Scala constructor overload?

Question

How do you provide overloaded constructors in Scala?

Answer 1

While looking at my code, I suddenly realized that I did kind of an overload a constructor. I then remembered that question and came back to give another answer:

In Scala, you can’t overload constructors, but you can do this with functions.

Also, many choose to make the apply function of a companion object a factory for the respective class.

Making this class abstract and overloading the apply function to implement-instantiate this class, you have your overloaded “constructor”:

abstract class Expectation[T] extends BooleanStatement {
    val expected: Seq[T]
    …
}

object Expectation {
    def apply[T](expd:     T ): Expectation[T] = new Expectation[T] {val expected = List(expd)}
    def apply[T](expd: Seq[T]): Expectation[T] = new Expectation[T] {val expected =      expd }

    def main(args: Array[String]): Unit = {
        val expectTrueness = Expectation(true)
        …
    }
}

Note that I explicitly define each apply to return Expectation[T], else it would return a duck-typed Expectation[T]{val expected: List[T]}.