scala - 与下划线一起使用时，部分函数应用程序过早地运行代码块

Question

鉴于:

def save(f: => Any)(run:Boolean) { if (run) { println("running f"); f } else println("not running f") } 

我可以这样称呼它:

save("test")(true) -> running f
save("test")(false) -> not running f
save(throw new RuntimeException("boom!"))(false) -> not running f
save(throw new RuntimeException("boom!"))(true) -> running f and then exception thrown

这是部分应用的奇怪行为:

save(throw new RuntimeException("boom!"))(_) -> (Boolean) => Unit = <function1> //as expected
save(throw new RuntimeException("boom!")) _ -> exception thrown

代码块会立即被评估，而不是作为函数传入。以上2种说法有什么区别？

Answer 1

第一个案例 ,

save(throw new RuntimeException("boom!")) _ 

根据"Scala Reference" (第 6.7 节)，使用尾随下划线代替参数列表，并将表达式转换为

val f: (Boolean) => Unit = save(throw new RuntimeException("boom!"))

def save 的第一个参数在哪里立即被评估。

The expression e _ is well-formed if e is of method type or if e is a call-by-name parameter. If e is a method with parameters, e _ represents e converted to a function type by eta expansion (§6.26.5). If e is a parameterless method or call-by-name parameter of type =>T , e _ represents the function of type () => T , which evaluates e when it is applied to the empty parameterlist ().

为了使事情按您的预期工作，需要进行一些修改:

scala> def save(f:() => Any)(run:Boolean) { if (run) { println("running f"); f() } else println("not running f") }
save: (f: () => Any)(run: Boolean)Unit

scala> val f = save(() => throw new RuntimeException("boom!")) _
f: (Boolean) => Unit = <function1>

scala> f(true)
running f
java.lang.RuntimeException: boom!
        at $anonfun$1.apply(<console>:6)

第二种情况 ,

save(throw new RuntimeException("boom!"))(_)

根据"Scala Reference" (第 6.23 节)，当占位符用作参数的替换时，表达式将转换为

val f: (Boolean) => Unit = save(throw new RuntimeException("boom!"))(_)