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This is modver, a tool that helps you obey semantic versioning rules in your Go module.

It can read and compare two different versions of the same module, from two different directories, or two different Git commits, or the base and head of a Git pull request. It then reports whether the changes require an increase in the major-version number, the minor-version number, or the patchlevel.

Installation and usage

Modver can be used from the command line, or in your Go program, or with GitHub Actions.

Command-line interface

Install the modver command like this:

go install

Assuming the current directory is the root of a cloned Git repository, you can run it like this:

$ modver -git .git HEAD~1 HEAD

to tell what kind of version-number change is needed for the latest commit. The -git .git gives the path to the repository’s info; it can also be something like The arguments HEAD~1 and HEAD specify two Git revisions to compare; in this case, the latest two commits on the current branch. These could also be tags or commit hashes.

GitHub Action

You can arrange for Modver to inspect the changes on your pull-request branch as part of a GitHub Actions-based continuous-integration step. It will add a comment to the pull request with its findings, and will update the comment as new commits are pushed to the branch.

To do this, you’ll need a directory in your GitHub repository named .github/workflows, and a Yaml file containing (at least) the following:

name: Tests

    branches: [ main ]
    branches: [ main ]

    runs-on: ubuntu-latest
      - name: Checkout
        uses: actions/checkout@v3
          fetch-depth: 0

      - name: Set up Go
        uses: actions/setup-go@v4
          go-version: 1.19

      - name: Modver
        if: ${{ github.event_name == 'pull_request' }}
        uses: bobg/modver@v2.5.0
          github_token: ${{ secrets.GITHUB_TOKEN }}
          pull_request_url:${{ github.repository }}/pull/${{ github.event.number }}

This can be combined with other steps that run unit tests, etc. You can change Tests to whatever name you like, and should change main to the name of your repository’s default branch. If your pull request is on a GitHub server other than, change the hostname in the pull_request_url parameter to match.

Note the fetch-depth: 0 parameter for the Checkout step. This causes GitHub Actions to create a clone of your repo with its full history, as opposed to the default, which is a shallow clone. Modver requires enough history to be present in the clone for it to access the “base” and “head” revisions of your pull-request branch.

For more information about configuring GitHub Actions, see the GitHub Actions documentation.

Go library

Modver also has a simple API for use from within Go programs. Add it to your project with go get See the Go doc page for information about how to use it.

Semantic versioning

Briefly, a major-version bump is needed for incompatible changes in the public API, such as when a type is removed or renamed, or parameters or results are added to or removed from a function. Old callers cannot expect to use the new version without being updated.

A minor-version bump is needed when new features are added to the public API, like a new entrypoint or new fields in an existing struct. Old callers can continue using the new version without being updated, but callers depending on the new features cannot use the old version.

A patchlevel bump is needed for most other changes.

The result produced by modver is the minimal change required. The actual change required may be greater. For example, if a new method is added to a type, this function will return Minor. However, if something also changed about an existing method that breaks the old contract - it accepts a narrower range of inputs, for example, or returns errors in some new cases - that may well require a major-version bump, and this function can't detect those cases.

You can be assured, however, that if this function returns Major, a minor-version bump won't suffice, and if this function returns Minor, a patchlevel bump won't suffice, etc.

The modver command (in the cmd/modver subdirectory) can be used, among other ways, to test that each commit to a Git repository increments the module’s version number appropriately. This is done for modver itself using GitHub Actions, here.

(Note that the standard actions/checkout@v2 action, for cloning a repository during GitHub Actions, creates a shallow clone with just one commit’s worth of history. For the usage here to work, you’ll need more history: at least two commit’s worth and maybe more to pull in the latest tag for the previous revision. The clone depth can be overridden with the fetch-depth parameter, which modver does here.)