CODEWITHSUNDEEP
scala
dveim
dveim

Reputation: 3482

How derivatives of SeqView (like SeqViewRMS) appear?

In "Programming in Scala" (Section 24.15, Views), I saw next code:

scala> val v = Vector(1 to 10: _*)
v: scala.collection.immutable.Vector[Int] = Vector(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

v.view map (_ + 1)
res69: scala.collection.SeqView[Int,Seq[_]] = SeqViewM(...)

It's said

... is in essence a wrapper that records the fact that a map with function (_ + 1) needs to be applied on the vector v.

But I can't figure out where those SeqViewR*M*S* are. Are there any already created classes, like Function1..22 (what I didn't find, and it's a bit impossible to create all combinations), or these are created dynamically?

Upvotes: 1

Views: 69

Answers (1)

wingedsubmariner
wingedsubmariner

Reputation: 13667

SeqViewR, SeqViewRMS, etc. are not in fact classes, but only text descriptions for views. The same class will use a different description depending on how much transformation it represents, e.g. SeqViewR and SeqViewRR are usually the same class.

The relevant code is the viewToString method in TraversableViewLike:

protected[this] def viewIdString: String = ""
protected[this] def viewIdentifier: String = ""
def viewToString = stringPrefix + viewIdString + "(...)"

stringPrefix is the original type of the view, e.g. SeqView. viewIdString is the R/RMS/ whatever part. When you invoke a method like map or take or reverse, you obtain a class that implements TraversableViewLike.Transformed, which has the following definition:

final override protected[this] def viewIdString = self.viewIdString + viewIdentifier

and then in each implementation something like:

final override protected[this] def viewIdentifier = "R"

This viewIdString method takes the viewIdString of the original view and attaches the appropriate letter. This works recursively, so the result of view.reverse.reverse.reverse is a class with a viewIdString of "RRR". Note that no classes are being dynamically generated, only their descriptions.

Upvotes: 2

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