Enforcing non-emptyness of scala varargs at compile time - scala

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.

Answer 2

Let's start out with what I think is your base requirement: the ability to define myFunc in some way such that the following occurs at the Scala console when a user provides literals. Then maybe if we can achieve that, we can try to go for varargs.
myFunc(List(1)) // no problem
myFunc(List[Int]()) // compile error!
Moreover, we don't want to have to force users either to split a list into a head and tail or have them convert to a ::.
Well when we're given literals, since we have access to the syntax used to construct the value, we can use macros to verify that a list is non-empty. Moreover, there's already a library that'll do it for us, namely refined!
scala> refineMV[NonEmpty]("Hello")
res2: String Refined NonEmpty = Hello
scala> refineMV[NonEmpty]("")
<console>:39: error: Predicate isEmpty() did not fail.
refineMV[NonEmpty]("")
^
Unfortunately this is still problematic in your case, because you'll need to put refineMV into the body of your function at which point the literal syntactically disappears and macro magic fails.
Okay what about the general case that doesn't rely on syntax?
// Can we do this?
val xs = getListOfIntsFromStdin() // Pretend this function exists
myFunc(xs) // compile error if xs is empty
Well now we're up against a wall; there's no way a compile time error can happen here since the code has already been compiled and yet clearly xs could be empty. We'll have to deal with this case at runtime, either in a type-safe manner with Option and the like or with something like runtime exceptions. But maybe we can do a little better than just throw our hands up in the air. There's two possible paths of improvement.
Somehow provide implicit evidence that xs is nonempty. If the compiler can find that evidence, then great! If not, it's on the user to provide it somehow at runtime.
Track the provenance of xs through your program and statically prove that it must be non-empty. If this cannot be proved, either error out at compile time or somehow force the user to handle the empty case.
Once again, unfortunately this is problematic.
I strongly suspect this is not possible (but this is still only a suspicion and I would be happy to be proved wrong). The reason is that ultimately implicit resolution is type-directed which means that Scala gets the ability to do type-level computation on types, but Scala has no mechanism that I know of to do type-level computation on values (i.e. dependent typing). We require the latter here because List(1, 2, 3) and List[Int]() are indistinguishable at the type level.
Now you're in SMT solver land, which does have some efforts in other languages (hello Liquid Haskell!). Sadly I don't know of any such efforts in Scala (and I imagine it would be a harder task to do in Scala).
The bottom line is that when it comes to error checking there is no free lunch. A compiler can't magically make error handling go away (although it can tell you when you don't strictly need it), the best it can do is yell at you when you forget to handle certain classes of errors, which is itself very valuable. To underscore the no free lunch point, let's return to a language that does have dependent types (Idris) and see how it handles non-empty values of List and the prototypical function that breaks on empty lists, List.head.
First we get a compile error on empty lists
Idris> List.head []
(input):1:11:When checking argument ok to function Prelude.List.head:
Can't find a value of type
NonEmpty []
Good, what about non-empty lists, even if they're obfuscated by a couple of leaps?
Idris> :let x = 5
-- Below is equivalent to
-- val y = identity(Some(x).getOrElse(3))
Idris> :let y = maybe 3 id (Just x)
-- Idris makes a distinction between Natural numbers and Integers
-- Disregarding the Integer to Nat conversion, this is
-- val z = Stream.continually(2).take(y)
Idris> :let z = Stream.take (fromIntegerNat y) (Stream.repeat 2)
Idris> List.head z
2 : Integer
It somehow works! What if we really don't let the Idris compiler know anything about the number we pass along and instead get one at runtime from the user? We blow up with a truly gargantuan error message that starts with When checking argument ok to function Prelude.List.head: Can't find a value of type NonEmpty...
import Data.String
generateN1s : Nat -> List Int
generateN1s x = Stream.take x (Stream.repeat 1)
parseOr0 : String -> Nat
parseOr0 str = case parseInteger str of
Nothing => 0
Just x => fromIntegerNat x
z : IO Int
z = do
x <- getLine
let someNum = parseOr0 x
let firstElem = List.head $ generateN1s someNum -- Compile error here
pure firstElem
Hmmm... well what's the type signature of List.head?
Idris> :t List.head
-- {auto ...} is roughly the same as Scala's implicit
head : (l : List a) -> {auto ok : NonEmpty l} -> a
Ah so we just need to provide a NonEmpty.
data NonEmpty : (xs : List a) -> Type where
IsNonEmpty : NonEmpty (x :: xs)
Oh a ::. And we're back at square one.

Answer 3

Use scala.collection.immutable.::
:: is the cons of the list
defined in std lib
::[A](head: A, tail: List[A])
use :: to define myFunc
def myFunc[A](list: ::[A]): Int = 1
def myFunc[A](head: A, tail: A*): Int = myFunc(::(head, tail.toList))
Scala REPL
scala> def myFunc[A](list: ::[A]): Int = 1
myFunc: [A](list: scala.collection.immutable.::[A])Int
scala> def myFunc[A](head: A, tail: A*): Int = myFunc(::(head, tail.toList))
myFunc: [A](head: A, tail: A*)Int