exp/template: add builtin len function

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/4868045

src/pkg/exp/template/doc.go

@@ -232,6 +232,8 @@ Predefined global functions are named as follows.
 	js
 		Returns the escaped JavaScript equivalent of the textual
 		representation of its arguments.
+	len
+		Returns the integer length of its argument.
 	not
 		Returns the boolean negation of its single argument.
 	or

src/pkg/exp/template/exec_test.go

@@ -331,6 +331,12 @@ var execTests = []execTest{
 	{"map[WRONG]", "{{index .MSI 10}}", "", tVal, false},
 	{"double index", "{{index .SMSI 1 `eleven`}}", "11", tVal, true},
 
+	// Len.
+	{"slice", "{{len .SI}}", "3", tVal, true},
+	{"map", "{{len .MSI }}", "3", tVal, true},
+	{"len of int", "{{len 3}}", "", tVal, false},
+	{"len of nothing", "{{len .Empty0}}", "", tVal, false},
+
 	// With.
 	{"with true", "{{with true}}{{.}}{{end}}", "true", tVal, true},
 	{"with false", "{{with false}}{{.}}{{else}}FALSE{{end}}", "FALSE", tVal, true},

src/pkg/exp/template/funcs.go

@@ -27,6 +27,7 @@ var builtins = FuncMap{
 	"html":    HTMLEscaper,
 	"index":   index,
 	"js":      JSEscaper,
+	"len":     length,
 	"not":     not,
 	"or":      or,
 	"print":   fmt.Sprint,
@@ -140,6 +141,21 @@ func index(item interface{}, indices ...interface{}) (interface{}, os.Error) {
 	return v.Interface(), nil
 }
 
+// Length
+
+// length returns the length of the item, with an error if it has no defined length.
+func length(item interface{}) (int, os.Error) {
+	v, isNil := indirect(reflect.ValueOf(item))
+	if isNil {
+		return 0, fmt.Errorf("len of nil pointer")
+	}
+	switch v.Kind() {
+	case reflect.Array, reflect.Chan, reflect.Map, reflect.Slice, reflect.String:
+		return v.Len(), nil
+	}
+	return 0, fmt.Errorf("len of type %s", v.Type())
+}
+
 // Boolean logic.
 
 func truth(a interface{}) bool {