What is the #:: operator in a scala Stream? [duplicate]

Question

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Searching the Scala documentation for #::

I'm looking through the docs of Stream

The filter method has this code:

def naturalsFrom(i: Int): Stream[Int] = i #:: naturalsFrom(i + 1)
naturalsFrom(1)  10 } filter { _ % 5 == 0 } take 10 mkString(", ")

What is the #:: operator? Does this map to a function call somewhere?

Answer 1

As SHildebrandt says, #:: is the cons operator for Streams.

In other words, #:: is to streams what :: is to Lists

val x = Stream(1,2,3,4)                   //> x  : scala.collection.immutable.Stream[Int] = Stream(1, ?)
10#::x                                    //> res0: scala.collection.immutable.Stream[Int] = Stream(10, ?)

val y = List(1,2,3,4)                     //> y  : List[Int] = List(1, 2, 3, 4)
10::y                                     //> res1: List[Int] = List(10, 1, 2, 3, 4)

Answer 2

x #:: xs

returns

Stream.cons(x, xs)

which returns a Stream of an element x followed by a Stream xs.