Add retryUnless policy

Facilitates creating a RetryPolicy that only retries if a given predicate is
not satisfied. Analogous to takeUnless in the stdlib.

Author: michaelbull

src/main/kotlin/com/github/michaelbull/retry/policy/RetryIf.kt renamed to src/main/kotlin/com/github/michaelbull/retry/policy/Predicate.kt

@@ -17,3 +17,17 @@ inline fun <E> retryIf(crossinline predicate: RetryFailure<E>.() -> Boolean): Re
         StopRetrying
     }
 }
+
+/**
+ * Creates a [RetryPolicy] that returns an [instruction][RetryInstruction] to
+ * [ContinueRetrying] if the [RetryFailure] _does not_ satisfy the given
+ * [predicate], or an [instruction][RetryInstruction] to [StopRetrying] if it
+ * does.
+ */
+inline fun <E> retryUnless(crossinline predicate: RetryFailure<E>.() -> Boolean): RetryPolicy<E> = {
+    if (!predicate(this)) {
+        ContinueRetrying
+    } else {
+        StopRetrying
+    }
+}

src/test/kotlin/com/github/michaelbull/retry/policy/RetryIfTest.kt renamed to src/test/kotlin/com/github/michaelbull/retry/policy/PredicateTest.kt

@@ -8,14 +8,18 @@ import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.assertThrows
 
 @ExperimentalCoroutinesApi
-class RetryIfTest {
+class PredicateTest {
 
     private val retryIllegalState = retryIf<Throwable> {
         reason is IllegalStateException
     }
 
+    private val dontRetryOob = retryUnless<Throwable> {
+        reason is IndexOutOfBoundsException
+    }
+
     @Test
-    fun `retry with filter should retry specified exception`() = runBlockingTest {
+    fun `retryIf should retry specified exception`() = runBlockingTest {
         var attempts = 0
 
         retry(limitAttempts(5) + retryIllegalState) {
@@ -29,7 +33,7 @@ class RetryIfTest {
     }
 
     @Test
-    fun `retry with filter should not retry unspecified exception`() = runBlockingTest {
+    fun `retryIf should not retry unspecified exception`() = runBlockingTest {
         var attempts = 0
 
         assertThrows<UnsupportedOperationException> {
@@ -43,4 +47,34 @@ class RetryIfTest {
 
         assertEquals(1, attempts)
     }
+
+    @Test
+    fun `retryUnless should not retry specified exception`() = runBlockingTest {
+        var attempts = 0
+
+        assertThrows<IndexOutOfBoundsException> {
+            retry(limitAttempts(5) + dontRetryOob) {
+                attempts++
+                if (attempts < 5) {
+                    throw IndexOutOfBoundsException()
+                }
+            }
+        }
+
+        assertEquals(1, attempts)
+    }
+
+    @Test
+    fun `retryUnless should retry unspecified exception`() = runBlockingTest {
+        var attempts = 0
+
+        retry(limitAttempts(5) + dontRetryOob) {
+            attempts++
+            if (attempts < 5) {
+                throw IllegalStateException()
+            }
+        }
+
+        assertEquals(5, attempts)
+    }
 }