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elm-package

The package manager for Elm. The full catalog of community libraries is located at package.elm-lang.org.

Basic Usage

To install a library run:

elm-package install elm-lang/html         # Install latest version
elm-package install elm-lang/html 1.0.0   # Install version 1.0.0

elm-package is sandboxed by default, so the downloaded package will be placed in your project's elm-stuff/ directory. Sandboxing means it is easy for different projects to have different dependencies.

Installing a package will also create a file called elm-package.json which gives a structured overview of your project, including stuff like what license you use, what packages you depend on, and which directories contain source code. Take a look at this file and see if everything looks correct!

Version Rules

Many people use version numbers in different ways, making it hard to give reliable version bounds in your own package. With elm-package versions are determined based on API changes. The rules are:

  • Versions all have exactly three parts: MAJOR.MINOR.PATCH

  • All packages start with initial version 1.0.0

  • Versions are incremented based on how the API changes:

    • PATCH - the API is the same, no risk of breaking code
    • MINOR - values have been added, existing values are unchanged
    • MAJOR - existing values have been changed or removed
  • elm-package will bump versions for you, automatically enforcing these rules

This means that if your package works with elm-lang/html 1.0.0 it is very likely to work with everything up until 2.0.0. At that point, some breaking change has occurred that might break your code. It is conceivable that things break on a minor change if you are importing things unqualified and a newly added value causes a name collision, but that is not extremely likely.

Updating Dependencies

Say you know a new version of elm-lang/core has come out, but you are not sure if you want to update. You can see how big of a change it is by running the following command:

elm-package diff elm-lang/core 3.0.0 4.0.0

This will show you all of the changes from version 3.0.0 which you have and version 4.0.0 which you would like to have. This gives you some real basis for deciding if you should update right now.

If you like what you see, take the following steps.

  • Save a copy of elm-stuff/exact-dependencies.json so you can always come back to a working state.

  • Change your version bounds in elm-package.json to include the newest stuff.

  • Run elm-package install elm-lang/core 4.0.0 and see how things go!

Publishing Packages

This is a step by step discussion of how to make a nice package that will be useful, easy to learn, and pleasant to use.

Designing APIs

Before publishing, look through the design guidelines. Some key takeaways are:

  • Design for a concrete use case
  • Always give functions human readable names
  • Avoid gratuitous abstraction

Preparing for Publication

The information in elm-package.json determines what people will see when they browse your package on package.elm-lang.org. Here are some hints for filling in that information:

  • Keep the summary under 80 characters.

  • The recommended license is BSD3, but of course, you can use whatever license you want.

  • exposed-modules lets you expose some small set of modules. Use this to stop internal details from polluting your API and cluttering the docs with modules that are not meant for users.

You should also create a README.md for your project. It should explain the use case of your library along with some examples to help people get situated before deciding to use your library or diving into your documentation.

Finally, you should document all of the publicly exposed modules and functions based on this format. Examples are one of the most powerful ways to learn new APIs so do not be lazy, make your users' lives easy!

Publishing for the First Time

When you have finished the API, tested everything, and written up docs, it is time to share it with others!

All Elm packages start with version number 1.0.0 and then increase according to automatically enforced rules. For now, all you need to know is that 1.0.0 is where things start.

elm-package is currently backed by GitHub, so we use GitHub tags to refer to specific version numbers. Add a version tag to your repo like this:

git tag -a 1.0.0 -m "initial release"
git push --tags

This will add a tag 1.0.0 which matches the version number you are publishing. It also associates that tag with a message. You can make your message more helpful than "initial release".

Once that is done, run the following command:

elm-package publish

This will send all the relevant information to the package catalog and verify that everything is in order. You just published a package!

Publishing Updates

Once you have published 1.0.0 you enter into the world of automatically enforced versioning as described in the version rules section. elm-package provides a couple tools to make it easy to diff APIs and figure out new version numbers.

First we have API diffing with the following commands:

elm-package diff        # diff current API with most recently published API
elm-package diff 1.0.0  # diff current API with version 1.0.0

Both of these will help you review your changes and make sure everything is what you were expecting. From there, you can automatically bump your version number by running this command:

elm-package bump

Based on the version number listed in your elm-package.json file, it will run a diff and figure out the magnitude of the changes. It will then tell you your new version number!

From here, everything is the same as publishing for the first time. Tag it on GitHub and publish it.