scala - Scala 中定义函数的三种方式之间的差异

Question

给出表达同一函数的三种方式f(a) := a + 1:

val f1 = (a:Int) => a + 1
def f2 = (a:Int) => a + 1
def f3:(Int => Int) = a => a + 1

这些定义有何不同？ REPL 没有表明任何明显的差异:

scala> f1
res38: (Int) => Int = <function1>
scala> f2
res39: (Int) => Int = <function1>
scala> f3
res40: (Int) => Int = <function1>

Answer 1

在类内部，val 在初始化时求值，而 def 仅在调用函数时和每次求值。在下面的代码中，您将看到第一次使用对象时对 x 进行求值，但在访问 x 成员时不再求值。相反，y 在对象实例化时不求值，而是在每次访问成员时求值。

  class A(a: Int) {
    val x = { println("x is set to something"); a }
    def y = { println("y is set to something"); a }
  }

  // Prints: x is set to something
  val a = new A(1)

  // Prints: "1"
  println(a.x)

  // Prints: "1"                               
  println(a.x)

  // Prints: "y is set to something" and "1"                                  
  println(a.y)

  // Prints: "y is set to something" and "1"                                                                                   
  println(a.y)