One of the cool things about learning Scala is the concept of val. This keyword creates a “fixed value” which cannot be modified.

Or so I thought…

While working today in a Scala project using a 3rd party Java library, I was bitten by a gotcha which I will demonstrate here:

case class DemoClass(var mutableProp: String, immutableProp: String)

val demo = new DemoClass("watch me go", "watch me stay")

demo.mutableProp = "gone"

demo
// res3: DemoClass = DemoClass("gone", "watch me stay")

demo.immutableProp = "not gonna happen"
// cmd4.sc:1: reassignment to val
// val res4 = demo.immutableProp = "not gonna happen"
//

So what’s happening here?

A val is created (demo) which is an instance of DemoClass.

DemoClass contains a property that is a var, meaning that it is mutable in the Scala world.

So while demo is a “fixed value”, we are still able to modify its internal properties.

A heavy-handed example, to be sure. But I think it demonstrates my point that just cause a top-level object is defined as a val doesn’t mean it is frozen in stone.

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special note: today marks my 1 year anniversary as a software developer at Uptake! I’m so happy and fortunate to be able to work for such a great company, full of totally brilliant people, where I get to learn awesome stuff every day. Cheers to 1 year.