go-mod-tricks

go.mod file and go mod command tricks to make using modules "a breeze".

1. go.mod file tricks

   • Replace a module with local copy
   • List modules used in build

2. go mod command tricks

   • Updating an untagged module to latest version
   • Why am I importing x module?

3. Workspace tricks

   • Workspace initialization
   • Replacing third party modules with local modules
   • Pitfalls

4. Other tools

   • gomod dependency graph visualization

"a breeze", really?

No, of course not. Go modules is by far the most complex part of learning Go. The only person who has reached module enlightenment may be Bryan Mills. This document will serve as a cheatsheet to alleviate pain points caused by the complexity for the rest of us. We hope it helps you!

go.mod file

Replace a module with local copy

The following line in a go.mod file has the effect of compiling the module with a local version of the module.

replace github.com/author/dependency => /path/to/local/copy

List modules used in build

The following command lists all modules and their versions used in your build excluding test-only dependencies.

go list -deps -f '{{with .Module}}{{.Path}} {{.Version}}{{end}}' ./... | sort -u

Where sort -u excludes repeated entries. Thanks to thepudds on slack for command.

go mod Command

Updating an untagged module to latest version

To update an untagged module (no vX.X.X semantic versioning in place) in a module run the following:

cd my-module
go get -u github.com/author/dependency@branch-name

where branch-name is the name of the branch with the changes you wish to have.If you have recent pushes to the module repository the go tool might miss them since it searches a proxy. To avoid this you can tell the go tool to search the direct source:

GOPROXY=direct go get -u github.com/author/dependency@branch-name

You can also specify a certain commit by replacing branch-name with the commit hash:

go get -u github.com/author/dependency@commit-hash

Why am I importing x module?

To find out how a import is being pulled in you may run the following

cd my-module
go mod why -m github.com/author/dependency

The output of go mod why are newline separated import paths where the last one is the package being used in your project:

# github.com/author/dependency
my-module
github.com/foo/bar
github.com/author/dependency/some-package

In the case above, your module is importing github.com/foo/bar which in turn imports github.com/author/dependency/some-package.

Workspace tricks

Go workspaces solve the following problem: When developing there may come a time when you wish to edit a module you have imported to fix a bug on the side of a third party library, or to add a print statement. Go workspaces enable you to do this in a straightforward way.

Workspaces: Initialization

First run the following command to initialize your go.work file:

go work init

This generates a file named "go.work". Its only contents are a single line with the go version i.e: go 1.21.0. A newly created "go.work" file has no effect on your program compilation, you must add directives to it first (see next section).

Workspaces: Replacing a third party module with a local version

1. First you must have an initialized go.work file next to your existing go.mod file (see first section of workspace tricks).

2. You should have also cloned the repository of the go module to your local machine and have it in a nearby directory.

3. Replace the module by running:

go work use $RELATIVE_PATH_TO_MODULE

4. You are done! You may start editing the module in $RELATIVE_PATH_TO_MODULE freely and changes will be reflected in your program!

Practical example

We look at a practical example to build more intuition.

Below is an example directory structure.

parent
├── ours
│   ├── .gitignore
│   ├── go.mod
│   ├── go.work
│   └── main.go
├── module1
│   ├── go.mod
│   └── logic1.go
└── module2
    ├── go.mod
    └── logic2.go

For our example we are working in our project inside the ours directory. We've already initialized the go.work file in preparation to leverage Go's

We are currently using a remote version of module1 which we wish to edit. So we've cloned module1 to the same parent directory where our ours module resides.

We run the following command from inside ours directory

go work use ../module1

The go tool will find the go.mod file within this directory, check we are actually using it in our project and finally once it confirms it can actually replace it, edit our go.work file adding a use directive entry.

Once this is done we may start editing the module1/logic1.go file and changes to it will be observed in the ours module.

If we wish to also replace module2 we can just replace ../module1 with ../module2 in the command above.

Workspaces: Pitfalls

It is important to realize once you start editing your workspaced modules your project will no longer have the same behaviour when someone clones your changes without also uploading your workspaced modules. It is generally frowned upon to track versions of workspaced changes without including these workspaced modules in your versioning. And even then, tracking workspaced changes is a very complex task which can lead to unecessary code churn and pain if not done correctly.

Ideally one matches versioning between branches in the module one is primarily working on and the workspaced module, trying to match commit names so that changes can be tracked and reproduced easily. When finally ready to generate a PR one proceeds to create a PR for both the workspaced module and the module using the former.

It is suggested to not include the go.work file within versioning unless it is internally agreed upon in your organization. Add the following lines to your .gitignore to this effect:

go.work
go.work.sum
vendor # Excludes vendor directory from `go vendor` command.

Other tools

gomod dependency graph visualization

Generate visual dependency graphs using https://github.com/Helcaraxan/gomod.