added support for fold() along with a handy expect() test method (like scalatest)

stdlib/ktSrc/JavaIterables.kt

@@ -81,6 +81,20 @@ inline fun <T> java.lang.Iterable<T>.foreach(operation: (element: T) -> Unit) {
     operation(elem)
 }
 
+/**
+ * Folds all the values from from left to right using the initial value to perform the operation on sequential pairs of values
+ *
+ * For example to sum together all numeric values in a collection of numbers it would be
+ * {code}numbers.fold(0){(a, b) -> a + b}{code}
+ */
+inline fun <T> java.lang.Iterable<T>.fold(initial: T, operation: (it: T, it2: T) -> T): T {
+  var answer = initial
+  for (elem in this) {
+    answer = operation(answer, elem)
+  }
+  return answer
+}
+
 /**
  * Iterates through the collection performing the transformation on each element and using the result
  * as the key in a map to group elements by the result

testlib/test/CollectionTest.kt

@@ -127,6 +127,21 @@ class CollectionTest() : TestSupport() {
     assertEquals(6, count)
   }
 
+  fun testFold() {
+    expect(10) {
+      val numbers = arrayList(1, 2, 3, 4)
+
+      // TODO would be nice to be able to write this as this
+      //numbers.fold(0){it + it2}
+      numbers.fold(0){(it, it2) -> it + it2}
+    }
+
+    expect(0) {
+      val numbers = arrayList<Int>()
+      numbers.fold(0){(it, it2) -> it + it2}
+    }
+  }
+
   fun testGroupBy() {
     val words = arrayList("a", "ab", "abc", "def", "abcd")
     /*

testlib/test/Test.kt

@@ -97,6 +97,15 @@ fun assertNull(actual: Any?, message: String = "") {
   Assert.assertNull(message, actual)
 }
 
+fun <T> expect(expected: T, block: ()-> T) {
+  expect(expected, block.toString(), block)
+}
+
+fun <T> expect(expected: T, message: String, block: ()-> T) {
+  val actual = block()
+  assertEquals(expected, actual, message)
+}
+
 fun fails(block: ()-> Any) {
   try {
     block()