scala - 在 Scala 函数中按名称参数调用

Question

我有一个关于这两个函数之间区别的问题:

def getFunction(checkpointPath: String,
                  sparkConf: SparkConf,
                  creatingFunc: () => StreamingContext): StreamingContext = {
    function body
  }


def getFunction(checkpointPath: String,
                  sparkConf: SparkConf,
                  creatingFunc:  => StreamingContext): StreamingContext = {
    function body
  }

所以 called by name 参数是相同的:

creatingFunc:  => StreamingContext 

和

creatingFunc: () => StreamingContext

还是不？

Answer 1

两者不一样。第一种情况指定方法参数是按名称调用

creatingFunc:  => StreamingContext 

而第二种情况指定按值传递方法参数，其中参数恰好是 () => StreamingContex 类型的函数

creatingFunc: () => StreamingContext

例如，考虑以下两种方法

def foo(arg: () => String) = ""
def foo(arg: => String) = ""

然后

foo(() => "") // resolves to call first foo
foo("")       // resolves to call second foo