The default textual representation is determined by how the fmt package prints the value. So you were barking up the right tree.
See this example:
t := template.Must(template.New("").Parse("{{.}}"))
m := map[string]interface{}{"a": "abc", "b": 2}
t.Execute(os.Stdout, m)
It outputs:
map[a:abc b:2]
Now if we use a custom map type with a String() method:
type MyMap map[string]interface{}
func (m MyMap) String() string { return "custom" }
mm := MyMap{"a": "abc", "b": 2}
t.Execute(os.Stdout, mm)
Output is:
custom
Try these (and below examples) on the Go Playground.
What to look out for?
Note that MyMap.String() has a value receiver (not a pointer). And I pass a value of MyMap, so it works. If you change the receiver type to pointer to MyMap, it won't work. And it is because then only a value of type *MyMap will have a String() method, but not a value of MyMap.
If the String() method has a pointer receiver, you have to pass &mm (a value of type *MyMap) if you want your custom representation to work.
Also note that in case of html/template, the template engine does contextual escaping, so the result of the fmt package may further be escaped.
For example if your custom String() method would return something "unsafe":
func (m MyMap2) String() string { return "<html>" }
Trying to insert it:
mm2 := MyMap2{"a": "abc", "b": 2}
t.Execute(os.Stdout, mm2)
Gets escaped:
<html>
Implementation
This is where it is implemented in the text/template package: text/template/exec.go, unexported function state.PrintValue(), currently line #848:
_, err := fmt.Fprint(s.wr, iface)
If you're using the html/template package, it's implemented in html/template/content.go, unexported function stringify(), currently line #135:
return fmt.Sprint(args...), contentTypePlain
Further options
Also note that if the value implements error, the Error() method will be called and it takes precedence over String():
type MyMap map[string]interface{}
func (m MyMap) Error() string { return "custom-error" }
func (m MyMap) String() string { return "custom" }
t := template.Must(template.New("").Parse("{{.}}"))
mm := MyMap{"a": "abc", "b": 2}
t.Execute(os.Stdout, mm)
Will output:
custom-error
Instead of custom. Try it on the Go Playground.
