Add retryIf function

Facilitates creating a RetryPolicy that only retries if the failure
matches a given predicate.

Author: gnefedev

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

@@ -0,0 +1,19 @@
+package com.github.michaelbull.retry.policy
+
+import com.github.michaelbull.retry.ContinueRetrying
+import com.github.michaelbull.retry.RetryFailure
+import com.github.michaelbull.retry.RetryInstruction
+import com.github.michaelbull.retry.StopRetrying
+
+/**
+ * Creates a [RetryPolicy] that returns an [instruction][RetryInstruction] to
+ * [ContinueRetrying] if the [RetryFailure] satisfies the given [predicate],
+ * or an [instruction][RetryInstruction] to [StopRetrying] if it doesn't.
+ */
+inline fun <E> retryIf(crossinline predicate: RetryFailure<E>.() -> Boolean): RetryPolicy<E> = {
+    if (predicate(this)) {
+        ContinueRetrying
+    } else {
+        StopRetrying
+    }
+}

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

@@ -0,0 +1,46 @@
+package com.github.michaelbull.retry.policy
+
+import com.github.michaelbull.retry.retry
+import kotlinx.coroutines.ExperimentalCoroutinesApi
+import kotlinx.coroutines.test.runBlockingTest
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.assertThrows
+
+@ExperimentalCoroutinesApi
+class RetryIfTest {
+
+    private val retryIllegalState = retryIf<Throwable> {
+        reason is IllegalStateException
+    }
+
+    @Test
+    fun `retry with filter should retry specified exception`() = runBlockingTest {
+        var attempts = 0
+
+        retry(limitAttempts(5) + retryIllegalState) {
+            attempts++
+            if (attempts < 5) {
+                throw IllegalStateException()
+            }
+        }
+
+        assertEquals(5, attempts)
+    }
+
+    @Test
+    fun `retry with filter should not retry unspecified exception`() = runBlockingTest {
+        var attempts = 0
+
+        assertThrows<UnsupportedOperationException> {
+            retry(limitAttempts(5) + retryIllegalState) {
+                attempts++
+                if (attempts < 5) {
+                    throw UnsupportedOperationException()
+                }
+            }
+        }
+
+        assertEquals(1, attempts)
+    }
+}