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Make Show inherit from a contravariant base trait #1649

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Jul 7, 2017
Merged

Make Show inherit from a contravariant base trait #1649

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9 changes: 7 additions & 2 deletions core/src/main/scala/cats/Show.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -10,11 +10,16 @@ import cats.functor.Contravariant
* made a toString method, a Show instance will only exist if someone
* explicitly provided one.
*/
@typeclass trait Show[T] {
@typeclass trait Show[T] extends Show.ContravariantShow[T] {
// duplicated so simulacrum knows it requires an instance of this trait
def show(t: T): String
}

object Show {
trait ContravariantShow[-T] {
def show(t: T): String
}

/** creates an instance of [[Show]] using the provided function */
def show[A](f: A => String): Show[A] = new Show[A] {
def show(a: A): String = f(a)
Expand All @@ -27,7 +32,7 @@ object Show {

final case class Shown(override val toString: String) extends AnyVal
object Shown {
implicit def mat[A](x: A)(implicit z: Show[A]): Shown = Shown(z show x)
implicit def mat[A](x: A)(implicit z: ContravariantShow[A]): Shown = Shown(z show x)
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I wonder if we can get the test to pass by asking for Show[_ >: A] which is effectively what this trick is doing, and not having to introduce the new type. I think we can (at least on 2.12).

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Unfortunately type inference looks to be just slightly shy, like it was when I asked for [A, B >: A](x: A)(implicit z: Show[B]): it never converts anything to a Shown.

[error] /Users/edmund/workspace/scala/cats/tests/src/test/scala/cats/tests/ShowTests.scala:29: type mismatch;
[error]  found   : TimeOfDay
[error]  required: cats.Show.Shown
[error]     assertResult("Good morning, Whiskers!")(show"Good $tod, $cat!")
[error]                                                        ^
[error] /Users/edmund/workspace/scala/cats/tests/src/test/scala/cats/tests/ShowTests.scala:29: type mismatch;
[error]  found   : Cat
[error]  required: cats.Show.Shown
[error]     assertResult("Good morning, Whiskers!")(show"Good $tod, $cat!")
[error]                                                              ^
[error] /Users/edmund/workspace/scala/cats/tests/src/test/scala/cats/tests/ShowTests.scala:31: type mismatch;
[error]  found   : TimeOfDay
[error]  required: cats.Show.Shown
[error]     assertResult("Good morning, Whiskers!")(show"Good $tod, ${List(cat).head}!")
[error]                                                        ^
[error] /Users/edmund/workspace/scala/cats/tests/src/test/scala/cats/tests/ShowTests.scala:31: type mismatch;
[error]  found   : Cat
[error]  required: cats.Show.Shown
[error]     assertResult("Good morning, Whiskers!")(show"Good $tod, ${List(cat).head}!")
[error]                                                                         ^
[error] /Users/edmund/workspace/scala/cats/tests/src/test/scala/cats/tests/ShowTests.scala:46: type mismatch;
[error]  found   : Cat
[error]  required: cats.Show.Shown
[error]     assertResult("Good morning, Whiskers!")(show"Good morning, $cat!")
[error]                                                                 ^
[error] 5 errors found

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when I did a simple test in the repl this worked when I did implicitly[Foo[_ >: A]] did you try that form (not putting a specific type)? you never know with scala which minor translation of the code will work.

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An implicit in the repl is like you've imported it. His is in the Shown companion object.

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right. That clarifies it for me.

}

final case class ShowInterpolator(_sc: StringContext) extends AnyVal {
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19 changes: 12 additions & 7 deletions tests/src/test/scala/cats/tests/ShowTests.scala
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Expand Up @@ -10,23 +10,28 @@ class ShowTests extends CatsSuite {
checkAll("Contravariant[Show]", ContravariantTests[Show].contravariant[Int, Int, Int])
checkAll("Contravariant[Show]", SerializableTests.serializable(Contravariant[Show]))

sealed trait TimeOfDay
case object Morning extends TimeOfDay
object TimeOfDay {
implicit val showTimeOfDay: Show[TimeOfDay] = Show.show { case Morning => "morning" }
}

test("show string interpolator") {
case class Cat(name: String)
object Cat {
implicit val showCat: Show[Cat] = Show.show(_.name)
}

sealed trait TimeOfDay
case object Morning extends TimeOfDay
object TimeOfDay {
implicit val showTimeOfDay: Show[TimeOfDay] = Show.show { case Morning => "morning" }
}

val tod: TimeOfDay = Morning
val cat = Cat("Whiskers")

assertResult("Good morning, Whiskers!")(show"Good $tod, $cat!")

assertResult("Good morning, Whiskers!")(show"Good $tod, ${List(cat).head}!")
}

test("show string interpolator and contravariance") {
val tod: Morning.type = Morning

assertResult("Good morning")(show"Good $tod")
}
}