Golang override embedded functions

Question

I wonder if is there solution for apply "polymorphism" in embedded functions when one pointing to other. For example I have following interface:

type Client interface {
    Get(string) string
    GetResource() string
}

and default implementation:

type ClientImpl struct {
}

func (c ClientImpl) Get(s string) string {
    return fmt.Sprintf("Impl [%s]", s)
}

func (c ClientImpl) GetResource() string {
    return c.Get("resource") # points to Get
}

And in other implementation (tests for example) I want to replace default Get function with other response but leave GetResource method untouched

type TestImpl struct {
    ClientImpl
}

func (t TestImpl) Get(s string) string {
    return fmt.Sprintf("Test [%s]", s)
}

Override function works when it's called directly but doesn't when called from embedded function. Look output produced from following test case:

c := ClientImpl{}
t := TestImpl{}
fmt.Println("WORKS:")
fmt.Println("Client Get:", c.Get("aaa"))
fmt.Println("Test   Get:", t.Get("aaa"))
fmt.Println("DOSN'T WORK :(")
fmt.Println("Client GetRes:", c.GetResource())
fmt.Println("Test   GetRes:", t.GetResource())

# WORKS:
# Client Get: Impl [aaa]
# Test   Get: Test [aaa]
# DOSN'T WORK :(
# Client GetRes: Impl [resource]
# Test   GetRes: Impl [resource]

How to make last print to output string Test [resource]?

Go Playground example: https://play.golang.org/p/b-vM1_W3oB

"How to make last print to output string Test [resource]?" No way. — Volker
– Volker, Commented Apr 6, 2017 at 9:04
Ok so I have to duplicate code or extract Get function from Client interface? — SZMER
– SZMER, Commented Apr 6, 2017 at 9:06
The best advice is: Stop trying to mimic "traditional OOP" with embedding. It just doesn't work out. If you need a test implementation of an interface: Write one. — Volker
– Volker, Commented Apr 6, 2017 at 9:13
Go features polymorphism via interfaces exclusively. It does not support inheritance or overriding, and embedding is not a way of faking these features. Any time you try to force OOP into Go, you're going to run into problems; you have to write Go as Go, not as any other language. — Adrian
– Adrian, Commented Apr 6, 2017 at 14:56

LeGEC · Accepted Answer · 2017-04-06 12:23:01Z

In your example, you need to isolate the Get function in its own interface.

Here are two similar ways to do this :

// one way to do this :
type Getter interface {
    Get(string) string
}

type ClientImpl struct {
}

func (c ClientImpl) Get(s string) string {
    return fmt.Sprintf("Impl [%s]", s)
}

type TestImpl struct {
}

func (t TestImpl) Get(s string) string {
    return fmt.Sprintf("Test [%s]", s)
}

// write a bare GetResource function :
func GetResource(c Getter) string {
    return c.Get("resource")
}

https://play.golang.org/p/R2XciBx_yk

// another way : store the "Getter" as a field in a struct
type Getter interface {
    Get(string) string
}

type ProdGetter struct {
}

func (c ProdGetter) Get(s string) string {
    return fmt.Sprintf("Impl [%s]", s)
}

type TestGetter struct {
}

func (t TestGetter) Get(s string) string {
    return fmt.Sprintf("Test [%s]", s)
}

// create a struct, which holds a Getter, and has methods to do stuff with it :
type Client struct {
    Getter
}

func (c *Client) GetResource() string {
    return c.Get("resource")
}

https://play.golang.org/p/ZMI5PlAo4L

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