scala> println (Nil == List())
true
scala> println (Nil eq List())
true
scala> println (Nil equals List())
true
scala> System.identityHashCode(Nil)
374527572
scala> System.identityHashCode(List())
374527572
Nil is more idiomatic and can be preferred in most cases. Questions?
User unknown has shown that the run time value of both Nil
and List()
are the same. However, their static type is not:
scala> val x = List()
x: List[Nothing] = List()
scala> val y = Nil
y: scala.collection.immutable.Nil.type = List()
scala> def cmpTypes[A, B](a: A, b: B)(implicit ev: A =:= B = null) = if (ev eq null) false else true
cmpTypes: [A, B](a: A, b: B)(implicit ev: =:=[A,B])Boolean
scala> cmpTypes(x, y)
res0: Boolean = false
scala> cmpTypes(x, x)
res1: Boolean = true
scala> cmpTypes(y, y)
res2: Boolean = true
This is of particular importance when it is used to infer a type, such as in a fold's accumulator:
scala> List(1, 2, 3).foldLeft(List[Int]())((x, y) => y :: x)
res6: List[Int] = List(3, 2, 1)
scala> List(1, 2, 3).foldLeft(Nil)((x, y) => y :: x)
<console>:10: error: type mismatch;
found : List[Int]
required: scala.collection.immutable.Nil.type
List(1, 2, 3).foldLeft(Nil)((x, y) => y :: x)
^
As user unknown's answer shows, they are the same object.
Idiomatically Nil should be preferred because it is nice and short. There's an exception though: if an explicit type is needed for whatever reason I think
List[Foo]()
is nicer than
Nil : List[Foo]
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