Add Comparison trait.

modules/core/shared/src/main/scala/weaver/Comparison.scala

@@ -0,0 +1,77 @@
+package weaver
+
+import cats.Eq
+import cats.Show
+import com.eed3si9n.expecty._
+import scala.annotation.implicitNotFound
+
+/**
+ * A type class used to compare two instances of the same type and construct an
+ * informative report.
+ *
+ * If the comparison succeeds with [[Result.Success]] then no report is printed.
+ * If the comparison fails with [[Result.Failure]], then the report is printed
+ * with the test failure.
+ *
+ * The report is generally a diff of the `expected` and `found` values. It may
+ * use ANSI escape codes to add color.
+ */
+@implicitNotFound("Could not find an implicit Comparison[${A}]. Does ${A} have an associated cats.Eq[${A}] instance?")
+trait Comparison[A] {
+
+  def diff(expected: A, found: A): Comparison.Result
+}
+
+object Comparison {
+  sealed trait Result
+  object Result {
+    case object Success                extends Result
+    case class Failure(report: String) extends Result
+  }
+
+  /**
+   * Create a [[Comparison]] instance from an [[cats.kernel.Eq]] implementation.
+   *
+   * Uses the [[cats.Show]] instance or [[cats.Show.fromToString]] to construct
+   * a string diff of the `expected` and `found` values on failure.
+   */
+  implicit def fromEq[A](
+      implicit eqv: Eq[A],
+      showA: Show[A] = Show.fromToString[A]
+  ): Comparison[A] = {
+    new Comparison[A] {
+      def diff(expected: A, found: A): Result = {
+        if (eqv.eqv(found, expected)) {
+          Result.Success
+        } else {
+          val expectedLines = showA.show(expected).linesIterator.toSeq
+          val foundLines    = showA.show(found).linesIterator.toSeq
+          val report = DiffUtil
+            .mkColoredLineDiff(expectedLines, foundLines)
+            .linesIterator
+            .toSeq
+            .map(str => Console.RESET.toString + str)
+            .mkString("\n")
+          Result.Failure(report)
+        }
+      }
+    }
+  }
+
+  /**
+   * Create a [[Comparison]] instance from a `diff` implementation.
+   */
+  def instance[A](f: (A, A) => Option[String]): Comparison[A] =
+    new Comparison[A] {
+      def diff(expected: A, found: A): Result = f(expected, found) match {
+        case None         => Result.Success
+        case Some(report) => Result.Failure(report)
+      }
+    }
+
+  /**
+   * Create a [[Comparison]] instance from a `diff` implementation.
+   */
+  def instance[A](f: PartialFunction[(A, A), String]): Comparison[A] =
+    instance((expected, found) => f.lift((expected, found)))
+}

modules/core/shared/src/main/scala/weaver/internals/ExpectSame.scala

@@ -5,35 +5,21 @@ import cats.Show
 import cats.data.{ NonEmptyList, Validated }
 import cats.kernel.Eq
 
-import com.eed3si9n.expecty._
-
 private[weaver] trait ExpectSame {
 
   def eql[A](
       expected: A,
       found: A)(
-      implicit eqA: Eq[A],
-      showA: Show[A] = Show.fromToString[A],
+      implicit comparisonA: Comparison[A],
       loc: SourceLocation): Expectations = {
-
-    if (eqA.eqv(expected, found))
-      Expectations(Validated.validNel(()))
-    else {
-      val header = "Values not equal:"
-
-      val expectedLines = showA.show(expected).linesIterator.toSeq
-      val foundLines    = showA.show(found).linesIterator.toSeq
-      val sourceLocs    = NonEmptyList.of(loc)
-      val diff = DiffUtil
-        .mkColoredLineDiff(expectedLines, foundLines)
-        .linesIterator
-        .toSeq
-        .map(str => Console.RESET.toString + str)
-        .mkString("\n")
-
-      Expectations(
-        Validated.invalidNel[AssertionException, Unit](
-          new AssertionException(header + "\n\n" + diff, sourceLocs)))
+    comparisonA.diff(expected, found) match {
+      case Comparison.Result.Success => Expectations(Validated.validNel(()))
+      case Comparison.Result.Failure(report) =>
+        val header     = "Values not equal:"
+        val sourceLocs = NonEmptyList.of(loc)
+        Expectations(
+          Validated.invalidNel[AssertionException, Unit](
+            new AssertionException(header + "\n\n" + report, sourceLocs)))
     }
   }
 
@@ -43,7 +29,8 @@ private[weaver] trait ExpectSame {
   def same[A](
       expected: A,
       found: A)(
-      implicit eqA: Eq[A] = Eq.fromUniversalEquals[A],
-      showA: Show[A] = Show.fromToString[A],
-      loc: SourceLocation): Expectations = eql(expected, found)
+      implicit comparisonA: Comparison[A] =
+        Comparison.fromEq[A](Eq.fromUniversalEquals, Show.fromToString),
+      loc: SourceLocation): Expectations =
+    eql(expected, found)(comparisonA, loc)
 }

modules/framework-cats/shared/src/test/scala/DogFoodTests.scala

@@ -255,16 +255,41 @@ object DogFoodTests extends IOSuite {
   }
 
   test(
-    "expect.same delegates to show when an instance is found") {
+    "expect.eql delegates to Comparison show when an instance is found") {
     _.runSuite(Meta.Rendering).map {
       case (logs, _) =>
         val actual =
           extractLogEventAfterFailures(logs) {
-            case LoggedEvent.Error(msg) if msg.contains("(cats.Show)") => msg
+            case LoggedEvent.Error(msg) if msg.contains("(eql Comparison)") =>
+              msg
           }.get
 
         val expected = """
-        |- (cats.Show) 0ms
+        |- (eql Comparison) 0ms
+        |  Values not equal: (src/main/DogFoodTests.scala:5)
+        |
+        |  Foo {     |  Foo {
+        |    s: foo  |    s: foo
+        |    i: [1]  |    i: [2]
+        |  }         |  }
+        """.stripMargin.trim
+
+        expect.same(actual, expected)
+    }
+  }
+
+  test(
+    "expect.same delegates to Comparison show when an instance is found") {
+    _.runSuite(Meta.Rendering).map {
+      case (logs, _) =>
+        val actual =
+          extractLogEventAfterFailures(logs) {
+            case LoggedEvent.Error(msg) if msg.contains("(same Comparison)") =>
+              msg
+          }.get
+
+        val expected = """
+        |- (same Comparison) 0ms
         |  Values not equal: (src/main/DogFoodTests.scala:5)
         |
         |  Foo {     |  Foo {

modules/framework-cats/shared/src/test/scala/ExpectationsTests.scala

@@ -61,9 +61,14 @@ object ExpectationsTests extends SimpleIOSuite {
       })
   }
 
-  pureTest("expect.same respects cats.kernel.Eq") {
+  pureTest("expect.eql respects cats.kernel.Eq") {
     implicit val eqInt: Eq[Int] = Eq.allEqual
-    expect.same(0, 1)
+    expect.eql(0, 1)
+  }
+
+  pureTest("expect.eql respects weaver.Comparison") {
+    implicit val comparison: Comparison[Int] = Comparison.fromEq(Eq.allEqual)
+    expect.eql(0, 1)
   }
 
   pureTest("when success") {

modules/framework-cats/shared/src/test/scala/Meta.scala

@@ -40,21 +40,26 @@ object Meta {
       cancel("I was cancelled :(")
     }
 
-    pureTest("(cats.Show)") {
-      import cats.Show
-      case class Foo(s: String, i: Int)
-      object Foo {
-        implicit val show: Show[Foo] = Show.show[Foo] {
-          case Foo(s, i) =>
-            s"""
+    import cats.Show
+    case class Foo(s: String, i: Int)
+    object Foo {
+      val show: Show[Foo] = Show.show[Foo] {
+        case Foo(s, i) =>
+          s"""
           |Foo {
           |  s: ${Show[String].show(s)}
           |  i: ${Show[Int].show(i)}
           |}
           """.stripMargin.trim()
-        }
       }
+      implicit val comparison: Comparison[Foo] =
+        Comparison.fromEq[Foo](cats.Eq.fromUniversalEquals, show)
+    }
 
+    pureTest("(eql Comparison)") {
+      expect.eql(Foo("foo", 1), Foo("foo", 2))
+    }
+    pureTest("(same Comparison)") {
       expect.same(Foo("foo", 1), Foo("foo", 2))
     }
   }