Collection type generated by for with yield

Question

When I evaluate a for in Scala, I get an immutable IndexedSeq (a collection with array-like performance characteristics, such as efficient random a

Answer 1

Thanks michael.kebe for your comment.

This explains how for is translated to operations with map, flatMap, filter and foreach. So my example:

val s = for (i <- 0 to 9) yield math.random + i

is translated to something like this (I'm not sure if it's translated to map or flatMap in this case):

val s = (0 to 9) map { math.random + _ }

The result type of operations like map on collections depends on the collection you call it on. The type of 0 to 9 is a Range.Inclusive:

scala> val d = 0 to 9
d: scala.collection.immutable.Range.Inclusive with scala.collection.immutable.Range.ByOne = Range(0, 1, 2, 3, 4, 5, 6, 7, 8, 9)

The result of the map operation on that is an IndexedSeq (because of the builder stuff inside the collections library).

So, to answer my question: the result of a for (...) yield ... depends on what type is inside the parantheses. If I want a List as the result, I could do this:

scala> val s = for (i <- List.range(0, 9)) yield math.random + i
s: List[Double] = List(0.05778968639862214, 1.6758775042995566, ...

Answer 2

You can always transform a range to a list using toList:

> val s = for (i <- (0 to 9).toList) yield math.random + i
> s  : List[Double]