Simple Scala Keyword Parameters

Everything here is mostly intuitive, but I'm putting it here for personal reference. Should be helpful to a lot of people though. There's probably a whole ton of interesting cases I'm leaving out, and I'll try to keep this updated when I think of them. For now I'm covering Default Values, No Default Values, and Overloading. (Note, this doesn't come out until 2.8, I'm working off trunk)

Default Values

• Define a class that takes two keyword args, name and lives, and provide default values.
  

  
  scala> case class Cat(name:String="kitty", lives:Int=9)
  defined class Cat
  

  
  
• Instantiate the cat providing no arguments.
  

  
  scala> new Cat
  res1: Cat = Cat(kitty,9)
  

  
  
• Instantiate a cat providing both params, unnamed.
  

  
  scala> Cat("Java", 1)
  res2: Cat = Cat(Java,1)
  

  
  
• Instantiate a cat providing just the first param, unnamed.
  

  
  scala> Cat("Scala")
  res3: Cat = Cat(Scala,9)
  

  
  
• Instantiate a cat providing both params, named.
  

  
  scala> new Cat(name="newspeak", lives=20)
  res4: Cat = Cat(newspeak,20)
  

  
  
• Instantiate a cat providing both params in reverse order. (Only works if names the argument names are given.)
  

  
  scala> new Cat(lives=20, name="newspeak")
  res5: Cat = Cat(newspeak,20)
  

  
  
• Instantiate a cat providing the first param, named.
  

  
  scala> new Cat(name="newspeak")
  res6: Cat = Cat(newspeak,9)
  

  
  
• Instantiate a cat providing the second param, named.
  

  
  scala> Cat(lives=4)                     
  res7: Cat = Cat(kitty,4)
  

  
  
• Instantiate a cat providing the first argument unnamed, and the second argument named!
  

  
  scala> new Cat("Lua", lives=1)
  res8: Cat = Cat(Lua,1)
  

  
  
• Attempt to instantiate a cat providing the first argument named, and the second argument unnamed. You can't do it! After you name a parameter, the parameters that follow must be named.
  

  
  scala> new Cat(name="Lua", 1)     
  :7: error: positional after named argument.
        new Cat(name="Lua", 1)
                            ^
  

No Default Values

• Redefine class Cat without supplying default values. This means values must be provided when instantiating a cat (more generically, calling a function).
  

  
  scala> case class Cat(name:String, lives:Int)
  defined class Cat
  

  
  
• Instantiate a cat providing both params, unnamed.
  

  
  scala> Cat("Java", 1)
  res9: Cat = Cat(Java,1)
  

  
  
• Instantiate a cat providing both params, named.
  

  
  scala> new Cat(name="Douglas", lives=42)          
  res10: Cat = Cat(Douglas,42)
  

  
  
• Attempt to instantiate a cat, not providing values for the arguments.
  

  
  :7: error: not enough arguments for constructor Cat:
  (name: String,lives: Int)Cat, unspecified parameters:
  value name, value lives
        new Cat
        ^
  

Overloading

• Redefine the class, overloading the constructor.
  

  
  scala> case class Cat(name:String="kitty", lives:Int=9){
      | def this(name:String) = this(name, -54)
      | }
  

  
  
• Instantiate a cat providing just the first argument, unnamed. Since the compiler does find a method with the exact signature (in this case - one String), it calls it.
  

  
  scala> new Cat("Martin")
  res11: Cat = Cat(Martin,-54)
  

  
  
• Here's a couple of other similar, and interesting cases. First, overload the constructor giving the argument a different name than the one defined in the primary constructor.
  

  
  scala> case class Cat(name:String="kitty", lives:Int=9){
      | def this(x:Int) = this("hello", x+99)   
      | }
  defined class Cat
  

  
  
• Instantiate a cat, using the keyword 'lives'. Since the overloading method uses the name x, and x!=lives, and the original constructor uses 'lives', that is the method that is invoked.
  

  
  scala> new Cat(lives=8)                                
  res12: Cat = Cat(kitty,8)
  

  
  
• Instantiate a cat - same as the String overloading case above.
  

  
  scala> new Cat(8)     
  res13: Cat = Cat(hello,107)