如何在Go文本/模板用法中使用.Key

Question

I don't understand the document paragraph in http://golang.org/pkg/text/template/

- The name of a key of the data, which must be a map, preceded
  by a period, such as
    .Key
  The result is the map element value indexed by the key.
  Key invocations may be chained and combined with fields to any
  depth:
    .Field1.Key1.Field2.Key2
  Although the key must be an alphanumeric identifier, unlike with
  field names they do not need to start with an upper case letter.
  Keys can also be evaluated on variables, including chaining:
    $x.key1.key2

Here is my test code, but failed. code: http://play.golang.org/p/lbLJ4yoL2T.

var season = map[int]string{1: "spring", 2: "summer",              
        3: "autumn", 4: "winter"}                                  

func main() {                                                      
        const greeting = `Welcome, {{.Key}}`                       
        t := template.Must(template.New("greet").Parse(greeting))  
        err := t.Execute(os.Stdout, season)                        
        if err != nil {                                            
                fmt.Println(err)                                   
        }                                                          
}                                                                  

Output

Welcome, template: greet:1:11: executing "greet" at <.Key>: can't evaluate field Key in type map[int]string

Answer 1

I'd assume "Key" is the key (as in key/value) of the map. Also, the map keys can't be an int to use it in the template like that. So instead of {{.Key}} try {{.a}} as shown in this fork of your playground:

var season = map[string]string{"a": "spring", "b": "summer",              
        "c": "autumn", "d": "winter"}                                  

func main() {                                                      
        const greeting = `Welcome, {{.a}}`                       
        t := template.Must(template.New("greet").Parse(greeting))  
        err := t.Execute(os.Stdout, season)                        
        if err != nil {                                            
                fmt.Println(err)                                   
        }                                                          
}                                                                  

Output:

Welcome, spring