Example of Mocking in Golang, and Monkey Patch

gistfile1.go

package main

import "fmt"

type Something struct {}
func (s Something) Test() bool {
	return false
}

type SomethingInterface interface {
	Test() bool
}

type MockSomething struct {
	MockTest func() bool
}
func (ms MockSomething) Test () bool {
	if ms.MockTest != nil {
		return ms.MockTest()
	}
	return false
}

func main() {
	regularSomething := Something{}
	mockSomething := MockSomething{
		MockTest: func() bool {
			// this is where your mock happens
			return true
		},
	}
	fmt.Println("result of do something is: ",
		doSomething(regularSomething))
		
	fmt.Println("result of mocked do something is: ",
		doSomething(mockSomething))

	// now a patch example
	oldDoSomething := doSomething
	// reset func after completing method
	defer func () { doSomething = oldDoSomething}()
	doSomething = func(s SomethingInterface) bool {
		return true
	}
	// monkey patched version of doSomething will return true,
	// instead of what it should return, false.
	fmt.Println("result of do something is: ",
		doSomething(regularSomething))
	
}

var doSomething = func(s SomethingInterface) bool {
	return s.Test()
}

Nitpick about the naming. "Monkey patching" proper is forcefully modifying the compiled machine code, either on disk or in memory. (There are a few Go modules for that.) I'm struggling however to find a better name for that pattern with doSomething var. Maybe "monkey substitution"?

@jabielecki I think you want to call this a stub.