Switch to Scala 3 universal equality for assertEquals()
#223
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Previously, MUnit had a subtyping constraint on `assertEquals(a, b)` so that it would fail to compile if `a` was not a subtype of `b`. This was a suboptimal solution because the compile error messages could become cryptic in some cases. Now, MUnit uses the new `Eql[A, B]` universal equality in Scala 3. For Scala 2, we drop the subtyping constraint so that it's no longer a compile error to compare unrelated types. If the values are different, then users will see a runtime error instead.
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| * http://dotty.epfl.ch/docs/reference/contextual/multiversal-equality.html | ||
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| sealed abstract class Eql[A, B] |
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cc/ @OlivierBlanvillain @smarter what is the recommended Eql "polyfill" for Scala 2 in cross-built library APIs?
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This is the implementation in the docs
package scala
import annotation.implicitNotFound
@implicitNotFound("Values of types ${L} and ${R} cannot be compared with == or !=")
sealed trait Eql[-L, -R]
object Eql {
object derived extends Eql[Any, Any]
}
We can probably copy paste this, and add implicit to the derived object. We can also probably drop the implicitNotFound annotation for Scala 2 (since it will never be triggered)
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You're the first person to do it so there's no recommendation yet :). Personally I think I would use separate source files for Scala 2 and 3 in the places where I would want to use Eql.
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I went with a separate Compare[A, B] type-class that I use in both Scala 2 and Scala 3, and in Scala 3 I provide default instances based on Eql[A, B].
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I should probably have marked this PR as a draft. I’m still experimenting with different alternatives to see if we can support custom equality type classes like cats.Eq |
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Superseded by #225 |
Previously, MUnit had a subtyping constraint on
assertEquals(a, b)so that it would fail to compile ifawas not a subtype ofb. This was a suboptimal solution because the compile error messages could become cryptic in some cases. Now, MUnit uses the newEql[A, B]multiversal equality in Scala 3 (http://dotty.epfl.ch/docs/reference/contextual/multiversal-equality.html). For Scala 2, we drop the subtyping constraint so that it's no longer a compile error to compare unrelated types. If the values are different, then users will see a runtime error instead.Fixes #189