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Category[Is] and related tests #2759

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Apr 5, 2019
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Category[Is] and related tests #2759

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2e7bc8a
Category[Is] instance and related tests
ctongfei Mar 26, 2019
cde902c
Category[Is] tests
ctongfei Mar 26, 2019
6db4245
scalafmt dislike my usage of infix op
ctongfei Mar 29, 2019
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15 changes: 14 additions & 1 deletion core/src/main/scala/cats/evidence/Is.scala
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Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
package cats.evidence

import cats.Id
import cats.arrow._

/**
* A value of `A Is B` is proof that the types `A` and `B` are the same. More
Expand Down Expand Up @@ -65,7 +66,19 @@ abstract class Is[A, B] extends Serializable {
substitute[A =:= ?](implicitly[A =:= A])
}

object Is {
sealed abstract class IsInstances {
import Is._

/**
* The category instance on Leibniz categories.
*/
implicit val leibniz: Category[Is] = new Category[Is] {
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Can we give thise one a more verbose name? Something like catsEvidenceCategoryForLeibniz? :)

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The Category instance for As is named as liskov -- so I copied that :-)

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Good call :)

def id[A]: A Is A = refl[A]
def compose[A, B, C](bc: B Is C, ab: A Is B): A Is C = bc.compose(ab)
}
}

object Is extends IsInstances {

/**
* In truth, "all values of `A Is B` are `refl`". `reflAny` is that
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16 changes: 16 additions & 0 deletions tests/src/test/scala/cats/tests/IsSuite.scala
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Original file line number Diff line number Diff line change
@@ -1,9 +1,25 @@
package cats
package tests

import cats.arrow._
import cats.kernel.laws.discipline.SerializableTests
import cats.laws.discipline.CategoryTests
import org.scalacheck.{Arbitrary, Gen}

class IsSuite extends CatsSuite {
import evidence._

implicit def arbIs[A, B](implicit ev: A Is B): Arbitrary[A Is B] = Arbitrary(Gen.const(ev))
implicit def eqIs[A, B]: Eq[A Is B] = Eq.fromUniversalEquals

trait Top {
def foo: String = this.getClass.getName
}
case class Bottom() extends Top

checkAll("Is[Bottom, Bottom]", CategoryTests[Is].category[Bottom, Bottom, Bottom, Bottom])
checkAll("Category[Is]", SerializableTests.serializable(Category[Is]))

test("syntax") {
trait Bar

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