How to define an interface using another interface - go

Question

In Go, how to define an interface using another interface?
Here is an example:
package main
import (
"fmt"
)
// Interfaces
type Message interface {
Read() string
}
type MessageReader interface {
ReceiveMessages([]Message)
}
// Structs
type SQSMessage struct {
Content string
}
type SQSMessageReader struct {
Name string
}
// Implements
func (reader *SQSMessageReader) ReceiveMessages([]SQSMessage) {
}
func (msg *SQSMessage) Read() string {
return msg.Content
}
// A function needs a reader interface
func FuncNeedsReader(MessageReader) {
fmt.Println("get reader")
}
func main() {
var reader SQSMessageReader
FuncNeedsReader(reader)
}
I got error:
./prog.go:40:17: cannot use reader (type SQSMessageReader) as type MessageReader in argument to FuncNeedsReader:
SQSMessageReader does not implement MessageReader (wrong type for ReceiveMessages method)
have ReceiveMessages([]SQSMessage)
want ReceiveMessages([]Message)
Do anyone know if this design is conflicting with Go's design?
I know we have "accept interfaces, return structs"
So I also tried https://play.golang.org/p/qdGaKRYAqw7, but still fail with similar error.

Answer 1

I see two problems here.
First:
You have defined your MessageReader interface as:
type MessageReader interface {
ReceiveMessages([]Message)
}
But you have defined your ReceiveMessages method on SQSMessageReader like this:
func (reader *SQSMessageReader) ReceiveMessages([]SQSMessage) {
}
Because it takes an []SQSMessage parameter rather than []Message, it does not implement the MessageReader interface. You would need to write:
func (reader *SQSMessageReader) ReceiveMessages([]Message) {
}
Second:
You have written:
func main() {
var reader SQSMessageReader
FuncNeedsReader(reader)
}
But ReceiveMessages has a pointer receiver (func (reader *SQSMessageReader) ReceiveMessages...), so you would need:
func main() {
var reader SQSMessageReader
FuncNeedsReader(&reader)
}
With these two changes, your code builds without errors.