for-comprehensions are desugared to a series of map
, flatMap
and filter
operations.
When you use map
on a Range
, you get a Vector
output:
scala> 0 to 2 map (x => x * x)
res12: scala.collection.immutable.IndexedSeq[Int] = Vector(0, 1, 4)
This is because a Range
is a very simple sort of collection, that is essentially just two three numbers: a start value, an end value and a step. If you look at the result of the mapping above, you can see that the resulting values cannot be represented by something of the Range
type.