A slice function for Golang templates (text/template and html/template).

template-slice.go

func slice(item interface{}, indices ...interface{}) (interface{}, error) {
	v := reflect.ValueOf(item)

	switch v.Kind() {
	case reflect.Array, reflect.Slice, reflect.String:
	case reflect.Invalid:
		return nil, errors.New("index of untyped nil")
	default:
		return nil, fmt.Errorf("can't index item of type %s", v.Type())
	}

	if len(indices) == 0 {
		return nil, fmt.Errorf("too few arguments, accepted 2..3 but got 1")
	}

	index := reflect.ValueOf(indices[0])

	var i int64
	switch index.Kind() {
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		i = index.Int()
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
		i = 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 i < 0 || i >= int64(v.Len()) {
		return nil, fmt.Errorf("index out of range: %d", i)
	}

	switch len(indices) {
	case 1:
		return v.Slice(int(i), v.Len()).Interface(), nil
	case 2:
		index1 := reflect.ValueOf(indices[1])

		var j int64
		switch index1.Kind() {
		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
			j = index1.Int()
		case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
			j = int64(index1.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", index1.Type())
		}

		if j < 0 || j >= int64(v.Len()) {
			return nil, fmt.Errorf("index out of range: %d", j)
		}

		if i > j {
			return nil, fmt.Errorf("invalid index %d:%d", i, j)
		}

		return v.Slice(int(i), int(j)).Interface(), nil
	default:
		return nil, fmt.Errorf("too many arguments, accepted 2..3 but got %d", 1+len(indices))
	}
}