Go from the beginning - Go modules

There are two interesting use cases with modules:

Consuming a module, you will most likely use a combination oof core modules and external third party modules
Creating a module, in some case you will create code that you or someone else will be able to use. For this scenario, you can create a module and upload it to GitHub.

Consume internal files

You might want to split up your app in many different files. Let's say you have the following files:

/app
  main.go
  /helper
    helper.go

Create the module like so:
```
go mod init
```

create the helper directory and helper.go file and give it the following content:

// helper.go

package helper

import "fmt"

func Help() {
 fmt.Println("This is a helper function")
}

Create the main.go file and give it the following content:

   package main

   import (
     "log-tester/helper"

     "github.com/softchris/logger"
   )

   func main() {
     logger.Log("hey there")
     helper.Help()
   }

Note this import "log-tester/helper", it ensures the helper package is in scope.

Compile and run

   go run main.go

Consume an external module

To consume an external module, you need to:

Import it, this involves using the import instruction and fully qualifying the address to the module's location.
Use it in code, call the code from the module that you mean to use
Ensure it's downloaded, so your code can be run.

Create a project

Let's create a new project

Run go mod init:
```
go mod init hello
```
Note how go.mod was created with the following content:
```
module hello

go 1.16
```

Add reference to an external lib

Next, lets create some code that will use the external library:

Create the file main.go

package main

import (
  "fmt"
  "github.com/softchris/math"
)

To the same file, add main() function and use external function, from package:
```
func main() {
  sum += math.Add(1,2)
  fmt.Println(sum)
}
```

Fetch the lib

Now, we need to resolve the external library.

Run go mod tidy:

go mod tidy

Your go.mod is updated:

require github.com/softchris/math v0.2.0

There's also go.sum file with the following content:

github.com/softchris/math v0.2.0 h1:88L6PLRBGygS3LY5KGIJhyw9IZturmd2ksU+p13OPa4=

github.com/softchris/math v0.2.0/go.mod h1:v8WzhjKC+ipuH+i9IZ0Ta2IArniTP53gc5TgCINCqAo=

This is Go's way of keeping track of how to build the app by referencing to the go module in question.

Run go run:

   go run main.go

Create a module

When you build a module meant for sharing, there's some gotchas:

you need to create a package
your package won't be called main
there's concept of public and private parts of your code
you can't test it locally
upload your package to GitHub for wide distribution

Create the code for the module

To create a moodule meant for wider use you need to first initialize a module.

Create a directory logger for your new package:

   mkdir logger
   cd logger

Run go mod init <address at github>, for example:

   go mod init github.com/softchris/logger

This will create a go.mod file in your directory.

   logger/
     go.mod

The file looks like so:

   module github.com/softchris/logger

   go 1.16

It contains the package name and the version of Go it means to use.

Create a file to host your package code, for example log.go and give it the following content:

    package logger

    import (
     "fmt"
    )

    var Version string = "1.0"

    func Log(mess string) {
     fmt.Println("[LOG] " + mess)
    }

Note package logger instead of main.
The uppercase variables and methods makes the publicly available. Anything named with lowercase will be private for the package.

Test it locally

You can test your package locally. To do so you need a separate package that you can import your package from.

Move up a directory:

   cd ..

Create a new directory logger-test:

   mkdir logger-test
   cd logger-test

Create a package, it will be used for testing only:

   go mod init logger-test

Create a file main.go and add the following code:

    package main

    import "github.com/softchris/logger"

    func main() {
     logger.Log("hey there")
    }

At this point, you are consuming the "logger" package but it's pointing to GitHub and your package don't live there yet. However, you can repoint to a local address on your machine, lets do that next.

Open go.mod and add the following:

   require github.com/softchris/logger v0.0.0

   replace github.com/softchris/logger => ../logger

Two things are happening here:

you are asking for the "logger" package:

  require github.com/softchris/logger v0.0.0

you are making it point to your local system instead of GitHub
```
  replace github.com/softchris/logger => ../logger
```

Run the package with go run:
```
go run main.go
```
You should see:
```
[LOG] hey there
```

Publish a package

To publish your package, you can put it on GitHub.

Create a git repo with git init:

   git init

Create the repo at GitHub.
Make you do at least one commit:
```
git add .
git commit -m "first commit"
```
Do the following do upload your package to GitHub:

   git remote add origin https://github.com/softchris/logger.git

   git branch -M main
   git push -u origin main

Tag your package with git tag:

   git tag v0.1.0
   git push origin v0.1.0

Now your package has the tag 0.1.0

Test it out

Go to your project "logger-test":

   cd ..
   cd logger-test

Open up go.mod and remove these lines:

   require github.com/softchris/logger v0.1.0
   replace github.com/softchris/logger => ../logger

Run go mod tidy, this will force Go to go look for the package:

Your go.mod should now contain:

   require github.com/softchris/logger v0.1.0

Also, your go.sum should contain:

   github.com/softchris/logger v0.1.0 h1:Kqw7t9C3Y7BtHDLTx/KXEqHy5x8EJxrLian742S0di0=

   github.com/softchris/logger v0.1.0/go.mod h1:rrzWjMsM3tqjetDBDyezI8mFCjGucF/b5RSAqptKF/M=

Run the program with go run:

   go run main.go

You should see:

   [LOG] hey there

Summary

Great, you've managed to create a module locally, put it on GitHub and have one of your other Go projects use it.

Go from the beginning - working with packages and modules