Mark Needham

I’ve been playing around with Scala a bit and in particular following some of the code examples from Daniel Spiewak’s ‘Scala for Java Refugees’ article on Traits and Types.

One thing that I got a bit confused about in one of the examples was the use of the ‘_’ at the end of one of the function definitions:

class MyContainer[T] {
  private var obj:T = null
 
  def value = obj
  def value_=(v:T) = obj = v
}
 
val cont = new MyContainer[String]
cont.value = "Daniel"
 
println(cont.value)

From my limited understanding of the language the ‘_’ (or placeholder syntax) is often passed to functions when we only want to partially apply the function to its arguments but when you use it in that context there’s usually a space between the function name and the ‘_’ so it wasn’t being used like that.

From Programming in Scala:

Remember that you need to leave a space between the function name and the underscore, because otherwise the compiler will think you are referring to a different symbol, such as for example, a method named println_, which likely does not exist.

In this example we were able to make use of ‘value’ without using the ‘_’ though so clearly this was some other syntax I was unaware of.

I came across the idea that it might be linked to ‘DynamicVariable‘ but I think that it’s just a coincidence that ‘DynamicVariable’ happens to define a function called ‘value_’ since the code described above doesn’t mention ‘DynamicVariable’ anywhere.

Eventually Lars Westergren set me back on track by pointing out that what’s being described is a mixed identifier which is a much simpler explanation than what I’d been thinking!

In this case the mixed identifier is ‘value_=’ which defines an assignment operator which takes in a value ‘v’ of type ‘T’ and then assigns it to ‘obj’.

It seems quite similar to the way we would use properties in C#.

I’m still finding having so many ‘=’ on the same line to be a bit confusing at the moment so I’d probably rewrite that function like this so it’s easier for me to understand:

  def value_=(v:T) = { obj = v }

Channing Walton also linked me to a post which explains how Scala properties work.