如何将可选的闭包参数传​​递给函数？

Question

我想将一个闭包传递给一个方法作为一个选项，我正在做如下所示。我得到一个编译错误，如下所示。是否可以将一个可选的闭包参数传​​递给一个函数？

def sampleMethod(a: String, b: String, optionalMethod: Option[(String, Int) => Unit]) { 
    // do some processing with a and b 
    optionalMethod match { 
     case Some(optionalMethod) => { 
     optionalMethod("a",3) 
     } 
     case _ 
     log("no optional method passed") 
    } 
} 

// definition of optMethod in some other place 
val optMethod = (c: String, d: Int) => { 
    // some processing with c, d and external values 
} 

// invoke 
sampleMethod("hi", "bye", optMethod) => FAILS TO COMPILE 

ERROR = type mismatch. expecting Option[(String, Int) => Unit] found (String, Int) => Unit 

Answer 1

如何

sampleMethod("hi", "bye", Some(optMethod)) 

Answer 2

正如前面所指出的那样，你的方法需要包含optionalMethod的Option值。如果你想避免Option值

// invoke with method 
sampleMethod("hi", "bye", Some(optMethod)) 
// invoke without method 
sampleMethod("hi", "bye", None) 

，你可以尝试以下方法：必须将Option值，因此传递给它（ESP避免None）：

def sampleMethod(a: String, b: String, optionalMethod: (String, Int) => Unit = (_, _) => log("no optional method passed")) { 
    optionalMethod("a", 3) 
} 

// invoke with method 
sampleMethod("hi", "bye", optMethod) 
// invoke without method 
sampleMethod("hi", "bye") 

Answer 3

更加清晰：

scala> def sampleMethod(a: String, b: String, optionalMethod: Option[(String, Int) => Unit]) { 
    | optionalMethod.map(f => f("a", 3)) 
    | } 
sampleMethod: (a: String, b: String, optionalMethod: Option[(String, Int) => Unit])Unit 


scala> sampleMethod("A", "A", Some((c:String, d:Int) => println(s"Hello wolrd $c...$d"))) 
Hello wolrd a...3 

您必须只需添加 “有些（）” 在你optinal功能

Answer 4

错误消息非常明确：sampleMethod需要Option，但是您传递的是直函数值（未包含在Some中）。

解决这个问题的最简单的方法是包装optMethod成Some：

sampleMethod("hi", "bye", Some(optMethod)) 

但是，如果你希望能够简单地做sampleMethod("hi", "bye", optMethod)，你可以添加重载的sampleMethod定义：

object Test { 
    def sampleMethod(a: String, b: String, optionalMethod: Option[(String, Int) => Unit]) { 
    // do some processing with a and b 
    optionalMethod match { 
     case Some(optionalMethod) => { 
     optionalMethod("a",3) 
     } 
     case _ => log("no optional method passed") 
    } 
    } 
    def sampleMethod(a: String, b: String) { sampleMethod(a, b, None) } 
    def sampleMethod(a: String, b: String, optionalMethod: (String, Int) => Unit) { 
    sampleMethod(a, b, Some(optionalMethod)) 
    } 
} 

val optMethod = (c: String, d: Int) => { 
    // some processing with c, d and external values 
} 

// invoke 
Test.sampleMethod("hi", "bye", optMethod) // Now Compiles fine 
Test.sampleMethod("hi", "bye") // This too