Map support for template.Execute().

Allows the developer to pass a map either by itself for
evaluation, or inside a struct. Access to data inside
maps is identical to the current system for structs, ie.

-Psuedocode-

mp map[string]string = {
	"header" : "A fantastic header!",
	"footer" : "A not-so-fantastic footer!",
}
template.Execute(mp)

...can be accessed using {header} and {footer} in
the template. Similarly, for maps inside structs:

type s struct {
	mp map[string]string,
}
s1 = new s
s1.mp["header"] = "A fantastic header!";
template.Execute(s1)

...is accessed using {mp.header}. Multi-maps, ie.
map[string](map[string]string) and maps of structs
containing more maps are unsupported, but then, I'm
not even sure if that's supported by the language.

Map elements can be of any type that can be written
by the formatters. Keys should really only be strings.

Fixes #259.

R=r, rsc
https://golang.org/cl/157088

src/pkg/template/template.go

@@ -10,10 +10,11 @@
 
 	Templates are executed by applying them to a data structure.
 	Annotations in the template refer to elements of the data
-	structure (typically a field of a struct) to control execution
-	and derive values to be displayed.  The template walks the
-	structure as it executes and the "cursor" @ represents the
-	value at the current location in the structure.
+	structure (typically a field of a struct or a key in a map)
+	to control execution and derive values to be displayed.
+	The template walks the structure as it executes and the
+	"cursor" @ represents the value at the current location
+	in the structure.
 
 	Data items may be values or pointers; the interface hides the
 	indirection.
@@ -605,20 +606,24 @@ func (st *state) findVar(s string) reflect.Value {
 	data := st.data;
 	elems := strings.Split(s, ".", 0);
 	for i := 0; i < len(elems); i++ {
-		// Look up field; data must be a struct.
+		// Look up field; data must be a struct or map.
 		data = reflect.Indirect(data);
 		if data == nil {
 			return nil
 		}
-		typ, ok := data.Type().(*reflect.StructType);
-		if !ok {
+
+		switch typ := data.Type().(type) {
+		case *reflect.StructType:
+			field, ok := typ.FieldByName(elems[i]);
+			if !ok {
+				return nil
+			}
+			data = data.(*reflect.StructValue).FieldByIndex(field.Index);
+		case *reflect.MapType:
+			data = data.(*reflect.MapValue).Elem(reflect.NewValue(elems[i]))
+		default:
 			return nil
 		}
-		field, ok := typ.FieldByName(elems[i]);
-		if !ok {
-			return nil
-		}
-		data = data.(*reflect.StructValue).FieldByIndex(field.Index);
 	}
 	return data;
 }

src/pkg/template/template_test.go

@@ -21,6 +21,10 @@ type T struct {
 	value	string;
 }
 
+type U struct {
+	mp map[string]int;
+}
+
 type S struct {
 	header		string;
 	integer		int;
@@ -35,6 +39,8 @@ type S struct {
 	vec		*vector.Vector;
 	true		bool;
 	false		bool;
+	mp		map[string]string;
+	innermap	U;
 }
 
 var t1 = T{"ItemNumber1", "ValueNumber1"}
@@ -275,6 +281,20 @@ var tests = []*Test{
 
 		out: "1\n4\n",
 	},
+
+	// Maps
+
+	&Test{
+		in: "{mp.mapkey}\n",
+
+		out: "Ahoy!\n",
+	},
+
+	&Test{
+		in: "{innermap.mp.innerkey}\n",
+
+		out: "55\n",
+	},
 }
 
 func TestAll(t *testing.T) {
@@ -293,6 +313,10 @@ func TestAll(t *testing.T) {
 	s.vec.Push("elt2");
 	s.true = true;
 	s.false = false;
+	s.mp = make(map[string]string);
+	s.mp["mapkey"] = "Ahoy!";
+	s.innermap.mp = make(map[string]int);
+	s.innermap.mp["innerkey"] = 55;
 
 	var buf bytes.Buffer;
 	for _, test := range tests {
@@ -318,6 +342,24 @@ func TestAll(t *testing.T) {
 	}
 }
 
+func TestMapDriverType(t *testing.T) {
+	mp := map[string]string{"footer": "Ahoy!"};
+	tmpl, err := Parse("template: {footer}", nil);
+	if err != nil {
+		t.Error("unexpected parse error:", err)
+	}
+	var b bytes.Buffer;
+	err = tmpl.Execute(mp, &b);
+	if err != nil {
+		t.Error("unexpected execute error:", err)
+	}
+	s := b.String();
+	expected := "template: Ahoy!";
+	if s != expected {
+		t.Errorf("failed passing string as data: expected %q got %q", "template: Ahoy!", s)
+	}
+}
+
 func TestStringDriverType(t *testing.T) {
 	tmpl, err := Parse("template: {@}", nil);
 	if err != nil {