How to do mock a function using gomock in Go?

I am new to Go and doing small simple project + doing testing habit to learn.. but I am having trouble in setting up test using mock. Specifically in setting up the mock object

sample/sample.go

package sample
import (
    "fmt"
    "net/http"
)

func GetResponse(path, employeeID string) string {
    url := fmt.Sprintf("http://example.com/%s/%s", path, employeeID)
    // made some request here
    // then convert resp.Body to string and save it to value
    return value
}

func Process(path, employeeID string) string {
    return GetResponse(path, employeeID)
}

sample/sample_test.go

package sample_test
import (
     "testing"
     
     "github.com/example/sample" // want to mock some method on this package
)

func TestProcess(t *testing.T) {
     ctrl := gomock.NewController(t)
     defer ctrl.Finish()
     
     sample.MOCK()SetController(ctrl)
     sample.EXPECT().GetResponse("path", "employeeID").Return("abc")

     v := sample.GetResponse("path", "employeeID")
     fmt.Println(v)
}

Everytime I run this with

go test

I always get error

undefined: sample.MOCK
undefined: sample.EXPECT

1. Anyone could point out what am I doing wrong? do I need to initialize a mock object first? before mocking the method?

2. If I make GetResponse to be private [getResponse], I wont be able to mock it right?

Appreciate the all help.. Thanks!

I am not a gomock expert, but after reading the gomock GoDoc page, I see several issues with your code. First, you apparently can't use gomock to mock package functions (as you are trying to do with sample.GetResponse), only interfaces. Second, according to "standard usage", you have to

1. define an interface (e.g. FooInterface) that you want to mock
2. use mockgen to generate a mock from that interface
3. create a mockObj using the function NewMockFooInterface()
4. mockObj will then have the methods you want to call (i.e. EXPECT())

Testing and Coverage :

1. Interfaces --> Interfaces are named collections of method signatures. Interfaces are a key part of Go.
   Using an interface based structure is a great practice to ensure your code is less coupled to any specific dependency. Also, interfaces allow for efficient ,easy and structured testing.
2. GoMock is a mocking framework for the Go programming language.It enjoys a somewhat official status as part of the github.com/golang organization. It integrates well with Go's built-in testing package, and provides a flexible expectation API.
3. We need gomock package(handled by our dependency management mechanism) as well as the mockgen code generation tool(folder containing mockgen.exe to be set to PATH variable) for our testing
   Usage of GoMock follows four basic steps:
4. Use mockgen to generate a mock for the interface you wish to mock.
5. In your test, create an instance of gomock.Controller and pass it to your mock object’s constructor to obtain a mock object.
6. Call EXPECT() on your mocks to set up their expectations and return values
7. Call Finish() on the mock controller to assert the mock’s expectations(we use testify to make assertions)

For more on how to generate mocks for our interfaces using mockgen , please refer → https://github.com/golang/mock
an example of mockgen command to generate mock interfaces file ->

mockgen -destination={put the generated mocks in the file} -package={generate mocks for this package} {generate mocks for this interface}

For more info refer --> https://blog.codecentric.de/en/2017/08/gomock-tutorial/