Scala pattern matching and type inference
Can someone explain why the following code compiles?
Option("foo") match {
case x: List[String] => println("A")
case _ => println("B")
}
This gives me a (expected) warning about erasing styles, but it still compiles. I expected this to throw a type error as if I matched with "foo"instead Option("foo").
Thank!
+5
3 answers
The code is commented out, so let's think that:
/** If we can absolutely rule out a match we can fail early.
* This is the case if the scrutinee has no unresolved type arguments
* and is a "final type", meaning final + invariant in all type parameters.
*/
Note that None is not final, for example. I know, right?
If you ever try scalac -Ypatmat-debug, a comment here might help:
https://github.com/scala/scala/pull/650
Reachability is almost achievable:
https://issues.scala-lang.org/browse/SI-6146
promises , - . ? , Of [Foo [_]]?
8.2 - 8.4 , Foo [a] (- a). , . .
trait Foo[+A]
final class Fuzz[+A] extends Foo[A]
final object Fooz extends Foo[Nothing]
object Futz extends Foo[Nothing]
//error
Fooz match {
case x: List[_] => println("A")
case _ => println("B")
}
//no error
Futz match { ... }
+4
, Option, List Product, . , . , :
scala> Option("foo") match {
| case x: Tuple2[String,String] => println("TUPLE")
| case x: List[String] => println("LIST")
| case _ => println("OTHER")
| }
<console>:9: warning: non variable type-argument String in type pattern (String, String) is unchecked since it is eliminated by erasure
case x: Tuple2[String,String] => println("TUPLE")
^
<console>:10: warning: non variable type-argument String in type pattern List[String] is unchecked since it is eliminated by erasure
case x: List[String] => println("LIST")
^
UPDATE w/r/t case classes (- ):
scala> case class Foo(bar: Int)
defined class Foo
scala> val y: Product = Foo(123)
y: Product = Foo(123)
+2