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

First, you are assuming that => A and () => A are the same. However, they are not. For example, the => A can only be used in the context of passing parameters by-name - it is impossible to declare a val of type => A. As case class parameters are always vals (unless explicitly declared vars), it is clear why case class Cons[+A](h: => A, t: => Stream[A]) would not work.

Second, just wrapping a by-name parameter into a function with an empty parameter list is not the same as what the code above accomplishes: using lazy vals, it is ensured that both hd and tl are evaluated at most once. If the code read

Cons(() => hd, () => tl)

the original hd would be evaluated every time the h method (field) of a Cons object is invoked. Using a lazy val, hd is evaluated only the first time the h method of this Cons object is invoked, and the same value is returned in every subsequent invocation.

Demonstrating the difference in a stripped-down fashion in the REPL:

> def foo = { println("evaluating foo"); "foo" }
> val direct : () => String = () => foo
> direct()
evaluating foo
res6: String = foo
> direct()
evaluating foo
res7: String = foo
> val lzy : () => String = { lazy val v = foo; () => v }
> lzy()
evaluating foo
res8: String = foo
> lzy()
res9: String = foo

Note how the "evaluating foo" output in the second invocation of lzy() is gone, as opposed to the second invocation of direct().