Using different levels of interfaces - go

Question

I'm trying to organize my code using interfaces in Go.
I have 2 sources of data: FTP and API. In each source, I have several structs that make the logic vary depending on the case.
In this question, I will omit API and stick with FTP.
My problem comes from the impossibility to say: FTPAcq is also an Acquisition
If FetchMeters(), when I do ftp.Decrypt(nil) I would like ftp to be "compatible" with FTPAcq
Here is my code:
package main
import (
"github.com/dutchcoders/goftp"
log "github.com/sirupsen/logrus"
"os"
)
type Acquisition interface {
FetchMeters() ([]Meter, error)
Name() string
}
type FTPAcq interface {
Unzip(file string) string
Decrypt(file string) string
}
//type APIAcq interface {
// FetchMeter(meterID string) (Meter, error)
//}
func main() {
var acqs []Acquisition
ftp, err := NewFTPDriver(os.Getenv("FTP_USER"), os.Getenv("FTP_PASSWD"), os.Getenv("FTP_ADDR"), os.Getenv("FTP_PORT"))
if err != nil {
panic(err)
}
ftp1 := NewFTPDriverSGE(*ftp)
ftp2 := NewFTPDriverTA(*ftp)
acqs = append(acqs, ftp1, ftp2)
for _, acq := range acqs {
tmpMeters, err := acq.FetchMeters()
if err != nil {
log.Warn(acq.Name(), " got error :", err)
}
log.Info(tmpMeters)
}
}
type Meter struct {
ID string
OperationID string
Unit string
}
//FtpSGE is a implementation of acquisition Interface (see driver.go)
type FTP struct {
Username string
Password string
Url string
Port string
client *goftp.FTP
}
type FTPSGE struct {
FTP
}
type FTPTA struct {
FTP
}
func (f FTPSGE) Unzip(path string) []string {
return nil
}
func (f FTPTA) Unzip(path string) []string {
return nil
}
func (f FTPSGE) Decrypt(path string) []string {
return nil
}
func (f FTPTA) Decrypt(path string) []string {
return nil
}
func (ftp FTP) FetchMeters() ([]Meter, error) {
log.Info(ftp.Name(), " is running")
files := ftp.Download(nil)
files = ftp.Decrypt("") // I have several implementation of Decrypt
files = ftp.Unzip("") // I have several implementation of Unzip
log.Info(files)
return nil, nil
}
func (ftp FTP) Name() string {
panic("FTP ")
}
func (ftp FTP) Download(files []string) []string {
panic("implement me")
}
func NewFTPDriver(user, password, url, port string) (*FTP, error) {
var err error
ftp := &FTP{
Username: user,
Password: password,
Url: url,
Port: port,
}
if ftp.client, err = goftp.Connect(url + ":" + port); err != nil {
return nil, err
}
if err = ftp.client.Login(user, password); err != nil {
return nil, err
}
return ftp, nil
}
func NewFTPDriverSGE(f FTP) *FTPSGE {
return &FTPSGE{f}
}
func NewFTPDriverTA(f FTP) *FTPTA {
return &FTPTA{f}
}
In my case, I get:
ftp.Decrypt undefined (type FTP has no field or method Decrypt)
How should I do?

Answer 1

FTP does not implement FTPAcq. It implements only Acquisition. It doesn't even have Decrypt() as a method, interface or not.
FTPSGE and FTPTA implement FTPAcq, but they are not the same type as FTP.
I don't know what you're trying to accomplish, but perhaps something to try is embedding FTP in FTPSGE and FTPTA. This gives those 2 types the fields and methods of the embedded type and still allows you to define additional methods on those types (the methods for FTPAcq in your case).
For example
type FTPSGE {
FTP
}
// OR
type FTPSGE {
*FTP
}
Which you then create as so: x := FTPSGE{ftp1}. Keep in mind that this will create a copy of ftp1 inside x. If ftp1 is type FTP (not a pointer), the entire struct gets copied. If ftp1 is type *FTP (a pointer, which seems to be what you're using), the pointer is copied and x.FTP still points to the same data as ftp1.
This means FTPSGE will implement both Acquisition and FTPAcq.
You'll have to be careful of whether or not interfaces are implemented on the value or pointer: func (a A) Something() vs func (a *A) Somthing().
Here is some reading on methods, interfaces, and embedding.
https://www.ardanlabs.com/blog/2014/05/methods-interfaces-and-embedded-types.html
https://travix.io/type-embedding-in-go-ba40dd4264df
https://golang.org/doc/effective_go.html (and lots of other info)