Import Instance Properties

A truly fantastic aspect of Scala is the uniform principle that Scala attempts to adhere to. In other words Scala tries to not make any rules that only apply to a single case when it can be applied generally.

One example is matching you can see several uses of matching in the following topics:

• Assignments
• Assignment and Parameter Objects

But matching it applies to today's topic as well. This topic covers a cool trick that helps assist with parameter objects and complex return types.

This topic is another take on Assignment and Parameter Objects. There are cases when a method has a large number of parameters and the API can be cleaned up by introducing a parameter object. Or perhaps an object with several public proprties are passed to a method.

1. scala> case class Params(p1:Int, p2:Int, p3:Int)
2. defined class Params
4. scala>  def method(params:Params) = {
5.      |   println(params.p1, params.p2, params.p3)
6.      | }
7. method: (Params)Unit
9. scala> method(Params(1,2,3))
10. (1,2,3)
11. }

The symbol 'params' introduces noise into the code. The noise can be reduced further by assigned the properties of the parameter object to local variables:

1. scala>  case class Params(p1:Int, p2:Int, p3:Int)
2. defined class Params
4. scala> 
6. scala>  def method(params:Params) = {
7.      |   val Params(p1,p2,p3) = params
8.      |   println(p1,p2,p3)
9.      | }
10. method: (Params)Unit
12. scala> method(Params(1,2,3))
13. (1,2,3)
14. }

But we can do better remember that we can import methods and properties from an object:

1. scala> object Obj {
2.      |  val prop = 10
3.      | }
4. defined module Obj
6. scala> import Obj._
7. import Obj._
9. scala> println(prop)
10. 10

Since all instance are objects it is possible to import fields and methods from instances as well:

1. scala>  case class Params(p1:Int, p2:Int, p3:Int)
2. defined class Params
4. scala> 
6. scala>  def method(params:Params) = {
7.      |   import params._
8.      |   println(p1, p2, p3)
9.      | }
10. method: (Params)Unit
11. }

The same technique is extremely useful when a method needs to return multiple values:

1. scala>  def method() = {
2.      |   (1,2,3)
3.      | }
4. method: ()(Int, Int, Int)
6. scala> val retVal = method()
7. retVal: (Int, Int, Int) = (1,2,3)
8. /*
9.  retVal is a tuple so we can import the tuple
10.  properties.  Becareful to not do this multiple times in
11.  the same scope
12. */
13. scala> import retVal._
14. import retVal._
16. scala> println(_1,_2,_3)
17. (1,2,3)
19. scala> def method2={
20.        // Notice case class declaration can be contained in method
21.      | case class Return(v1:Int,v2:Int)
22.      | Return(6,7)
23.      | }
24. method2: java.lang.Object with ScalaObject with Product{def v1: Int; def v2: Int}
26. scala> val r = method2
27. r: java.lang.Object with ScalaObject with Product{def v1: Int; def v2: Int} = Return(6,7)
29. scala> import r._
30. import r._
32. scala> println(v1,v2)
33. (6,7)
34. }