How Do Go Modules Handle Internal Packages to Prevent External Imports?

Organization of Go Code with Internal Packages

Understanding Internal Packages

Internal packages, denoted by the "internal" directory, are intended for grouping related code that should not be exposed outside the project. In Go modules, internal packages cannot be imported by external projects.

Module Structure

In the provided example, the project structure is as follows:

project/
  internal/
    foo/
      foo.go  # package foo
    bar/
      bar.go  # package bar
  main.go

where main.go imports the internal packages project/internal/foo and project/internal/bar.

Import Issue

The issue you encountered was trying to import internal packages from outside the project directory. Internal packages can only be imported from within the same project, so using the paths project/internal/foo and project/internal/bar from main.go would not work.

Solution with Go Modules

However, with the introduction of Go modules in Go v1.11, it's no longer necessary to specify your project path in $GOPATH/src. Create a go.mod file and define the internal package modules as follows:

project/
  go.mod
  main.go
  
\---internal
    +---bar
    |       bar.go
    |       go.mod
    |
    \---foo
            foo.go
            go.mod

project/internal/bar/go.mod

module bar

go 1.14

project/internal/bar/bar.go

package bar

import "fmt"

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

project/internal/foo/go.mod

module foo

go 1.14

project/internal/foo/foo.go

package foo

import "fmt"

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

project/main.go

package main

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

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

project/go.mod

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

Key Points

• Internal packages are restricted to use within the same project.
• Go modules provide a way to structure projects with internal packages using go.mod files and replace directives.
• Versioning in require and replace does not affect the functionality, as long as the package is found in the correct location.

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