/Crystal 0.31

Writing Shards

How to write and release Crystal Shards.

What's a Shard?

Simply put, a Shard is a package of Crystal code, made to be shared-with and used-by other projects.

See the Shards command for details.


In this tutorial, we'll be making a Crystal library called palindrome-example.

For those who don't know, a palindrome is a word which is spelled the same way forwards as it is backwards. e.g. racecar, mom, dad, kayak, madam


In order to release a Crystal Shard, and follow along with this tutorial, you will need the following:

Creating the Project

Begin by using the Crystal compiler's init lib command to create a Crystal library with the standard directory structure.

In your terminal: crystal init lib <YOUR-SHARD-NAME>


 $  crystal init lib palindrome-example
      create  palindrome-example/.gitignore
      create  palindrome-example/.editorconfig
      create  palindrome-example/LICENSE
      create  palindrome-example/README.md
      create  palindrome-example/.travis.yml
      create  palindrome-example/shard.yml
      create  palindrome-example/src/palindrome-example.cr
      create  palindrome-example/src/palindrome-example/version.cr
      create  palindrome-example/spec/spec_helper.cr
      create  palindrome-example/spec/palindrome-example_spec.cr
Initialized empty Git repository in /<YOUR-DIRECTORY>/.../palindrome-example/.git/

...and cd into the directory:


cd palindrome-example

Then add & commit to start tracking the files with Git:

 $  git add -A
 $  git commit -am "First Commit"
[master (root-commit) 77bad84] First Commit
 10 files changed, 102 insertions(+)
 create mode 100644 .editorconfig
 create mode 100644 .gitignore
 create mode 100644 .travis.yml
 create mode 100644 LICENSE
 create mode 100644 README.md
 create mode 100644 shard.yml
 create mode 100644 spec/palindrome-example_spec.cr
 create mode 100644 spec/spec_helper.cr
 create mode 100644 src/palindrome-example.cr
 create mode 100644 src/palindrome-example/version.cr

Writing the Code

The code you write is up to you, but how you write it impacts whether people want to use your library and/or help you maintain it.

Testing the Code

  • Test your code. All of it. It's the only way for anyone, including you, to know if it works.
  • Crystal has a built-in testing library. Use it!


Run crystal docs to convert your code and comments into interlinking API documentation. Open the files in the /docs/ directory with a web browser to see how your documentation is looking along the way.

See below for instructions on hosting your compiler-generated docs on GitHub Pages.

Once your documentation is ready and available, add this documentation badge below the description in your README.md so users know that it exists. (Be sure to replace <LINK-TO-YOUR-DOCUMENTATION> accordingly)


Writing a README

A good README can make or break your project. Awesome README is a nice curation of examples and resources on the topic.

Most importantly, your README should explain:

  1. What your library is
  2. What it does
  3. How to use it

This explanation should include a few examples along with subheadings.

NOTE: Be sure to replace all instances of [your-github-name] in the Crystal-generated README template with your GitHub username.

Coding Style


crystal tool format

To check if your code is formatted correctly, or to check if using the formatter wouldn't produce any changes, simply add --check to the end of this command.


crystal tool format --check

See the Travis CI section below to implement this in your build.

Writing a shard.yml

The spec is your rulebook. Follow it.


Your shard.yml's name property should be concise and descriptive.


name: palindrome-example


Add a description to your shard.yml.

A description is a single line description used to search for and find your shard.

A description should be:

  1. Informative
  2. Discoverable


It's hard for anyone to use your project if they can't find it. crystalshards.xyz is currently the go-to place for Crystal libraries, so that's what we'll optimize for.

There are people looking for the exact functionality of our library and the general functionality of our library. e.g. Bob needs a palindrome library, but Felipe is just looking for libraries involving text and Susan is looking for libraries involving spelling.

Our name is already descriptive enough for Bob's search of "palindrome". We don't need to repeat the palindrome keyword. Instead, we'll catch Susan's search for "spelling" and Felipe's search for "text".

description: |
  A textual algorithm to tell if a word is spelled the same way forwards as it is backwards.


  • Create a repository with the same name and description as specified in your shard.yml.

  • Add and commit everything:

    $ git add -A && git commit -am "shard complete"
  • Add the remote: (Be sure to replace <YOUR-GITHUB-USERNAME> and <YOUR-REPOSITORY-NAME> accordingly)

NOTE: If you like, feel free to replace public with origin, or a remote name of your choosing.

$ git remote add public https://github.com/<YOUR-GITHUB-NAME>/<YOUR-REPOSITORY-NAME>.git
  • Push it:
    $ git push public master

GitHub Releases

It's good practice to do GitHub Releases.

Add the following markdown build badge below the description in your README to inform users what the most current release is: (Be sure to replace <YOUR-GITHUB-USERNAME> and <YOUR-REPOSITORY-NAME> accordingly)

[![GitHub release](https://img.shields.io/github/release/<YOUR-GITHUB-USERNAME>/<YOUR-REPOSITORY-NAME>.svg)](https://github.com/<YOUR-GITHUB-USERNAME>/<YOUR-REPOSITORY-NAME>/releases)

Start by navigating to your repository's releases page.

  • This can be found at https://github.com/<YOUR-GITHUB-NAME>/<YOUR-REPOSITORY-NAME>/releases

Click "Create a new release".

According to the Crystal Shards README,

When libraries are installed from Git repositories, the repository is expected to have version tags following a semver-like format, prefixed with a v. Examples: v1.2.3, v2.0.0-rc1 or v2017.04.1

Accordingly, in the input that says tag version, type v0.1.0. Make sure this matches the version in shard.yml. Title it v0.1.0 and write a short description for the release.

Click "Publish release" and you're done!

You'll now notice that the GitHub Release badge has updated in your README.

Follow Semantic Versioning and create a new release every time your push new code to master.

Travis CI and .travis.yml

If you haven't already, sign up for Travis CI.

Insert the following markdown build badge below the description in your README.md: (be sure to replace <YOUR-GITHUB-USERNAME> and <YOUR-REPOSITORY-NAME> accordingly)

[![Build Status](https://travis-ci.org/<YOUR-GITHUB-USERNAME>/<YOUR-REPOSITORY-NAME>.svg?branch=master)](https://travis-ci.org/<YOUR-GITHUB-USERNAME>/<YOUR-REPOSITORY-NAME>)

Build badges are a simple way to tell people whether your Travis CI build passes.

Add the following lines to your .travis.yml:

  - crystal spec

This tells Travis CI to run your tests. Accordingly with the outcome of this command, Travis CI will return a build status of "passed", "errored", "failed" or "canceled".

If you want to verify that all your code has been formatted with crystal tool format, add a script for crystal tool format --check. If the code is not formatted correctly, this will break the build just as failing tests would.


  - crystal spec
  - crystal tool format --check

Commit and push to GitHub.

Follow these guidelines to get your repo up & running on Travis CI.

Once you're up and running, and the build is passing, the build badge will update in your README.

Hosting your docs on GitHub-Pages

Add the following script to your .travis.yml:

  - crystal docs

This tells Travis CI to generate your documentation.

Next, add the following lines to your .travis.yml. (Be sure to replace all instances of <YOUR-GITHUB-REPOSITORY-NAME> accordingly)

  provider: pages
  skip_cleanup: true
  github_token: $GITHUB_TOKEN
    branch: master
  local_dir: docs

Set the Environment Variable, GITHUB_TOKEN, with your personal access token.

If you've been following along, your .travis.yml file should look something like this:

language: crystal
  - crystal spec
  - crystal docs
  provider: pages
  skip_cleanup: true
  github_token: $GITHUB_TOKEN
    branch: master
  local_dir: docs

Click Here for the official documentation on deploying to GitHub-Pages with Travis CI.

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