How do I easily convert from one collection type to another during a filter, map, flatMap in Scala?

Using breakOut

Use breakOut as the CanBuildFrom and let the typer know what you want your result type to be (unfortunately you need to specify String here)

scala> import collection.breakOut
import collection.breakOut

scala> List(1, 2, 3)
res0: List[Int] = List(1, 2, 3)

scala> res0.map(_.toString)(breakOut) : Vector[String]
res2: Vector[String] = Vector(1, 2, 3)

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Using to[Collection] (starting with Scala 2.10.0)

Scala 2.10.0 introduced an easy way to convert a collection to another collection:

scala> List(1, 2, 3).map(_.toString).to[Vector]
res0: Vector[String] = Vector(1, 2, 3)

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Using toIndexedSeq

Alternatively ask for an IndexedSeq explicitly:

scala> res0.map(_.toString).toIndexedSeq
res4: scala.collection.immutable.IndexedSeq[String] = Vector(1, 2, 3)

If you want to do this without creating the intermediate List, then:

scala> res0.view.map(_.toString).toIndexedSeq
res5: scala.collection.immutable.IndexedSeq[String] = Vector(1, 2, 3)

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Using Natural Transformations

You could do it (awkwardly, but more generally) using natural transformations

scala> trait Trans[F[_], G[_]] {
 | def f2g[A](f : F[A]) : G[A]
 | }
defined trait Trans

Now provide a typeclass instance from the List ~> Vector transformation:

scala> implicit val List2Vector = new Trans[List, collection.immutable.Vector] {
 | def f2g[A](l : List[A]) : Vector[A] = l.map(identity[A])(collection.breakOut)
 | }
List2Vector: java.lang.Object with Trans[List,scala.collection.immutable.Vector] = $anon$1@56329755

Define a wrapper and an implicit conversion:

scala> class Clever[M[_], A](ma : M[A]) { def to[N[_]](implicit t : Trans[M, N]) : N[A] = t.f2g(ma) }
defined class Clever

scala> implicit def ma2clever[M[_], A](ma : M[A]) = new Clever[M, A](ma)
ma2clever: [M[_],A](ma: M[A])Clever[M,A]

Then:

scala> List(1, 2, 3).map(_.toString).to[Vector]
res4: Vector[java.lang.String] = Vector(1, 2, 3)