Possible to make scala require a non-Nothing generic method parameter and return to type-safety

Question

In scala, the shit can hit the fan if the caller of a generic method omits to explicitly specify the type parameter. For example:

class Expression[+T] // Wil         


        

Answer 1

I've found a somewhat hacky solution, but it works. Create a NotNothing type that's contravariant in its type parameter, then provide an implicit object for both Any and Nothing. Now if you try to use a value of NotNothing with Nothing the compiler will complain about ambiguity. Case in point:

sealed trait NotNothing[-T] 

object NotNothing {
  implicit object YoureSupposedToSupplyAType extends NotNothing[Nothing]
  implicit object notNothing extends NotNothing[Any] 
}

Then constrain your makey function with the NotNothing type:

def makey[T : NotNothing]() = { ... }

And voila now you'll get a compile time error if you forget to supply a type!