How to Effectively Organize Go Code Using Internal Packages with Modules?

Understanding Internal Packages and Their Usage

Question:

How do I effectively organize Go code using "internal" packages?

Initial Problem:

Despite placing the project outside the GOPATH, importing internal packages from main.go fails, unless the relative path is used (e.g., "./internal/foo|bar").

Introduction of Modules:

With the introduction of modules in Go v1.11 and above, placing the project inside $GOPATH/src is no longer necessary. Instead, you need to create a go.mod file to specify the location of each module.

Example:

Consider the following project structure:

project/
  go.mod
  main.go

  internal/
    bar/
      bar.go
      go.mod
    foo/
      foo.go
      go.mod

go.mod Files:

The go.mod files for the internal packages are simple:

module bar

go 1.14

module foo

go 1.14

Code in Internal Packages:

// bar.go
package bar

import "fmt"

// Bar prints "Hello from Bar"
func Bar() {
    fmt.Println("Hello from Bar")
}

// foo.go
package foo

import "fmt"

// Foo prints "Hello from Foo"
func Foo() {
    fmt.Println("Hello from Foo")
}

main.go:

// main.go
package main

import (
    "internal/bar"
    "internal/foo"
)

func main() {
    bar.Bar()
    foo.Foo()
}

Project go.mod File:

The project go.mod file informs Go about the internal packages and their locations:

module project

go 1.14

require internal/bar v1.0.0
replace internal/bar => ./internal/bar
require internal/foo v1.0.0
replace internal/foo => ./internal/foo

Important Notes:

• The names in the require section are for Go's internal use and can be arbitrary.
• The version numbers in the require section don't matter in this context.
• The replace lines specify where Go can find the internal packages.

Execution:

Running the main.go file will now print:

Hello from Bar
Hello from Foo

This example demonstrates how to effectively use internal packages within a Go project using modules.

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