[add-getRight-and-getLeft-methods] Added asLeft() assertion to EitherAssert

Author: rcardin

src/main/kotlin/in/rcard/assertj/arrowcore/AbstractEitherAssert.kt

@@ -168,6 +168,18 @@ abstract class AbstractEitherAssert<
         return myself
     }
 
+    /**
+     * Verifies that the actual [Either] is not null and contains a left-sided value and returns an Object assertion
+     * that allows chaining (object) assertions on the value.
+     *
+     * @since 0.2.0
+     * @return a new [AbstractObjectAssert] for assertions chaining on the left-sided value of the [Either].
+     */
+    fun asLeft(): AbstractObjectAssert<*, LEFT> {
+        assertIsLeft()
+        return Assertions.assertThat(actual.leftOrNull())
+    }
+
     private fun assertIsLeft() {
         isNotNull
         if (!actual.isLeft()) {

src/test/kotlin/in/rcard/assertj/arrowcore/EitherAssert_asLeft_Test.kt

@@ -0,0 +1,35 @@
+package `in`.rcard.assertj.arrowcore
+
+import arrow.core.Either
+import arrow.core.left
+import arrow.core.right
+import `in`.rcard.assertj.arrowcore.EitherAssert.Companion.assertThat
+import `in`.rcard.assertj.arrowcore.errors.EitherShouldBeLeft.Companion.shouldBeLeft
+import org.assertj.core.api.Assertions
+import org.assertj.core.util.FailureMessages
+import org.junit.jupiter.api.Test
+
+internal class EitherAssert_asLeft_Test {
+
+    @Test
+    internal fun `should return a valid Object assert if the either contains a left-sided value`() {
+        val actualLeftValue: Either<Int, Nothing> = 42.left()
+        assertThat(actualLeftValue).asLeft().isEqualTo(42)
+    }
+
+    @Test
+    internal fun `should fail if the either contains a right-sided value`() {
+        val actualRightValue: Either<Nothing, String> = "42".right()
+        Assertions.assertThatThrownBy { assertThat(actualRightValue).asLeft() }
+            .isInstanceOf(AssertionError::class.java)
+            .hasMessage(shouldBeLeft(actualRightValue).create())
+    }
+
+    @Test
+    internal fun `should fail if the either is null`() {
+        val actualLeftValue: Either<Int, Nothing>? = null
+        Assertions.assertThatThrownBy { assertThat(actualLeftValue).asLeft() }
+            .isInstanceOf(AssertionError::class.java)
+            .hasMessage(FailureMessages.actualIsNull())
+    }
+}