Enforcing non-emptyness of scala varargs at compile time

Question

I have a function that expects a variable number of parameters of the same type, which sounds like the textbook use case for varargs:

def myFunc[A](as: A*) = ???

The problem I have is that myFunc cannot accept empty parameter lists. There's a trivial way of enforcing that at runtime:

def myFunc[A](as: A*) = {
  require(as.nonEmpty)
  ???
}

The problem with that is that it happens at runtime, as opposed to compile time. I would like the compiler to reject myFunc().

One possible solution would be:

def myFunc[A](head: A, tail: A*) = ???

And this works when myFunc is called with inline arguments, but I'd like users of my library to be able to pass in a List[A], which this syntax makes very awkward.

I could try to have both:

def myFunc[A](head: A, tail: A*) = myFunc(head +: tail)
def myFunc[A](as: A*) = ???

But we're right back where we started: there's now a way of calling myFunc with an empty parameter list.

I'm aware of scalaz's NonEmptyList, but in as much as possible, I'd like to stay with stlib types.

Is there a way to achieve what I have in mind with just the standard library, or do I need to accept some runtime error handling for something that really feels like the compiler should be able to deal with?

Answer 1

What about something like this?

scala> :paste
// Entering paste mode (ctrl-D to finish)

def myFunc()(implicit ev: Nothing) = ???
def myFunc[A](as: A*) = println(as)

// Exiting paste mode, now interpreting.

myFunc: ()(implicit ev: Nothing)Nothing <and> [A](as: A*)Unit
myFunc: ()(implicit ev: Nothing)Nothing <and> [A](as: A*)Unit

scala> myFunc(3)
WrappedArray(3)

scala> myFunc(List(3): _*)
List(3)

scala> myFunc()
<console>:13: error: could not find implicit value for parameter ev: Nothing
       myFunc()
             ^

scala> 

Replacing Nothing with a class that has an appropriate implicitNotFound annotation should allow for a sensible error message.