From 95767bc9925f328c1b73a0672d82afb39ae95a6b Mon Sep 17 00:00:00 2001
From: Simon Vergauwen <nomisRev@users.noreply.github.com>
Date: Tue, 20 Sep 2022 18:28:55 +0200
Subject: [PATCH] Make isRight smart contracted

---
 .../commonMain/kotlin/arrow/core/Either.kt    | 21 ++++++++++---------
 .../kotlin/arrow/core/EitherTest.kt           | 14 ++++++++-----
 2 files changed, 20 insertions(+), 15 deletions(-)

diff --git a/arrow-libs/core/arrow-core/src/commonMain/kotlin/arrow/core/Either.kt b/arrow-libs/core/arrow-core/src/commonMain/kotlin/arrow/core/Either.kt
index 345877d8618..65a8f074a3c 100644
--- a/arrow-libs/core/arrow-core/src/commonMain/kotlin/arrow/core/Either.kt
+++ b/arrow-libs/core/arrow-core/src/commonMain/kotlin/arrow/core/Either.kt
@@ -787,17 +787,18 @@ public sealed class Either<out A, out B> {
   @JsName("_isLeft")
   internal abstract val isLeft: Boolean
   
-  @Deprecated(
-    RedundantAPI + "Use `is Either.Left<*>`, `when`, or `fold` instead",
-    ReplaceWith("(this is Either.Left<*>)")
-  )
-  public fun isLeft(): Boolean = isLeft
+  @OptIn(ExperimentalContracts::class)
+  public fun isLeft(): Boolean {
+    contract { returns(true) implies (this@Either is Left<A>) }
+    return this@Either is Left<A>
+  }
   
-  @Deprecated(
-    RedundantAPI + "Use `is Either.Right<*>`, `when`, or `fold` instead",
-    ReplaceWith("(this is Either.Right<*>)")
-  )
-  public fun isRight(): Boolean = isRight
+  
+  @OptIn(ExperimentalContracts::class)
+  public fun isRight(): Boolean {
+    contract { returns(true) implies (this@Either is Right<B>) }
+    return this@Either is Right<B>
+  }
   
   /**
    * Transform an [Either] into a value of [C].
diff --git a/arrow-libs/core/arrow-core/src/commonTest/kotlin/arrow/core/EitherTest.kt b/arrow-libs/core/arrow-core/src/commonTest/kotlin/arrow/core/EitherTest.kt
index 3a847c12abf..1a50893d197 100644
--- a/arrow-libs/core/arrow-core/src/commonTest/kotlin/arrow/core/EitherTest.kt
+++ b/arrow-libs/core/arrow-core/src/commonTest/kotlin/arrow/core/EitherTest.kt
@@ -12,6 +12,7 @@ import arrow.core.test.generators.suspendFunThatReturnsEitherAnyOrAnyOrThrows
 import arrow.core.test.generators.suspendFunThatThrows
 import arrow.core.test.laws.MonoidLaws
 import arrow.typeclasses.Monoid
+import io.kotest.assertions.fail
 import io.kotest.assertions.throwables.shouldThrow
 import io.kotest.matchers.shouldBe
 import io.kotest.property.Arb
@@ -48,15 +49,18 @@ class EitherTest : UnitSpec() {
 
     "isLeft should return true if Left and false if Right" {
       checkAll(Arb.int()) { a: Int ->
-        Left(a).isLeft() && !Right(a).isLeft()
-        (Left(a) is Left<*>) && !(a.right() is Left<*>)
-      }
+        val x = Left(a)
+        if(x.isLeft()) x.value shouldBe a
+        else fail("Left(a).isLeft() cannot be false")
+        x.isRight() shouldBe false
     }
 
     "isRight should return false if Left and true if Right" {
       checkAll(Arb.int()) { a: Int ->
-        !Left(a).isRight() && Right(a).isRight()
-        (a.left() !is Right<*>) && (Right(a) is Right<*>)
+        val x = Right(a)
+        if(x.isRight()) x.value shouldBe a
+        else fail("Right(a).isRight() cannot be false")
+        x.isRight() shouldBe false
       }
     }