Local go modules

I have the following files :

$GOPATH/src/example.com
├── a
│   ├── a.go
│   └── go.mod
├── b
│   ├── b.go
│   └── go.mod
└── c
    ├── go.mod
    └── main.go

Contents are :

a/a.go :

package a

func NewA() int {
    return 1
}

a/go.mod :

module example.com/a

go 1.17

b/b.go :

package b

import A "example.com/a"

func NewB() int {
    return A.NewA()
}

b/go.mod :

module example.com/b

go 1.17

replace example.com/a => ../a

require example.com/a v0.0.0-00010101000000-000000000000 // indirect

c/main.go :

package main

import "fmt"
import B "example.com/b"

func main() {
    fmt.Println(B.NewB())
}

c/go.mod :

module example.com/c

go 1.17

replace example.com/b => ../b

require example.com/b v0.0.0-00010101000000-000000000000 // indirect

Then when I type in the c directory : go run main.go I get :

../b/b.go:3:8: missing go.sum entry for module providing package example.com/a (imported by example.com/b); to add:
        go get example.com/b@v0.0.0-00010101000000-000000000000

And go get example.com/b@v0.0.0-00010101000000-000000000000 says :

go: downloading example.com/a v0.0.0-00010101000000-000000000000
example.com/b imports
        example.com/a: unrecognized import path "example.com/a": reading https://example.com/a?go-get=1: 404 Not Found

For sure this is a local package and not available on internet and it’s written in all mod files with replace.

How to work with local packages ?
If I rename example.com to example I get : missing dot in first path element.

>Solution :

Quoting from Go Modules Reference: replace directive:

replace directives only apply in the main module’s go.mod file and are ignored in other modules. See Minimal version selection for details.

The replace directive in b/go.mod has no effect when you build the main package / module. You must add that replace directive to the main module’s go.mod.

So add it to c/go.mod. After running go mod tidy in the c folder, it’ll look like this:

module example.com/c

go 1.17

replace example.com/b => ../b

replace example.com/a => ../a

require example.com/b v0.0.0-00010101000000-000000000000

require example.com/a v0.0.0-00010101000000-000000000000 // indirect

Without this replace in c/go.mod you were seeing the error message because the go tool tried to get the package from example.com (which is an existing domain) but it does not contain a go module.