Revive "Strict Equality" for assertEquals()
#521
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Previously, FailException had some custom nice-to-have features that ComparisonFailException didn't have.
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. Additionally, this API didn't integrate with other libaries like Cats that has its own `cats.Eq[A,B]` type-class. Now, MUnit uses a new `munit.Compare[A,B]` type-class for comparing values of different types. By default, MUnit provides a "universal" instance that permits comparison between all types and uses the built-in `==` method. Users can optionally enable "strict equality" by adding the compiler option `"-Xmacro-settings.munit.strictEquality"` in Scala 2. In Scala 3, we use the `Eql[A, B]` type-classes instead to determine type equality.
This is a fourth attempt at improving strict equality in MUnit `assertEquals()` assertions. * First attempt (current release version): require second argument to be a supertype of the first argument. This has the flaw that the compile error message is cryptic and that the ordering of the arguments affects compilation. * Second attempt: use `Eql[A, B]` in Scala 3 and allow comparing any types in Scala 2. This has the flaw that it's a regression in some cases for Scala 2 users and that `Eql[A, B]` is not really usable in its current form, see related discussion https://contributors.scala-lang.org/t/should-multiversal-equality-provide-default-eql-instances/4574 * Third attempt: implement "strict equality" for Scala 2 with a macro and `Eql[T, T]` in Scala. This improves the situation for Scala 2, but would mean relying on a feature that we can't easily port to Scala 3. * Fourth attempt (this commit): improve the first attempt (current release) by allowing `Compare[A, B]` as long as `A <:< B` OR `B <:< A`. This is possible thanks to an observation by Gabriele Petronella that it's possible to layer the implicits to avoid diverging implicit search. The benefit of the fourth approach is that it works the same way for Scala 3 and Scala 3. It's very nice that we can avoid macros as well.
The Scala 3 (dotty) tests now use compareSubtypeWithSupertype instead of compareSupertypeWithSubtype. Additionally, the "unrelated" test was not seeing the context code above and so I've moved all the code into compileErrors.
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olafurpg
approved these changes
Apr 16, 2022
| @@ -28,9 +28,6 @@ object FullStackTraceFrameworkSuite | |||
| extends BaseStackTraceFrameworkSuite( | |||
| Array("-F"), | |||
| """|at munit.Assertions:failComparison | |||
| | at munit.Assertions:failComparison$ | |||
| | | ||
| |The following import might make progress towards fixing the problem: | ||
| | | ||
| | import munit.CustomCompare.fromCustomEquality |
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It's a shame these suggestions have false positives like this 🙈
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Mmmm, yeah this is a bummer.
I think it's related to scala/scala3#9685
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Nice find! I left a comment in that issue about the fact that we can reproduce without inline scala/scala3#9685 (comment)
Co-authored-by: Ólafur Páll Geirsson <olafurpg@gmail.com>
Co-authored-by: Ólafur Páll Geirsson <olafurpg@gmail.com>
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This PR is mostly a freshly rebased version of #225.
It is absolutely worth reviewing the discussion in the original PR. However, I've copy and pasted its description here:
Binary Incompatibility
This PR introduces a binary breaking changes to the following methods:
munit.Assertions.assertNotEqualsmunit.Assertions.assertEqualsmunit.FunSuite.assertNotEqualsmunit.FunSuite.assertEqualsRebasing notes:
The rebase got a little hairy. During it I ditched special casing for array comparisons, so 1503667 reintroduces it. Similarly, b28073f reintroduces the better string inequality error messages.