withFirst(predicate) mapping

robfletcher · May 11, 2020 · 8502303 · 8502303
1 parent 2660321
commit 8502303
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diff --git a/strikt-core/src/main/kotlin/strikt/assertions/Iterable.kt b/strikt-core/src/main/kotlin/strikt/assertions/Iterable.kt
@@ -68,10 +68,24 @@ fun <T : Collection<E>, E> Builder<T>.single(): Builder<E> =
  * iterable that matches [predicate].
  *
  * @see Iterable.first
+ * @throws strikt.internal.opentest4j.MappingFailed if no elements match [predicate].
  */
 infix fun <T : Iterable<E>, E> Builder<T>.first(predicate: (E) -> Boolean): Builder<E> =
   get("first matching element %s") { first(predicate) }
 
+/**
+ * Runs a group of assertions on the first element in the subject iterable that
+ * matches [predicate].
+ *
+ * @see Iterable.first
+ * @throws strikt.internal.opentest4j.MappingFailed if no elements match [predicate].
+ */
+fun <T : Iterable<E>, E> Builder<T>.withFirst(
+  predicate: (E) -> Boolean,
+  block: Builder<E>.() -> Unit
+): Builder<T> =
+  with("first matching element %s", { first(predicate) }, block)
+
 /**
  * Maps this assertion to an assertion over the last element in the subject
  * iterable.

diff --git a/strikt-core/src/test/kotlin/strikt/assertions/IterableAssertions.kt b/strikt-core/src/test/kotlin/strikt/assertions/IterableAssertions.kt
@@ -5,6 +5,7 @@ import dev.minutest.rootContext
 import org.junit.jupiter.api.assertThrows
 import org.opentest4j.MultipleFailuresError
 import strikt.api.expectThat
+import strikt.internal.opentest4j.MappingFailed
 
 internal object IterableAssertions : JUnit5Minutests {
 
@@ -816,6 +817,34 @@ internal object IterableAssertions : JUnit5Minutests {
         }
     }
 
+    context("withFirst(predicate) function") {
+      listOf("catflap", "rubberplant", "marzipan")
+        .permute()
+        .forEach { subject ->
+          test("runs assertions on the first element of a ${subject.javaClass.simpleName} that matches the predicate") {
+            expectThat(subject).withFirst({ it.startsWith('r') }) {
+              isEqualTo("rubberplant")
+            }
+          }
+
+          test("fails if the nested assertions fail on the first element of a ${subject.javaClass.simpleName} that matches the predicate") {
+            assertThrows<MultipleFailuresError> {
+              expectThat(subject).withFirst({ it.startsWith('r') }) {
+                isEqualTo("catflap")
+              }
+            }
+          }
+
+          test("fails if nothing in a ${subject.javaClass.simpleName} matches the predicate") {
+            assertThrows<MappingFailed> {
+              expectThat(subject).withFirst({ it.startsWith('z') }) {
+                isEqualTo("catflap")
+              }
+            }
+          }
+        }
+    }
+
     context("withLast function") {
       listOf("catflap", "rubberplant", "marzipan")
         .permute()