From 8239eb9b0f360c2e8abfb76345fb3ef81635e8a1 Mon Sep 17 00:00:00 2001
From: Hugo van Rijswijk <git@hugovr.nl>
Date: Wed, 25 Oct 2023 16:45:30 +0200
Subject: [PATCH] Add Invariant and Semigroupal typeclasses for ViewF

---
 shared/src/main/scala/crystal/package.scala   | 10 +++++++
 shared/src/main/scala/crystal/syntax.scala    |  1 +
 .../test/scala/crystal/ViewFLawsSpec.scala    | 28 +++++++++++++++++++
 .../src/test/scala/crystal/arbitraries.scala  |  5 ++++
 4 files changed, 44 insertions(+)
 create mode 100644 shared/src/test/scala/crystal/ViewFLawsSpec.scala

diff --git a/shared/src/main/scala/crystal/package.scala b/shared/src/main/scala/crystal/package.scala
index d2e43d33..630aae5b 100644
--- a/shared/src/main/scala/crystal/package.scala
+++ b/shared/src/main/scala/crystal/package.scala
@@ -6,7 +6,9 @@ package crystal
 import cats.Applicative
 import cats.Eq
 import cats.FlatMap
+import cats.Invariant
 import cats.Monad
+import cats.Semigroupal
 import cats.effect.Ref
 import cats.syntax.all.*
 
@@ -36,3 +38,11 @@ object throwable {
         (x ne null) == (y ne null)
     }
 }
+
+given [F[_]: Monad]: Semigroupal[ViewF[F, *]] with
+  def product[A, B](fa: ViewF[F, A], fb: ViewF[F, B]): ViewF[F, (A, B)] =
+    ViewF.apply(fa.get -> fb.get, (f, cb) => cb(f(fa.get, fb.get)))
+
+given [F[_]: Monad]: Invariant[ViewF[F, *]] with
+  def imap[A, B](fa: ViewF[F, A])(f: A => B)(g: B => A): ViewF[F, B] =
+    fa.zoom(f)(mod => a => g(mod(f(a))))
diff --git a/shared/src/main/scala/crystal/syntax.scala b/shared/src/main/scala/crystal/syntax.scala
index 3a894e68..aede2f66 100644
--- a/shared/src/main/scala/crystal/syntax.scala
+++ b/shared/src/main/scala/crystal/syntax.scala
@@ -28,6 +28,7 @@ trait syntax {
     def toPot: Pot[A]             = Pot.fromTry[A](a)
     @targetName("tryToPotOption")
     def toPotOption: PotOption[A] = PotOption.fromTry(a)
+
 }
 
 object syntax extends syntax
diff --git a/shared/src/test/scala/crystal/ViewFLawsSpec.scala b/shared/src/test/scala/crystal/ViewFLawsSpec.scala
new file mode 100644
index 00000000..000b9713
--- /dev/null
+++ b/shared/src/test/scala/crystal/ViewFLawsSpec.scala
@@ -0,0 +1,28 @@
+// Copyright (c) 2016-2023 Association of Universities for Research in Astronomy, Inc. (AURA)
+// For license information see LICENSE or https://opensource.org/licenses/BSD-3-Clause
+
+package crystal
+
+import cats.Eq
+import cats.Id
+import cats.Invariant
+import cats.laws.discipline.InvariantTests
+import cats.laws.discipline.SemigroupalTests
+import munit.DisciplineSuite
+
+import arbitraries.given
+
+class ViewFLawsSpec extends DisciplineSuite {
+
+  private given viewEq[A: Eq]: Eq[ViewF[Id, A]] = Eq.by(_.get)
+
+  checkAll(
+    "ViewF[Int].InvariantLaws",
+    InvariantTests[ViewF[Id, *]].invariant[Int, Int, String]
+  )
+
+  checkAll(
+    "ViewF[Int].SemigroupalLaws",
+    SemigroupalTests[ViewF[Id, *]].semigroupal[Int, Int, String]
+  )
+}
diff --git a/shared/src/test/scala/crystal/arbitraries.scala b/shared/src/test/scala/crystal/arbitraries.scala
index 16901efa..a056abde 100644
--- a/shared/src/test/scala/crystal/arbitraries.scala
+++ b/shared/src/test/scala/crystal/arbitraries.scala
@@ -3,6 +3,7 @@
 
 package crystal
 
+import cats.Id
 import org.scalacheck.*
 
 import scala.annotation.targetName
@@ -33,6 +34,10 @@ object arbitraries {
       )
     )
 
+  given viewFArb[A: Arbitrary]: Arbitrary[ViewF[Id, A]] = Arbitrary(
+    arbitrary[A].map(a => ViewF.apply[Id, A](a, (f, cb) => cb(f(a))))
+  )
+
   @targetName("potOptionFnArbitrary")
   given [A](using fArb: Arbitrary[A => A]): Arbitrary[PotOption[A] => PotOption[A]] =
     Arbitrary(arbitrary[A => A].map(f => _.map(f)))