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Why is the implicit conversion from Long to RichLong not applied if a RichLong supertype is expected?

The Scala 2.8 spec says in section 7.3 (the highlight is mine):

Implicit parameters and methods can also define implicit conversions called views. The view from type S to type T is determined by an implicit value that has the function type S => T or (=> S) => T or by a method that is converted to a value of this type. Representations apply in two situations.

  • If the expression e is of type T and T does not match the expressions, the expected type is pt. In this case, an implicit v is searched, which is applicable to e and , the result of which matches pt. The search continues, as in the case of implicit parameters, where the implicit region is one of T => pt. If this is the case, the expression e is converted to v (e).

[...]

Considering the above and the following facts:

  • Long not a subtype of java.lang.Comparable[Long] , i.e. does not match for input T , where T <: java.lang.Comaparable[Long]
  • Predef contains implicit def longWrapper (x: Long) : RichLong
  • RichLong is a subtype of java.lang.Comparable[Long] , that is, it matches for input T , where T <: java.lang.Comaparable[Long]

I expect that the implicit conversion will be applied where Long is encountered, and a subtype of java.lang.Comparable[Long] is expected. But:

 scala> def test[T <: java.lang.Comparable[Long]](c: T) = println(c) test: [T <: java.lang.Comparable[Long]](c: T)Unit scala> test(12L) <console>:7: error: inferred type arguments [Long] do not conform to method test type parameter bounds [T <: java.lang .Comparable[Long]] test(12L) ^

The result will be as expected if the value is explicitly converted:

 scala> test(longWrapper(12L)) 12

Why is the conversion function not applied implicitly?

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1 answer

You need to use view binding ( <% ) to search for the compiler and apply implicit conversion.

 scala> def test[T <% java.lang.Comparable[Long]](c: T) = println(c) test: [T](c: T)(implicit evidence$1: (T) => java.lang.Comparable[Long])Unit scala> test(12L) 12

More information about linking to viewing can be found on the page of this page (Ctrl + F for "viewing").

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