What does Predef.identity do in scala?

Question

Here is documentation about Predef, but there is no word about identity. What is this function used for? And what it does?

Answer 1

It's just an instance of the identity function, predefined for convenience, and perhaps to prevent people from redefining it on their own a whole bunch of times. identity simply returns its argument. It can be handy sometimes to pass to higher-order functions. You could do something like:

scala> def squareIf(test: Boolean) = List(1, 2, 3, 4, 5).map(if (test) x => x * x else identity)  squareIf: (test: Boolean)List[Int]  scala> squareIf(true) res4: List[Int] = List(1, 4, 9, 16, 25)  scala> squareIf(false) res5: List[Int] = List(1, 2, 3, 4, 5) 

I've also seen it used as a default argument value at times. Obviously, you could just say x => x any place you might use identity, and you'd even save a couple characters, so it doesn't buy you much, but it can be self-documenting.