Multiple assignment from a vector to individual variables

Question

In my test suite I am frequently writing code such as

val Vector(socialSci, humanities, pureSci, pureApplSci) = foo(someValue)

foo obviously returns a (short) Vector but I'd like to have names for each of the values. It's not always four items long, but since it's my test suite the correct number is known and I'm perfectly fine with an exception being thrown if there is a length mismatch.

The above code works -- except that I get a compiler error:

match may not be exhaustive.

Is there a way to get rid of the compiler warning? I've tried @nowarn and @nocheck in various positions with no luck.

Alternatively, is there a succinct way to write this so it doesn't cause a warning in the first place? The solution at Convert Vector to Tuple scala looks promising except that reflection isn't available for ScalaJS.

def toTuple(seq: Seq[_]): Product = {
  val clz = Class.forName("scala.Tuple" + seq.size)
  clz.getConstructors()(0).newInstance(seq.map(_.asInstanceOf[AnyRef]): _*).asInstanceOf[Product]
}

Is there an approach using a builder or something?

Update:

Thanks to Dmytro Mitin for reminding me to check my compiler options. Yes, I had completely forgotten about that and had -Xlint:valpattern enabled. Turning that off removes the warning.

That solves my immediate problem. But as a challenge, is there a way to implement toTuple without using reflection?

https://scastie.scala-lang.org/DmytroMitin/Jb0A5lWYQHKKoESw0v8T0Q As you can see by default there is no warning. Show your `scalacOptions` switched on. — Dmytro Mitin, Mar 28 '23 at 20:06
It looks like it should be returning a case class instead of a vector — michaJlS, Mar 28 '23 at 20:23
@DmytroMitin, thanks for the reminder. You're spot-on, as you can see in my update. Not sure how to credit you other than with this comment. Thanks! — bwbecker, Mar 28 '23 at 22:01
@michaJlS, I'm not returning a case class because the length can vary. In the main body of my code I process it as a vector. But in testing I want to look at the attributes of individual elements. Having them assigned to variables makes that easier. — bwbecker, Mar 28 '23 at 22:03

Dmytro Mitin · Accepted Answer · 2023-03-28T23:26:12.637

You can switch off -Xlint:valpattern.

But if you want to keep -Xlint:valpattern and switch off the warning match may not be exhaustive in this specific place you can use @unchecked

def foo(): Vector[Int] = Vector(1, 2, 3, 4)
val Vector(socialSci, humanities, pureSci, pureApplSci) = foo() : @unchecked

If a vector returned by foo can be longer than 4 elements you can additionally use vararg pattern

val Vector(socialSci, humanities, pureSci, pureApplSci, _*) = ...

You can also transform a vector into a tuple with Shapeless. I can't see direct Vector => Tuple, so I'm using Vector => HList => Tuple

import shapeless.syntax.std.traversable._ // libraryDependencies += "com.chuusai" %% "shapeless" % "2.3.10"
val (socialSci, humanities, pureSci, pureApplSci) = foo().toSizedHList(4).get.tupled

On contrary to runtime reflection, this mostly works at compile time (we're specifying 4 statically, otherwise .get will throw java.util.NoSuchElementException: None.get at runtime), so this should work in ScalaJS too

libraryDependencies += "com.chuusai" %%% "shapeless" % "2.3.10" // ScalaJS

Multiple assignment from a vector to individual variables

Update:

1 Answers1