Aman Aman - 1 year ago 78
Scala Question

What is the use of ::: (triple colons) in Scala

I am new to scala..I came across a concept where it says like below:

{ val x = a; b.:::(x) }

In this block a is still evaluated before b, and then the result of
this evaluation is passed as an operand to b’s ::: method

What is the meaning of above statement..

I tried like below:

var a =10

var b =20

What should be the result i should expect.

Can somebody please give me an example...

Thanks in advance....

Answer Source

The ::: operator is defined on List trait and concatenates two lists. Using it on Int like in your example (var a=10) shouldn't work (unless you define such operator yourself).

Here is how it works on lists:

val a = List(1, 2);
val b = List(3, 4);
val c1 = a ::: b  // List(1, 2, 3, 4) 
val c2 = a.:::(b) // List(3, 4, 1, 2) 

Calling ::: with the infix syntax (c1) and method call syntax (c2) differ in the order in which lists are concatenated (see Jörg's comment).

The statement "a is still evaluated before b" means that a is evaluated before passing it as an argument to the method call. Unless the method uses call by name, its arguments are evaluated before the call just like in Java.

This could give you some hint how to search for meaning of Scala operators and keywords.

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