Can Go's Reflection Create Functions Implementing Predefined Interfaces?

Extending Go's Reflection Capabilities

Question:

Is it possible to dynamically create a function that implements a predefined interface in Go using reflection?

Answer:

In the original version of Go, it was not possible to create new types with attached methods via reflection. However, with the release of Go 1.5, the reflect.FuncOf and reflect.MakeFunc functions were introduced to address this exact need.

These functions allow you to define a function type and create a function value that adheres to that type:

funcType := reflect.FuncOf([]reflect.Type{reflect.TypeOf("")}, []reflect.Type{reflect.TypeOf("")})
fn := reflect.MakeFunc(funcType, func(args []reflect.Value) []reflect.Value { ... })

The first argument to reflect.MakeFunc is the function type, while the second argument is a function value that implements that type.

In the context of the given example:

type MyService interface {
  Login(username, password string) (sessionId int, err error)
  HelloWorld(sessionId int) (hi string, err error)
}

You could create a function that implements this interface using reflection as follows:

fn := reflect.MakeFunc(reflect.TypeOf((*MyService)(nil)).Elem(), func(args []reflect.Value) []reflect.Value {
  switch args[0].String() {
  case "Login":
    return []reflect.Value{
      reflect.ValueOf(1),
      reflect.ValueOf(nil),
    }
  case "HelloWorld":
    return []reflect.Value{
      reflect.ValueOf("Hello, world!"),
      reflect.ValueOf(nil),
    }
  }
  return []reflect.Value{
    reflect.Value{},
    reflect.ValueOf(errors.New("Method not found")),
  }
})

This implementation uses a switch statement to determine which method of the interface to call based on the first argument. It then returns the appropriate values for the output arguments.

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