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// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// This file contains the code to handle template options.
package template
import "strings"
// missingKeyAction defines how to respond to indexing a map with a key that is not present.
type missingKeyAction int
const (
mapInvalid missingKeyAction = iota // Return an invalid reflect.Value.
mapZeroValue // Return the zero value for the map element.
mapError // Error out
type option struct {
missingKey missingKeyAction
// Option sets options for the template. Options are described by
// strings, either a simple string or "key=value". There can be at
// most one equals sign in an option string. If the option string
// is unrecognized or otherwise invalid, Option panics.
// Known options:
// missingkey: Control the behavior during execution if a map is
// indexed with a key that is not present in the map.
// "missingkey=default" or "missingkey=invalid"
// The default behavior: Do nothing and continue execution.
// If printed, the result of the index operation is the string
// "<no value>".
// "missingkey=zero"
// The operation returns the zero value for the map type's element.
// "missingkey=error"
// Execution stops immediately with an error.
func (t *Template) Option(opt ...string) *Template {
for _, s := range opt {
return t
func (t *Template) setOption(opt string) {
if opt == "" {
panic("empty option string")
elems := strings.Split(opt, "=")
switch len(elems) {
case 2:
// key=value
switch elems[0] {
case "missingkey":
switch elems[1] {
case "invalid", "default":
t.option.missingKey = mapInvalid
case "zero":
t.option.missingKey = mapZeroValue
case "error":
t.option.missingKey = mapError
panic("unrecognized option: " + opt)