Use of Scala by-name parameters

Question

I am going through the book \"Functional Programming in Scala\" and have run across an example that I don\'t fully understand.

In the chapter on strictness/laziness

Answer 1

In def cons[A](hd: => A, tl: => Stream[A]) : Stream[A]

the type of hd is A, tl is Stream[A]

whereas in case class Cons[+A](h: () => A, t: () => Stream[A]) extends Stream[A]

h is of type Function0[A] and t of type Function0[Stream[A]]

given the type of hd is A, the smart constructor invokes the case class as

 lazy val head = hd
 lazy val tail = tl
 Cons(() => head, () => tail) //it creates a function closure so that head is accessible within Cons for lazy evaluation