Comparing Manifests

Manifests are Scala's solution to Java's type erasure. It is not a complete solution as of yet but it does have several useful applications. Manifests were originally added to Scala 2.7 in an extremely limited form but have been improved a great deal for Scala 2.8. Now more powerful comparisons of manifests are possible. For another introduction to Manifests (2.7 manifests) see Manifests.

This post looks at a few ways to create manifests as well as how to compare manifests. The goal is to create a method that can be used in testing to require that a certain exception is thrown during a code block:

1. scala> intercepts[Exception] {println("hi :)")}                                            
2. hi :)
3. Expected java.lang.Exception but instead no exception was raised

The code snippet above is a failure because println does not throw an exception. In addition to requiring manifests this code snippet also is a custom control structure, for more information on those see Control Structure Tag

But before writing the intercepts methods a short inspection of the new manifest comparison operators:

1. scala> import scala.reflect.{
2.      |   Manifest, ClassManifest
3.      | }
4. import scala.reflect.{Manifest, ClassManifest}
6. // from class creates a manifest object given a class object
7. scala> import ClassManifest.fromClass
8. import ClassManifest.fromClass
10. // several comparisons using <:<
11. scala> fromClass(classOf[Exception]) <:< fromClass(classOf[Exception])
12. res4: Boolean = true
14. scala> fromClass(classOf[Exception]) <:< fromClass(classOf[RuntimeException])
15. res5: Boolean = false
17. scala> fromClass(classOf[Exception]) <:< fromClass(classOf[AssertionError])         
18. res6: Boolean = false
20. // now the opposite operator >:>
21. scala> fromClass(classOf[Exception]) >:> fromClass(classOf[AssertionError])
22. res7: Boolean = false
24. scala> fromClass(classOf[Exception]) >:> fromClass(classOf[RuntimeException])
25. res8: Boolean = true
27. scala> fromClass(classOf[Exception]) >:> fromClass(classOf[Error])           
28. res9: Boolean = false
30. scala> fromClass(classOf[Exception]) >:> fromClass(classOf[Throwable])
31. res10: Boolean = false
33. // the method singleType creates a manifest given an object
34. scala> ClassManifest.singleType(new RuntimeException())
35. res12: scala.reflect.Manifest[Nothing] = java.lang.RuntimeException.type
37. scala> ClassManifest.singleType(new RuntimeException()) <:< fromClass(classOf[Throwable])
38. res13: Boolean = true
40. scala> fromClass(classOf[Exception]) <:< fromClass(classOf[Throwable])                   
41. res14: Boolean = true

Now the implementation of intercepts:

1. scala> import scala.reflect.{
2.      |   Manifest, ClassManifest
3.      | }
4. import scala.reflect.{Manifest, ClassManifest}
6. scala> 
8. scala> import ClassManifest.singleType
9. import ClassManifest.singleType
11. scala>  def intercepts[E <: Throwable](test : => Unit)(implicit m:Manifest[E]) : Unit = {
12.      |     import Predef.{println => fail}
13.      |     try {
14.      |       test
15.      |       // this is a failure because test is expected to throw an exception
16.      |       fail("Expected "+m.toString+" but instead no exception was raised")
17.      |     }catch{
18.      |       // this checks that the expected type (m) is a superclass of the class of e
19.      |       case e if (m >:> singleType(e)) => ()
20.      |       // any other error is handled here
21.      |       case e => fail("Expected "+m.toString+" but instead got "+e.getClass)
22.      |     }
23.      |   }
24. intercepts: [E <: Throwable](test: => Unit)(implicit m: scala.reflect.Manifest[E])Unit
26. scala> intercepts[Exception] {println("hi :)")}                                            
27. hi :)
28. Expected java.lang.Exception but instead no exception was raised
30. scala> intercepts[Exception] { throw new IllegalArgumentException("why not throw this :)")}
32. scala> intercepts[Exception] { throw new AssertionError("The method should complain")}     
33. Expected java.lang.Exception but instead got class java.lang.AssertionError