tpl: Add custom index function

This commit adds a custom index template function that deviates from the stdlib
simply by not returning an "index out of range" error if an array, slice or
string index is out of range.  Instead, we just return nil values.  This should
help make the new default function more useful for Hugo users.

Fixes #1949

tpl/template_funcs.go

@@ -1290,6 +1290,89 @@ func dfault(dflt interface{}, given ...interface{}) (interface{}, error) {
 	return dflt, nil
 }
 
+// canBeNil reports whether an untyped nil can be assigned to the type. See reflect.Zero.
+//
+// Copied from Go stdlib src/text/template/exec.go.
+func canBeNil(typ reflect.Type) bool {
+	switch typ.Kind() {
+	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice:
+		return true
+	}
+	return false
+}
+
+// prepareArg checks if value can be used as an argument of type argType, and
+// converts an invalid value to appropriate zero if possible.
+//
+// Copied from Go stdlib src/text/template/funcs.go.
+func prepareArg(value reflect.Value, argType reflect.Type) (reflect.Value, error) {
+	if !value.IsValid() {
+		if !canBeNil(argType) {
+			return reflect.Value{}, fmt.Errorf("value is nil; should be of type %s", argType)
+		}
+		value = reflect.Zero(argType)
+	}
+	if !value.Type().AssignableTo(argType) {
+		return reflect.Value{}, fmt.Errorf("value has type %s; should be %s", value.Type(), argType)
+	}
+	return value, nil
+}
+
+// index returns the result of indexing its first argument by the following
+// arguments. Thus "index x 1 2 3" is, in Go syntax, x[1][2][3]. Each
+// indexed item must be a map, slice, or array.
+//
+// Copied from Go stdlib src/text/template/funcs.go.
+func index(item interface{}, indices ...interface{}) (interface{}, error) {
+	v := reflect.ValueOf(item)
+	if !v.IsValid() {
+		return nil, fmt.Errorf("index of untyped nil")
+	}
+	for _, i := range indices {
+		index := reflect.ValueOf(i)
+		var isNil bool
+		if v, isNil = indirect(v); isNil {
+			return nil, fmt.Errorf("index of nil pointer")
+		}
+		switch v.Kind() {
+		case reflect.Array, reflect.Slice, reflect.String:
+			var x int64
+			switch index.Kind() {
+			case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+				x = index.Int()
+			case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+				x = int64(index.Uint())
+			case reflect.Invalid:
+				return nil, fmt.Errorf("cannot index slice/array with nil")
+			default:
+				return nil, fmt.Errorf("cannot index slice/array with type %s", index.Type())
+			}
+			if x < 0 || x >= int64(v.Len()) {
+				// We deviate from stdlib here.  Don't return an error if the
+				// index is out of range.
+				return nil, nil
+			}
+			v = v.Index(int(x))
+		case reflect.Map:
+			index, err := prepareArg(index, v.Type().Key())
+			if err != nil {
+				return nil, err
+			}
+			if x := v.MapIndex(index); x.IsValid() {
+				v = x
+			} else {
+				v = reflect.Zero(v.Type().Elem())
+			}
+		case reflect.Invalid:
+			// the loop holds invariant: v.IsValid()
+			panic("unreachable")
+		default:
+			return nil, fmt.Errorf("can't index item of type %s", v.Type())
+		}
+	}
+	return v.Interface(), nil
+}
+
 // safeHTMLAttr returns a given string as html/template HTMLAttr content.
 //
 // safeHTMLAttr is currently disabled, pending further discussion
@@ -1583,6 +1666,7 @@ func init() {
 		"highlight":    highlight,
 		"humanize":     humanize,
 		"in":           in,
+		"index":        index,
 		"int":          func(v interface{}) int { return cast.ToInt(v) },
 		"intersect":    intersect,
 		"isSet":        isSet,

tpl/template_funcs_test.go

@@ -1907,6 +1907,7 @@ func TestDefault(t *testing.T) {
 		{map[string]string{"foo": "pop"}, `{{ index . "bar" | default "nada" }}`, `nada`, true},
 		{map[string]string{"foo": "cat"}, `{{ default "nope" .foo }}`, `cat`, true},
 		{map[string]string{"foo": "dog"}, `{{ default "nope" .foo "extra" }}`, ``, false},
+		{map[string]interface{}{"images": []string{}}, `{{ default "default.jpg" (index .images 0) }}`, `default.jpg`, true},
 	} {
 		tmpl, err := New().New("test").Parse(this.tpl)
 		if err != nil {