What does => and () => mean in Scala [duplicate]

Question

I am new to Scala and I really like it, but sometimes it surprises me. For instance:

clickedCallbacks: List[() => Unit]) 

Could anyone tell me what => and () => mean in Scala?

Answer 1

=> is syntactic sugar for creating instances of functions. Recall that every function in scala is an instance of a class.

For example, the type Int => String, is equivalent to the type Function1[Int,String] i.e. a function that takes an argument of type Int and returns a String.

  scala> val f: Function1[Int,String] = myInt => "my int: "+myInt.toString   f: (Int) => String = <function1>    scala> f(0)   res0: String = my int: 0    scala> val f2: Int => String = myInt => "my int v2: "+myInt.toString   f2: (Int) => String = <function1>    scala> f2(1)   res1: String = my int v2: 1 

Here myInt is bound to the argument value passed to f and f2.

() => T is the type of a function that takes no arguments and returns a T. It is equivalent to Function0[T]. () is called a zero parameter list I believe.

 scala> val f: () => Unit = () => { println("x")}  f: () => Unit = <function0>   scala> f()  x  scala> val f2: Function0[Unit] = () => println("x2") f: () => Unit = <function0>  scala> f2() x2 

Answer 2

=> has several meanings in Scala, all related to its mathematical meaning as implication.

• In a value, it introduces a function literal, or lambda. e.g. the bit inside the curly braces in List(1,2,3).map { (x: Int) => x * 2 }

• In a type, with symbols on both sides of the arrow (e.g. A => T, (A,B) => T, (A,B,C) => T, etc.) it's sugar for Function<n>[A[,B,...],T], that is, a function that takes parameters of type A[,B...], and returns a value of type T.

  • Empty parens on the left hand side (e.g. () => T) indicate that the function takes no parameters (also sometimes called a "thunk");

  • Empty parens on the right hand side denote that it returns ()—the sole value of type Unit, whose name can also be written ()—confused yet? :)

    A function that returns Unit is also known as a procedure, normally a method that's called only for its side effect.

• In the type declaration for a method or function parameter, with no symbol on the left hand side (e.g. def f(param: => T)) it's a "by-name parameter", meaning that is evaluated every time it's used within the body of the function, and not before. Ordinary "by-value" parameters are evaluated before entry into the function/method.

• In a case clause, they separate the pattern (and optional guard) from the result expression, e.g. case x => y.