golang modules and local packages

Question

I'm trying to understand how to organize my golang project using go1.11 modules. I tried several options, but none of them worked.

I have some code in the main package under the application folder and a local package that the main package uses.

$GOPATH
+ src
  + application/
    + main/
      + main.go
      + otherFileUnderMainPackage.go
    + aLocalPackage/
      + someCode.go
      + someCode_test.go
      + someMoreCode.go
      + someMoreCode_test.go

Files in the main package, imports ../aLocalPackage. When I compile by go build main/*.go it's working.

Then, I ran go mod init application: V.0.9.9 and got the go.mod file, but the build always fails. I always get error about not finding the local package: build application:V0.9.9/main: cannot find module for path _/.../src/application/aLocalPackage. I also tried to place the local package right under src/, place it under main/ etc. but none of these methods worked for me.

What is the way to use modules and local packages?

Thanks.

Answer 1

Relative import paths are not supported in module mode. You will need to update your import statements to use a full (absolute) import path.

You should also choose a module name other than application. Your module path should generally begin with a URL prefix that you control — either your own domain name, or a unique path such as github.com/$USER/$REPO.

Answer 2

I had some problems working with local packages myself. There are two tricks to make it work:

1. you run "go build" in the package directory

This compiles the package and places it in the build cache. This link about code organisation in go explains more. You can identify where the cache is using:

>go env GOCACHE
/home/<user>/.cache/go-build

2. Import using a path relative to the project

I puzzled loads over what the correct import path was and finally discovered that go doc or go list will tell you.

>go doc
package docs // import "tools/src/hello/docs"
>go list
tools/src/hello/docs

For example. I have a hello world API project and was using swaggo to generate documentation which it does in a docs sub-directory. To use it I add an import:

_ "tools/src/hello/docs"

For my case the _ is important as docs is not used directly but we its init() function to be invoked.

Now in hello/main.go I can add "tools/src/hello/docs" and it will import the correct package.

The path is relative to the location of go.mod if you have one. I have tools/ here as I have a go.mod declaring "modules tools".

Modules are a different kettle of fish - see https://github.com/golang/go/wiki/Modules. Recent versions of go (1.11 and later) can create a go.mod file which you may use to fix the version of a module that is used and avoid go's crazy default behaviour of just downloading the latest version of any package you import.