Obscure Scala: what does following mean in scala? ( : _* )

Question

Hi I have following code

CookieMock(response, email).cookies: _*

.cookies is type def cookies: scala.Seq[Cookie]. What does :_* mean in Scala?

Thanks

I believe `(:_*)` is the monkey face emoticon. – som-snytt May 09 '14 at 23:07 — som-snytt, May 09 '14 at 23:07

score 3 · Accepted Answer · edited May 23 '17 at 12:05

3

If you are familiar with Java
here is the same explanation in Java:

varargs

Because * is not a type, you add the underscore.

def printInts(ints: Int*) = ints.mkString(",") 
printInts(1,2,3)
//printInts(List(1,2,3)) //type mismatch; found : List[Int] required: Int
printInts(List(1,2,3): _*)

paste this to codebrew.io this will clarify.

edited May 23 '17 at 12:05

Community

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answered May 09 '14 at 19:35

JavaSheriff

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This makes sense! Thanks!!! – user_1357 May 09 '14 at 20:06
How do you execute the code inside that site? – tkroman May 09 '14 at 21:23
I don't know how this makes sense to the OP. "Because * is not a type, you add the underscore." is called "folk etymology" in the dictionary. BTW, `*` can name a type. – som-snytt May 09 '14 at 23:05
cdshines - How to execute the code inside that site? use codebrew.io – JavaSheriff May 10 '14 at 03:29

score 0 · Answer 2 · edited Jun 20 '20 at 09:12

0

The : is type ascription. _* is the type you ascribe when you need a Seq[A] to be treated as A*.

http://docs.scala-lang.org/style/types.html

The following are examples of ascription:
Nil: List[String]
Set(values: _*)
"Daniel": AnyRef
Ascription is basically just an up-cast performed at compile-time for the sake of the type checker. Its use is not common, but it does happen on occasion. The most often seen case of ascription is invoking a varargs method with a single Seq parameter. This is done by ascribing the _* type (as in the second example above).

edited Jun 20 '20 at 09:12

Community

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answered May 09 '14 at 19:29

Chris Martin

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What is Type Ascription? Could you give an example with actual values. Let's say we have Seq(1, 2, 3, 4) what would be the result of using :_*? Why would you use it? – user_1357 May 09 '14 at 19:41

Obscure Scala: what does following mean in scala? ( : _* )

2 Answers2