是否可以在Scala中的类型上进行模式匹配以进行表达？

Question

I'm trying to use a for expression to map over an Option, but I only want to match if the contents of the Option are of a specific type. What I thought would work is this:

for {
  vcs: Mercurial <- maybeVcs
} yield vcs

But that yields the following compile error:

<console>:76: error: type mismatch;
 found   : sbtrelease.Mercurial => sbtrelease.Mercurial
 required: sbtrelease.Vcs => ?
                vcs: Mercurial <- get (releaseVcs in Compile)
                               ^

Is it possible to pattern match on type in a for expression?

Answer 1

It's really straightforward if you use collect instead of for :

trait A
case class B(x: Int) extends A
case class C(y: Int) extends A

val someB: Option[A] = Some(B(2))
val someC: Option[A] = Some(C(2))
val noneA: Option[A] = None
someB.collect { case n: B => n }   // Some(B(2))
someC.collect { case n: B => n }   // None
noneA.collect { case n: B => n }   // None

Answer 2

The fact that this pattern match does not work is actually a bug (at least its not in accordance with the spec). See https://issues.scala-lang.org/browse/SI-900 .

However, there is a simple workaround. Define somewhere the following object:

object Id { unapply[T](x:T) = Some(x) }

Now you can use Id(x) as a pattern match that matches everything, and just binds x to whatever it matched. So basically a pointless construct, since Id(pattern) is the same as pattern .

However, it has one effect: A type annotation inside Id(...) will not be interpreted as a type annotation, but as a type pattern. Thus

for {
  Id(vcs: Mercurial) <- maybeVcs
} yield vcs

will have the effect you desire. (And differently from Bob's answer , the overall expression will have type Seq[Mercurial] and not Seq[Vcs] .)

Answer 3

You can use an ugly test:

for {
  vcs <- maybeVcs
  if vcs.instanceof[Mercurial]
} yield vcs