Playground link: http://play.golang.org/p/Ebf5AuJlcP
type Foo interface {}
type Bar interface {
ThisIsABar()
}
// Person implements both Foo and Bar
type Person struct {
Name string
}
func (p Person) ThisIsABar() {}
type FooContext struct {
Something Foo
}
type BarContext struct {
Something Bar
}
func main() {
t := template.Must(template.New("test").Parse("{{ .Something.Name }}
"))
// This works fine.
if err := t.Execute(os.Stdout, FooContext{Person{"Timmy"}}); err != nil {
fmt.Printf("Error: %s
", err)
}
// With BarContext, containing the exact same Person but wrapped in a
// different interface, it errors out.
if err := t.Execute(os.Stdout, BarContext{Person{"Timmy"}}); err != nil {
fmt.Printf("Error: %s
", err)
}
}
When I render a template (via the text/template package) containing {{ .Something.Name }}, I can go through interface Foo which contains no methods, and it works fine. But if I go through interface Bar instead, I get:
executing "test" at <.Something.Name>: can't evaluate field Name in type main.Bar
Why does the presence of an unrelated method on the interface, that isn't even used, affect the rendering of the template?
text/template is special casing interface{}, so called functions can have return type interface{}, etc. Adding a method to your interface means that detection no longer triggers.
http://golang.org/src/pkg/text/template/exec.go#L323
323 for _, cmd := range pipe.Cmds {
324 value = s.evalCommand(dot, cmd, value) // previous value is this one's final arg.
325 // If the object has type interface{}, dig down one level to the thing inside.
326 if value.Kind() == reflect.Interface && value.Type().NumMethod() == 0 {
327 value = reflect.ValueOf(value.Interface()) // lovely!
328 }
329 }
BarContext.Something is a Bar (an interface). A Bar has no field Name. If you want to use an interface there, you'll need to provide the data via a method that is part of the interface.