| [#516] | project: compiler | priority: low | category: bug | |
|---|---|---|---|---|
| submitter | assigned to | status | date submitted | |
| Sean | Martin | won't fix | 2006-01-17 19:07:01.0 | |
| subject | Type inference fails on compound types | |||
| code |
import scala.collection.mutable._;
object subscriber extends Subscriber[Message[String] with Undoable, Members] {
def notify(pub: Members, event: Message[String] with Undoable): Unit =
event match {
case Include(elem) => System.err.println("ADD: " + elem);
case Remove(elem) => System.err.println("REM: " + elem);
//case i : Include[HasTree] with Undoable =>
//case r : Remove [HasTree] with Undoable =>
}
}
|
|||
| what happened | Above code won't compile because type inferencing fails on message case classes. The commented cases must be use\ d instead to get the code to compile and work correctly. Additional output from the compiler (have no idea where\ this is coming from): infer constr (I)scala.collection.mutable.Include[I]:scala.collection.mutable.Include[I], pt = scala.collection.m\ utable.Message[Models.this.HasTree] with scala.collection.mutable.Undoable free type params = List() not a subtype scala.collection.mutable.Include[?I] of scala.collection.mutable.Message[Models.this.HasTree] with\ scala.collection.mutable.Undoable ici (I)scala.collection.mutable.Include[I] List(type I) scala.collection.mutable.Message[Models.this.HasTree] wi\ th scala.collection.mutable.Undoable infer constr (A)scala.collection.mutable.Remove[A]:scala.collection.mutable.Remove[A], pt = scala.collection.mut\ able.Message[Models.this.HasTree] with scala.collection.mutable.Undoable free type params = List() not a subtype scala.collection.mutable.Remove[?A] of scala.collection.mutable.Message[Models.this.HasTree] with \ scala.collection.mutable.Undoable ici (A)scala.collection.mutable.Remove[A] List(type A) scala.collection.mutable.Message[Models.this.HasTree] wit\ h scala.collection.mutable.Undoable infer constr (I)scala.collection.mutable.Include[I]:scala.collection.mutable.Include[I], pt = scala.collection.m\ utable.Message[Models.this.HasTree] with scala.collection.mutable.Undoable |
|||
| what expected | Expected type inference to determine that elem is a string in the pattern of both cases. Not sure if Scala's type system is this expressive though.... | |||
| [back to overview] | ||||
| Sean edited on 2006-01-17 19:08:05.0 |
| Sean edited on 2006-01-17 19:08:26.0 |
| Martin edited on 2006-01-20 18:59:32.0 |
This is how type inference is defined: The selector type must be supertype of the type of each pattern. In this case it isn't, becaus the selector type is Message with Undoable and
the patterns conform only to Message. You can get it to compile by widening the type of the selector:
(event: Message[String]) match {
case Include(elem) => System.err.println("ADD: " + elem);
case Remove(elem) => System.err.println("REM: " + elem);
//case i : Include[HasTree] with Undoable =>
//case r : Remove [HasTree] with Undoable =>
}
}
|
| Martin edited on 2006-01-20 18:59:53.0 |