The Matchable Trait

The Problem

The Scala 3 standard library has a type IArray for immutable arrays that is defined like this:

opaque type IArray[+T] = Array[_ <: T]

The IArray type offers extension methods for length and apply, but not for update; hence it seems values of type IArray cannot be updated.

However, there is a potential hole due to pattern matching. Consider:

val imm: IArray[Int] = ...
imm match
  case a: Array[Int] => a(0) = 1

The test will succeed at runtime since IArrays are represented as Arrays at runtime. But if we allowed it, it would break the fundamental abstraction of immutable arrays.

Aside: One could also achieve the same by casting:

imm.asInstanceOf[Array[Int]](0) = 1

But that is not as much of a problem since in Scala asInstanceOf is understood to be low-level and unsafe. By contrast, a pattern match that compiles without warning or error should not break abstractions.

Note also that the problem is not tied to opaque types as match selectors. The following slight variant with a value of parametric type T as match selector leads to the same problem:

def f[T](x: T) = x match
  case a: Array[Int] => a(0) = 0
f(imm)

Finally, note that the problem is not linked to just opaque types. No unbounded type parameter or abstract type should be decomposable with a pattern match.

The Solution

There is a new type scala.Matchable that controls pattern matching. When typing a pattern match of a constructor pattern C(...) or a type pattern _: C it is required that the selector type conforms to Matchable. If that's not the case a warning is issued. For instance when compiling the example at the start of this section we get:

> sc ../new/test.scala -source future
-- Warning: ../new/test.scala:4:12 ---------------------------------------------
4 |    case a: Array[Int] => a(0) = 0
  |            ^^^^^^^^^^
  |            pattern selector should be an instance of Matchable,
  |            but it has unmatchable type IArray[Int] instead

To allow migration from Scala 2 and cross-compiling between Scala 2 and 3 the warning is turned on only for -source future-migration or higher.

Matchable is a universal trait with Any as its parent class. It is extended by both AnyVal and AnyRef. Since Matchable is a supertype of every concrete value or reference class it means that instances of such classes can be matched as before. However, match selectors of the following types will produce a warning:

• Type Any: if pattern matching is required one should use Matchable instead.
• Unbounded type parameters and abstract types: If pattern matching is required they should have an upper bound Matchable.
• Type parameters and abstract types that are only bounded by some universal trait: Again, Matchable should be added as a bound.

Here is the hierarchy of top-level classes and traits with their defined methods:

abstract class Any:
  def getClass
  def isInstanceOf
  def asInstanceOf
  def ==
  def !=
  def ##
  def equals
  def hashCode
  def toString

trait Matchable extends Any

class AnyVal extends Any, Matchable
class Object extends Any, Matchable

Matchable is currently a marker trait without any methods. Over time we might migrate methods getClass and isInstanceOf to it, since these are closely related to pattern-matching.

Matchable and Universal Equality

Methods that pattern-match on selectors of type Any will need a cast once the Matchable warning is turned on. The most common such method is the universal equals method. It will have to be written as in the following example:

class C(val x: String):

  override def equals(that: Any): Boolean =
    that.asInstanceOf[Matchable] match
      case that: C => this.x == that.x
      case _ => false

The cast of that to Matchable serves as an indication that universal equality is unsafe in the presence of abstract types and opaque types since it cannot properly distinguish the meaning of a type from its representation. The cast is guaranteed to succeed at run-time since Any and Matchable both erase to Object.

For instance, consider the definitions

opaque type Meter = Double
def Meter(x: Double): Meter = x

opaque type Second = Double
def Second(x: Double): Second = x

Here, universal equals will return true for

Meter(10).equals(Second(10))

even though this is clearly false mathematically. With multiversal equality one can mitigate that problem somewhat by turning

import scala.language.strictEquality
  Meter(10) == Second(10)

into a type error.