Dynamic calls using Structural Types

Using reflection can be a real pain in Java since the API is a Java API and consists of many gets and searches through collections not to mention so many exceptions that need to be handled. In Scala there is a wonderful way to clean up a reflective call down to a single line (assuming you don't want to worry about handling exceptions.) Here structural typing can really be a pleasure.

1. // I am assigning a string to an Any reference
2. scala> val s:Any = "hello :D"
3. s: Any = hello :D
5. // Any does not have a length method
6. scala> s.length
7. < console>:6: error: value length is not a member of Any
8.        s.length
9.          ^
11. /*
12. But I can cast it to a structural type with a length method
13. */
14. scala> s.asInstanceOf[{def length:Int}].length
15. res2: Int = 8

There are restrictions to this. For example implicits will not work:

1. /*
2. The method r is part of StringLike (or RichString in Scala 2.7) 
3. and there is an implicit conversion from String to RichString/StringLike.
4. The structural type does not try to apply the implicits because implicits are a
5. compile time artifact and that information is not kept at run time.  Perhaps this
6. will be added in the future but it is not present now.
7. */
8. scala> s.asInstanceOf[{def r:util.matching.Regex}].r
9. java.lang.NoSuchMethodException: java.lang.String.r()

More examples:

1. scala> val s:Any = "hello :D"
2. s: Any = hello :D
4. scala> s.asInstanceOf[{def charAt(x:Int):Char}].charAt(2)
5. res9: Char = l
7. /*
8. This is interesting.  The actual method returns Char, but Char is
9. compatible with Any so this cast works.
10. */
11. scala> s.asInstanceOf[{def charAt(x:Int):Any}].charAt(2) 
12. res10: Any = l
14. // Now lets test to see if that lenience is available for parameters:
15. scala> class X 
16. defined class X
18. scala> class Y extends X
19. defined class Y
21. scala> class T {
22.      | def x(x:X):Int = 1
23.      | }
24. defined class T
26. scala> val a:Any = new T()
27. a: Any = T@687c3b99
29. scala> a.asInstanceOf[{def x(x:X):Any}].x(new Y())
30. res11: Any = 1
32. /*
33. Ok so return values can be subclasses but not params
34. */
35. scala> a.asInstanceOf[{def x(x:Y):Any}].x(new Y())
36. java.lang.NoSuchMethodException: T.x(Y)
37. at java.lang.Class.getMethod(Class.ja

Use with care :-D