的想(轻松)传递一堆懒惰评估的参数，那么你需要一点基础设施，据我所知库中并不存在(这是 2.8 的代码；将其视为对2.7 中的类似策略):

class Lazy[+T](t: () => T, lt: Lazy[T]) {
  val params: List[() => T] = (if (lt eq null) Nil else t :: lt.params)
  def ~[S >: T](s: => S) = new Lazy[S](s _,this)
}
object Lz extends Lazy[Nothing](null,null) {
  implicit def lazy2params[T : Manifest](lz: Lazy[T]) = lz.params.reverse.toArray
}

现在，您可以轻松创建一组惰性求值的参数:

scala> import Lz._  // To get implicit def
import Lz._

scala> def lazyAdder(ff: Array[()=>Int]) = {
     |   println("I'm adding now!");
     |   (0 /: ff){(n,f) => n+f()}
     | }
lazyAdder: (ff: Array[() => Int])Int

scala> def yelp = { println("You evaluated me!"); 5 }
yelp: Int

scala> val a = 3
a: Int = 3

scala> var b = 7
b: Int = 7

scala> lazyAdder( Lz ~ yelp ~ (a+b) )
I'm adding now!
You evaluated me!
res0: Int = 15

scala> val plist = Lz ~ yelp ~ (a+b)
plist: Lazy[Int] = Lazy@1ee1775

scala> b = 1
b: Int = 1

scala> lazyAdder(plist)
I'm adding now!
You evaluated me!
res1: Int = 9