Add support for flat extracting elements from an iterable

Author: Silvio D'Alessandro

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Added
 - Added `first` and `single` assertion for `Iterable`
+- Added `flatExtracting` assertion for `Iterable`
 
 ## [0.25] 2021-09-12
 

assertk/src/commonMain/kotlin/assertk/assertions/iterable.kt

@@ -181,6 +181,19 @@ fun <E, R1, R2, R3> Assert<Iterable<E>>.extracting(
     actual.map { Triple(f1(it), f2(it), f3(it)) }
 }
 
+/**
+ * Flat extracts a value from each item in the iterable, allowing you to assert on a flattened list of those values.
+ *
+ * ```
+ * assertThat(people)
+ *   .flatExtracting(Person::relatives)
+ *   .contains("Sue", "Bob")
+ * ```
+ */
+fun <E, R> Assert<Iterable<E>>.flatExtracting(f1: (E) -> Iterable<R>): Assert<List<R>> = transform { actual ->
+    actual.flatMap(f1)
+}
+
 /**
  * Asserts on each item in the iterable, passing if none of the items pass.
  * The given lambda will be run for each item.

assertk/src/commonTest/kotlin/test/assertk/assertions/IterableTest.kt

@@ -460,6 +460,27 @@ class IterableTest {
     }
     //region extracting
 
+    //region flatExtracting
+    @Test fun flat_extracting_function_passes() {
+        val thing = Thing("one", 2, '3', listOf("A", "B"))
+        assertThat(listOf(thing) as Iterable<Thing>).flatExtracting { it.four }.containsExactly("A", "B")
+    }
+
+    @Test fun flat_extracting_function_fails() {
+        val thing = Thing("one", 2, '3', listOf("A", "B"))
+        val error = assertFails {
+            assertThat(listOf(thing) as Iterable<Thing>).flatExtracting { it.four }.containsExactly("C", "D")
+        }
+        assertEquals(
+            """expected to contain exactly:<["C", "D"]> but was:<["A", "B"]>
+            | at index:0 expected:<"C">
+            | at index:0 unexpected:<"A">
+            | at index:1 expected:<"D">
+            | at index:1 unexpected:<"B"> ([Thing(one=one, two=2, three=3, four=[A, B])])""".trimMargin(), error.message
+        )
+    }
+    //region flatExtracting
+
     //region first
     @Test fun first_element_present_match_passes() {
         assertThat(listOf(1, 2)).first().isEqualTo(1)
@@ -507,5 +528,5 @@ class IterableTest {
     }
     //endregion
 
-    data class Thing(val one: String, val two: Int, val three: Char)
+    data class Thing(val one: String, val two: Int, val three: Char, val four: List<String> = listOf())
 }