letrec in Scala? (Immutable way to “Tie the knot?”)

Question

Suppose I have a stupid little case class like so:

case class Foo(name: String, other: Foo)

How can I define a and b

Answer 1

You want Foo to be unchanged, but laziness in Scala is on the declaration site. It is impossible for Foo to be non-strict without changing it, and the pattern indicated in Haskell only works because Foo, there, is non-strict (that is, Foo "a" b doesn't evaluate b immediately).

Otherwise the solution is pretty much the same, allowing for the hoops necessary to get everything non-strict:

class Foo(name: String, other0: => Foo) { // Cannot be case class, because that mandates strictness
  lazy val other = other0 // otherwise Scala will always reevaluate
}
object Foo {
  def apply(name: String, other: => Foo) = new Foo(name, other)
}

val (a: Foo, b: Foo) = (Foo("a", b), Foo("b", a))