Package Import error on Golang

My post has 2 question

I'm essentially trying to understand how to use gomock with gingko

GOPATH

/Users/Ratatouille/Desktop/test/goExample

The following project is located at

/Users/Ratatouille/Desktop/test/goExample/square

which has following project structure

.
├── area.go
└── area_test
    ├── area_mock.go
    ├── area_mock_test.go
    └── area_suite_test.go

what I'm not able to understand why I'm getting following error

Failed to compile area_test:

can't load package: package ./area_test: found packages area_test (area_mock.go) and area (area_mock_test.go) in /Users/Ratatouille/Desktop/test/goExample/square/area_test

Ginkgo ran 1 suite in 303.243871ms
Test Suite Failed

my second question is project repo has area.go file which look like this

package main

import "fmt"

type Square struct {
	side float32
}

func (s *Square) Area() float32 {
	return s.side * s.side
}

func (s *Square) CalculateArea(shaper Shaper) float32 {
	shaper.Area()
}

type Shaper interface {
	Area() float32
}

func main() {
	sq1 := new(Square)
	sq1.side = 2

	var areaIntf Shaper
	areaIntf = sq1
	fmt.Println(areaIntf.Area())
}

My question is how do I import main package inside area_mock_test.go

Your project structure isn't the one Go expects.

area_test/ are not the tests for area. I'm going to guess you ran go test ./area_test. That tells Go that ./area_test is a project. It tried to compile area_mock.go and tried to use area_mock_test.go and area_suite_test.go as its tests. The error you got is Go's awkward way of telling you that it did not expect to find a package named *_test in a project file.

area.go is also a problem. You have it using package main, but then try to test with package area_test. They don't match and Go won't like it. A package directory contains one package. Its tests must use either that package or thatpackage_test. This enforces that a package directory does one thing. Its either a library to import, or its a program to run. Not both.

Also the project directory is called square, but the package is area. It's ok to have files that don't match the project name, and its fine to have multiple files in one package, but using a package than the project directory name is not good practice.

There's another problem. Go expects source files to be in $GOPATH/src. You have them directly in $GOPATH. Import statements won't find them.

Go's project structure can take a bit of getting used to, and it has very firm ideas about how projects and packages are to be structured (Go has very firm ideas about how code is to be written, and how it isn't). Don't fight it. Do yourself a favor and use the default GOPATH of ~/go and put your code in ~/go/src/.

You'd write the square library like so.

~/go/src/square/
|--- square.go
|--- square_test.go

square.go might look like this.

package square

type Square struct {
    Side float32
}

func (s Square) Area() float32 {
    return s.Side * s.Side
}

Note that Side must be capitalized for it to be a public data member. Also note that its passed by value, in Go the style is to only pass by pointer if you intend to modify the struct.

square_test.go might look like this.

package square_test

import(
    "testing"
    "square"
    "github.com/stvp/assert"
)

func TestArea( t *testing.T ) {
    sq := square.Square{Side: 5}
    
    assert.Equal( t, sq.Area(), float32(25) )
}

Go does not come with any asserts. github.com/stvp/assert provides the basics and removes a lot of tedium. You can get it with go get github.com/stvp/assert.

Note that it is in package square_test to make this a blackbox test which can only use the public interface of square. If you wanted a glassbox/internal test, it would use package square and go in its own file like square_internal_test.go.

If you want a program that uses square, that would be in its own separate package directory. It would have a main.go with package main and import "square".

How To Write Go explains this in more detail.