Differences between these three ways of defining a function in Scala

Question

Given three ways of expressing the same function f(a) := a + 1:

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

How do these definitions differ? The REPL does not indicate any obvious differences:

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

You should note that in the 2nd block above, evaluating f1 in the REPL shows the value statically bound to f1 while evaluating f2 and f3 show the result of invoking those methods. In particular, a new Function1[Int, Int] instance is produced every time either f2 or f3 is invoked, while f1 is the same Function1[Int, Int] forever. — Randall Schulz
@RandallSchulz given that the val version does not require a new function instance, why would one ever use def in this case? — virtualeyes
@virtualeyes The only situation that I can recall where one sees defs yielding FunctionN[...] values is in the combinator parser library. It's not very common to write methods that yield functions and virtually never would one use a def to yield many copies of a semantically / functionally unchanging function. — Randall Schulz

Jack Jack · Accepted Answer · 2012-10-12T09:47:11

Inside a class, val is evaluated on initialization while def is evaluated only when, and every time, the function is called. In the code below you will see that x is evaluated the first time the object is used, but not again when the x member is accessed. In contrast, y is not evaluated when the object is instantiated, but is evaluated every time the member is accessed.

  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)

Differences between these three ways of defining a function in Scala

3 Answers