How to systematically avoid unsafe pattern matching in Scala?

Question

Consider the following broken function:

def sum (list : Seq[Int]) : Int = list match {
  case Nil => 0
  case head :: tail => head + sum(tail)
}

Here, the function was supposed to work with a List[Int], but was refactored to accept Seq[Int] instead, thus becoming broken without the compiler noticing.

This gaping hole in Scala's incomplete pattern match detection makes it next to useless.

I want to have a way of systematically detecting such problems. Specifically, I'd like the compiler to emit an error/warning on every instanceof-guided pattern match, i.e. I only want to allow pattern matches on sealed hierarchies and on custom matchers.

Are there existing compiler options/plugins for doing conservative (as opposed to arbitrary) checks of pattern matching safety?

Answer 1

Nil and :: are clearly ways to construct a List, but not all Sequences happen to be Lists, so one would expect the Scala type checker to reject this program as ill-typed. Right?

Wrong. Try this and you'll see what I mean:

def sum (list : Seq[Int]) : Int = list match {
  case Nil => 0
  case head :: tail => head + sum(tail)
  case _ => -1
}

> sum(Array(1,2,3).toSeq)
res1: Int = -1
> sum(List(1,2,3))
res2: Int = 6

So you see, some Sequences might be able to be deconstructed with Nil and ::, so those which can, will. Those which can't will fail the pattern match and move on, trying the next match. Nil and :: are sufficient to cover all the possibilities for List, but not for Seq. There is a tradeoff here between subtyping, convenience, and type safety. The solution for now is: be more careful when you refactor.

Answer 2

Have a look at this answer by M. Odersky.

Summary

Checks on matches of non-sealed hierarchies are doable, not trivial and not (yet) implemented.