Assignments in Scala

Question

I've been shown a weird snippet in Scala which I don't quite understand. To me, assignments in Scala return Unit, unlike in Java where it returns the type of the variable which has been affected a value. However, let's consider this class :

case class C(i: Int) { def f = C(i = i + 10) }

This compiles completely fine, which is pretty weird ! The factory method C.apply expects an Int, whereas I pass it what seems to be an assignment, of type Unit. By the way if I remove the assignment to just let the expression, it seems to have the exact same behaviour.

Let's try this now :

case class C(i: Int) { def f = { i = i + 10 C(i = i + 10) } }

Ok now this is the world I know : i is a val, then you cannot mutate it so i = i + 10 does not compile. However, C(i = i + 10) still compiles without complaining. What is this weirdness ? Is there a reason for thatto exist ?

Sean Vieira · Accepted Answer · 2015-07-17 03:13:27Z

This is because, in the case of C(i = i + 10) the left-hand i is not the field C#i but a named parameter. No assignment is being done at all.

C(i = i + 10) ^ ^ +---|-------- Parameter name | +- - - - - - - - - - - - - - Reference to the field `i` in the instance of the class `C`

Some places where named parameters make sense:

Avoiding the "what does this {boolean, integer} mean" moment:

someMethod(anObject, flagName=true) // As opposed to someMethod(anObject, true) ... what's `true` for? anotherMethod(arg1, arg2, totalMagic=33)

When using default values for parameters (to invoke the right constructor):

def withDefaults(i: Int, flag: Boolean = true, wrapper: Option[String] = None) = { wrapper.fold(s"$i with $flag")(_.format(i, flag)) } withDefaults(3, Some("stuff %s around %s")) // won't compile // error: type mismatch; // found : Some[String] // required: Boolean withDefaults(3, wrapper = Some("stuff %s around %s")) // works (returns "stuff 3 around true")

Oh, right I did not know this syntax. Is there any advantage to do this ? I just tried using it and it allows, for example, to pass the parameters in a random order which I don't find really good as quite often the order of the parameters has some semantic

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