Main package

All Go program has to be part of a package. That’s why the default package is the main. If a program is part of the main package, Go generates a binary file. When that file runs, it calls the main() function. But when a program is part of a package other than main, Go doesn’t generate a binary file

main package -> generate a binary file.
other package -> generate a package archive file (.a extension)

Package name convention

A package uses the last part of its import path as its name. E.g: the package called math/cmplx

// to import it
import "math/cmplx"

// to refer the objects in the package
cmplx.Inf()

Create a package

Create a new directory in the $GOPATH/src directory called calculator. Create a file called sum.go.

// file directory
src/
  calculator/
    sum.go

Then, in the sum.go file initialize the name of the package and add the code.

// initialize the package
package calculator

// add the code
var logMessage = "[LOG]"

// Version of the calculator
var Version = "1.0"

func internalSum(number int) int {
    return number - 1
}

// Sum two integer numbers
func Sum(number1, number2 int) int {
    return number1 + number2
}

To confirm that everything is working, you can run the go build command in the calculator directory. No executable is generated. 👍

Create a module

Go modules typically contain packages that offer related functionality. Go Module = multiple go packages. Modules also have a versions, and doesn’t need to be inside the $GOPATH/src directory.

Run this inside the $GOPATH/src/calculator directory

go mod init github.com/myuser/calculator

// github.com/myuser/calculator becomes the module's name 

The comand creates a go.mod file inside the calculator/ folder, that contains:

module github.com/myuser/calculator

go 1.14

Reference a local package (a module)

Working in a main package:

src/
  calculator/
    go.mod
    sum.go
  helloworld/
    main.go

The main.go want to use the calculator package:

package main

import (
  "fmt"
  // reference the module
  "github.com/myuser/calculator"
)

func main() {
    total := calculator.Sum(3, 5)
    fmt.Println(total)
    fmt.Println("Version: ", calculator.Version)
}

It doesn’t work as the package is local, so you need to tell the main.go where about the calculator package location. For that:

Init the helloworld package

> go mod init helloworld

Update the helloworld.go file to include where the calculator package location

module helloworld

go 1.14

require github.com/myuser/calculator v0.0.0

// refers to your local module
replace github.com/myuser/calculator => ../calculator

Now you can run the program: go run main.go

Publish a package

Offical page: PackagePublishing

We want to publish the calculator package, so we need to publish to GitHub. Then add the tag:

git tag v0.1.0
git push origin v0.1.0

Then, the developers want to use this package, should use:

import "github.com/myuser/calculator"