How to use “internal” packages?

With modules introduction in Go v1.11 and above you don’t have to specify your project path in $GOPATH/src

You need to tell Go about where each module located by creating go.mod file. Please refer to go help mod documentation.

Here is an example of how to do it:

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()
}

Now the most important module
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

Couple things here:

1. You can have any name in require. You can have project/internal/bar if you wish. What Go think it is URL address for the package, so it will try to pull it from web and give you error

go: internal/bar@v1.0.0: malformed module path "internal/bar": missing dot in first path element

That is the reason why you need to have replace where you tell Go where to find it, and that is the key!

replace internal/bar => ./internal/bar

2. The version doesn’t matter in this case. You can have v0.0.0 and it will work.

Now, when you execute your code you will have

Hello from Bar
Hello from Foo

Here is GitHub link for this code example