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Multiversal Equality

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Previously, Scala had universal equality: Two values of any types could be compared with each other with == and !=. This came from the fact that == and != are implemented in terms of Java's equals method, which can also compare values of any two reference types.

Universal equality is convenient but also dangerous since it undermines type safety. Say you have an erroneous program where a value y has type S instead of the expected type T.

val x = ... // of type T
val y = ... // of type S, but should be T
x == y      // typechecks, will always yield false

If all you do with y is compare it to other values of type T, the program will typecheck but probably give unexpected results.

Multiversal equality is an opt-in way to make universal equality safer. The idea is that by declaring an implicit value one can restrict the types that are legal in comparisons. The example above would not typecheck if an implicit was declared like this for type T (or an analogous one for type S):

implicit def eqT: Eq[T, T] = Eq

This definition effectively says that value of type T can (only) be compared with == or != to other values of type T. The definition is used only for type checking; it has no significance for runtime behavior, since == always maps to equals and != alwatys maps to the negation of equals. The right hand side of the definition is a value that has any Eq instance as its type. Here is the definition of class Eq and its companion object:

package scala
import annotation.implicitNotFound

@implicitNotFound("Values of types ${L} and ${R} cannot be compared with == or !=")
sealed trait Eq[-L, -R]

object Eq extends Eq[Any, Any]

One can have several Eq instances for a type. For example, the four definitions below make values of type A and type B comparable with each other, but not comparable to anything else:

implicit def eqA : Eq[A, A] = Eq
implicit def eqB : Eq[B, B] = Eq
implicit def eqAB: Eq[A, B] = Eq
implicit def eqBA: Eq[B, A] = Eq

(As usual, the names of the implicit definitions don't matter, we have chosen eqA, ..., eqBA only for illustration).

The scala.Eq object defines a number of Eq implicits that make values of types String, Boolean and Unit only comparable to values of the same type. They also make numbers only comparable to other numbers, sequences only comparable to other sequences and sets only comparable to other sets.

There's also a "fallback" instance named eqAny that allows comparisons over all types that do not themeselves have an Eq instance. eqAny is defined as follows:

def eqAny[L, R]: Eq[L, R] = Eq

Even though eqAny is not declared implicit, the compiler will still construct an eqAny instance as answer to an implicit search for the type Eq[L, R], provided that neither L nor R have Eq instances defined on them.

The primary motivation for having eqAny is backwards compatibility, if this is of no concern one can disable eqAny by enabling the language feature strictEquality. As for all language features this can be either done with an import

import scala.language.strictEquality

or with a command line option -language:strictEquality.

All enum types also come with Eq instances that make values of the enum type comparable only to other values of that enum type.

The precise rules for equality checking are as follows.

  1. A comparison using x == y or x != y between values x: T and y: U is legal if either T and U are the same, or one of the types is a subtype of the "lifted" version of the other type, or an implicit value of type scala.Eq[T, U] is found. See the description on Github for a definition of lifting.

  2. The usual rules for implicit search apply also to Eq instances, with one modification: An instance of scala.Eq.eqAny[T, U] is constructed if neither T nor U have a reflexive Eq instance themselves. Here, a type T has a reflexive Eq instance if the implicit search for Eq[T, T] succeeds and constructs an instance different from eqAny.

    Here lifting a type S means replacing all references to abstract types in covariant positions of S by their upper bound, and to replacing all refinement types in in covariant positions of S by their parent. The purpose if lifte

More on multiversal equality is found in a blog post and a Github issue.