CODEWITHSUNDEEP
scalascala-collections
jaksky
jaksky

Reputation: 3405

scala variable arguments :_*

could someone bring more shed on following piece of scala code which is not fully clear to me? I have following function defined

  def ids(ids: String*) = {
    _builder.ids(ids: _*)
    this
  }

Then I am trying to pass variable argument list to this function as follows:

def searchIds(kind: KindOfThing, adIds:String*) = {
...
ids(adIds)
}

Firstly, ids(adIds) piece doesn't work which is a bit odd at first as error message says: Type mismatch, expected: String, actual: Seq[String]. This means that variable arguments lists are not typed as collections or sequences.

In order to fix this use the trick ids(adIds: _*).

I am not 100% sure how :_* works, could someone put some shed on it? If I remember correctly : means that operation is applied to right argument instead to left, _ means "apply" to passed element, ... I checked String and Sequence scaladoc but wasn't able to find :_* method.

Could someone explain this?

Thx

Upvotes: 8

Views: 8020

Answers (1)

Ende Neu
Ende Neu

Reputation: 15783

You should look at your method definitions:

def ids(ids: String*)

Here you're saying that this method takes a variable number of strings, eg:

def ids(id1: String, id2: String, id3: String, ...)

Then the second method:

def searchIds(kind: KindOfThing, adIds:String*)

This also takes a variable number of string, which are packaged into a Seq[String], so adIds is actually a Seq, but your first method ids doesn't take a Seq, it takes N strings, that's why ids(adIds: _*) works.

: _* this is called the splat operator, what that's doing is splatting the Seq into N strings.

Upvotes: 12

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