CODEWITHSUNDEEP
javascalascala-2.8
Collin
Collin

Reputation: 1451

Why does the type parameter bound T <: Comparable[T] fail for T = Int?

scala> class Foo[T <: Comparable[T]](val x : T)
defined class Foo

scala> (3: Int).asInstanceOf[Comparable[Int]]  
res60: java.lang.Comparable[Int] = 3

scala> new Foo(3)                              
<console>:13: error: inferred type arguments [Int] do not conform to class Foo's type parameter bounds [T <: java.lang.Comparable[T]]
       new Foo(3)
       ^

Is the 2nd expression the result of type erasure?

How would I go about defining Foo so that I could parameterize it with Int but still be able to perform some ordering behavior with its instance variable?

Upvotes: 14

Views: 2652

Answers (3)

Aaron Novstrup
Aaron Novstrup

Reputation: 21017

Alternatively, you could use a context bound:

class Foo[T: Ordering](val v: T)

or 

class Foo[T: java.util.Comparator](val v: T)

The context bound represents an assertion that there is an implicit Ordering[T] (or java.util.Comparator[T]) in scope when the constructor is invoked and is equivalent to adding an implicit parameter:

class Foo[T](val v: T)(implicit ev: Ordering[T])

The advantage of this approach is that it will allow you to use an alternate ordering based on the context:

// by default, new Foo("a", "c", "b").items == List("a", "b", "c")
class Foo[T: Ordering](xs: T*) {
   val items = xs.toList.sorted
}

// with this object in scope, 
// new Foo("a", "c", "b").items == List("c", "b", "a")
implicit val descending = Ordering[String].reverse

Upvotes: 7

Daniel C. Sobral
Daniel C. Sobral

Reputation: 297205

The question, as stated, is still unanswered (though "use view bounds" solves the problem, which is more useful). The answer is simply that an Int in Scala is supposed to be equivalent to an int in Java, which is not a class at all, and, therefore, cannot even be a Comparable (though that could be solved in Java 7 with defender methods... I wonder if they'll do it).

The solution given, to use a view bound, is used throughout Scala to solve the problem of a class that could implement something but doesn't, because it is not under Scala's control -- ie, Java classes.

And, of course, it can be used by programmers themselves to deal with similar stuff from libraries and frameworks, or simply to produce wrappers around a library to give it a Scala-ish feeling.

Upvotes: 16

missingfaktor
missingfaktor

Reputation: 92056

Use a view bound.

Welcome to Scala version 2.8.0.final (Java HotSpot(TM) Client VM, Java 1.6.0_21).
Type in expressions to have them evaluated.
Type :help for more information.

scala> class Foo[T <% Comparable[T]](val x : T)
defined class Foo

scala> new Foo(3)
res0: Foo[Int] = Foo@9aca82

Upvotes: 17

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