· scala

Scala: The '_=' mixed identifier

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.

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