Consume an external module

In this chapter, we are looking at downloading and using external modules.

Introduction

This chapter will cover:

Creating a project.
Adding an external module to your project.
Use the external library in your app.

External module

To consume an external module, you need to:

Import it, 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.

Import module

To import a module, you can do one of two things:

go get <path to module>, this will fetch the module and download it and make it available for your project to use.
go mod tidy, this command checks the imports used in your program and fetches the module if not fetched already.

Use it in code

To use your module in code, you need to add it to the import section and then invoke it where you need it in the application code.

import (
  "github.com/<user>/<repo name>"
)

func main() {
  <repo name>.<Function>()
}

Here’s an example:

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

func main() {
  math.Add(1,1) // 2
}

Assignment - consume an external module

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 a main() function and call the external Add method from the math 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
```
Running the program gives you the following output:
```
3
```

Solution

go.sum

github.com/softchris/math v0.2.0 h1:88L6PLRBGygS3LY5KGIJhyw9IZturmd2ksU+p13OPa4=
github.com/softchris/math v0.2.0/go.mod h1:v8WzhjKC+ipuH+i9IZ0Ta2IArniTP53gc5TgCINCqAo=

go.mod

module hello

go 1.16

require github.com/softchris/math v0.2.0

main.go

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

func main() {
  sum := math.Add(1,2)
  fmt.Println(sum)
}

Challenge

See if you can find another module you want to use in your project. Add it to the project and use it in your code.