Why does the `is not a member of` error come while creating a list in scala using the :: operator

Question

I am learning scala and I've noticed that the following line of code doesn't work

     val worldFreq = ("India", 1) :: ("US", 2) :: ("Berlin", 10)

Results in the error : error: value :: is not a member of (String, Int) val worldFreq = ("India", 1) :: ("US", 2) :: ("Berlin", 10)

However this line of code works perfectly

val worldFreq = ("India", 1) :: ("US", 2) :: ("Berlin", 10) :: Nil
worldFreq: List[(String, Int)] = List((India,1), (US,2), (Berlin,10))

Can someone help me understand the error message and the fact the it works with Nil.

("India", 1) :: ("US", 2) :: ("Berlin", 10)::Nil do this it will work you are using :: operator on a tuple which is not the member of a tuple you can use it on List — Raman Mishra, Aug 31 '18 at 10:37
Possible duplicate of [Scala's '::' operator, how does it work?](https://stackoverflow.com/questions/2827293/scalas-operator-how-does-it-work) — Raman Mishra, Aug 31 '18 at 10:48

score 3 · Accepted Answer · answered Aug 31 '18 at 10:43

3

It happens because :: is right-associative operator.

So, when you type (1, 2) :: Nil it transforms to Nil.::((1,2)). And obviously, there is no :: method on tuples, so you can't write (1, 2) :: (3, 4).

You can read more here: Scala's '::' operator, how does it work?

answered Aug 31 '18 at 10:43

Yevhenii Popadiuk

1,024
7
18

Thanks @yevhenii. Also helpful to mention that Nil is an empty list which is quite different from null. – Bunny Rabbit Aug 31 '18 at 10:51

Why does the `is not a member of` error come while creating a list in scala using the :: operator

1 Answers1