Add Inject function to iterable package.

Fix a couple of style mistakes.

R=r,rsc
APPROVED=r
DELTA=34  (30 added, 1 deleted, 3 changed)
OCL=27623
CL=27623

src/lib/container/iterable.go

@@ -16,7 +16,7 @@ type Iterable interface {
 }
 
 func not(f func(interface {}) bool) (func(interface {}) bool) {
-  return func(e interface {}) bool { return !f(e) }
+	return func(e interface {}) bool { return !f(e) }
 }
 
 // All tests whether f is true for every element of iter.
@@ -79,6 +79,26 @@ func Find(iter Iterable, f func(interface {}) bool) interface {} {
 	return nil
 }
 
+// An injector function takes two arguments, an accumulated value and an
+// element, and returns the next accumulated value. See the Inject function.
+type Injector func(interface {}, interface {}) interface{};
+
+// Inject combines the elements of iter by repeatedly calling f with an
+// accumulated value and each element in order. The starting accumulated value
+// is initial, and after each call the accumulated value is set to the return
+// value of f. For instance, to compute a sum:
+//   var arr IntArray = []int{ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
+//   sum := iterable.Inject(arr, 0,
+//                          func(ax interface {}, x interface {}) interface {} {
+//                            return ax.(int) + x.(int) }).(int)
+func Inject(iter Iterable, initial interface {}, f Injector) interface {} {
+	acc := initial;
+	for e := range iter.Iter() {
+		acc = f(acc, e)
+	}
+	return acc
+}
+
 // mappedIterable is a helper struct that implements Iterable, returned by Map.
 type mappedIterable struct {
 	it Iterable;
@@ -95,7 +115,7 @@ func (m *mappedIterable) iterate(out chan<- interface {}) {
 func (m *mappedIterable) Iter() <-chan interface {} {
 	ch := make(chan interface {});
 	go m.iterate(ch);
-	return ch;
+	return ch
 }
 
 // Map returns an Iterable that returns the result of applying f to each
@@ -106,9 +126,8 @@ func Map(iter Iterable, f func(interface {}) interface {}) Iterable {
 
 // Partition(iter, f) returns Filter(iter, f) and Filter(iter, !f).
 func Partition(iter Iterable, f func(interface {}) bool) (Iterable, Iterable)  {
-  return Filter(iter, f), Filter(iter, not(f))
+	return Filter(iter, f), Filter(iter, not(f))
 }
 
 // TODO:
-// - Inject
 // - Zip

src/lib/container/iterable_test.go

@@ -42,6 +42,9 @@ func doubler(n interface {}) interface {} {
 func addOne(n interface {}) interface {} {
 	return n.(int) + 1
 }
+func adder(acc interface {}, n interface {}) interface {} {
+	return acc.(int) + n.(int)
+}
 
 // A stream of the natural numbers: 0, 1, 2, 3, ...
 type integerStream struct {}
@@ -107,6 +110,13 @@ func TestFind(t *testing.T) {
 	}
 }
 
+func TestInject(t *testing.T) {
+	res := Inject(oneToFive, 0, adder);
+	if res.(int) != 15 {
+		t.Errorf("Inject(oneToFive, 0, adder) = %v, want 15", res)
+	}
+}
+
 func TestMap(t *testing.T) {
 	res := Data(Map(Map(oneToFive, doubler), addOne));
 	assertArraysAreEqual(t, res, []int{ 3, 5, 7, 9, 11 })