Eggs Tutorial

  1. Eggs Tutorial
  2. Introduction
  3. Programming your extension
    1. Code layout
    2. Documentation
      1. Sections
    3. Egg build system and metadata
      1. Egg categories
    4. Licensing
    5. Tests
      1. Testing your extension
  4. Managing and hosting eggs
  5. Caveats
    1. License
  6. Post check in notes

Introduction

This document explains how to create an official CHICKEN Extension.

CHICKEN extensions can greatly enhance the functionality available in CHICKEN. They can define and export new convenient functions, wrap and make available libraries written in other languages (typically C) or even extend the basic language.

A good way to start getting involved in the CHICKEN community is to contribute new eggs. Not only this will benefit other users, who will now be able to use your egg, it will also benefit you as you'll gain access to all the infrastructure for managing eggs (eg. you will be able to install your eggs using chicken-install) and other users might start helping improve your eggs. You can create an egg from software you've written yourself, or else with free software libraries you've ported from other Schemes (or even other languages).

Programming your extension

Code layout

You should always use the module system for extensions to avoid creating name-conflicts if one of your exported toplevel identifiers clashes with an already existing definition. Modules also allow to export syntax definitions in a clean and easy-to-use way.

Documentation

Providing good documentation for your eggs is a fundamental part of their overall quality. Documentation for eggs is stored in wiki format in the CHICKEN subversion repository (see this document for information on how to request an account).

You can enter your entire documentation for your egg into this wiki. This has the advantage of inviting other members of the CHICKEN community to help improve the documentation for your eggs. Also, eggs documented in the CHICKEN wiki are automatically indexed by our API browser.

You can either use your favourite text editor to edit wiki files (then commit your changes to the subversion repository) or point your browser to http://wiki.call-cc.org/eggref/5/eggname-here and use it to edit the wiki contents.

If you decide to edit the wiki files locally using a text editor then commiting to the repository, you'll need to check out a copy of the subversion repository for wiki files:

 $ svn co https://code.call-cc.org/svn/chicken-eggs/wiki

Sections

We recommend that each page for an egg is given at least the following sections:

Your egg's name
The very first section of the documentation is taken as the title for your wiki page. Your egg's name is usually a good page title.
Description
Must briefly describe and state the purpose of the egg.
Authors
The egg authors and maintainers
Repository
Link to the code repository of the egg
Requirements
Should list other eggs that are required to build (compile-time) or load (runtime) this egg. Each entry should be linked to the respective egg.
API
The API description. Be sure to semantically format the procedures, macros, parameters, classes etc (see the Extensions for CHICKEN documentation section at the Editing help page).
Examples
Must provide simple examples of the most important functions in the egg. Note that all examples should be entirely self-contained; basically, pasting them in csi should work, which means, among other things, that they should include the use or require-extension lines loading the egg. Each example should be its own subsection and the actual code should follow a brief explanation of what it does.
License
The license for your egg (see the Eggs Licensing page)
Version History
Should list all releases of the egg and a description of their differences with the previous versions.

Egg build system and metadata

CHICKEN 5 introduced an new format for the system to build eggs, which is incompatible with the one for CHICKEN 4. Below you can find information on CHICKEN 5. For information on CHICKEN 4, see Eggs tutorial for CHICKEN 4.

The build system for eggs in CHICKEN 5 reads .egg files, which contain information about the egg and describe how eggs are to be built.

See the manual section on extensions for more information and simple examples on how to write .egg files and Egg specification format for the specification of .egg files.

For practical examples, see what existing eggs do.

Egg categories

For the category entry, used by .egg files you can use any of the following possibilities:

code-generation
Code generation
crypt
Cryptography
data
Algorithms and data-structures
db
Databases
debugging
Debugging tools
doc-tools
Documentation tools
egg-tools
Egg tools
ffi
Interfacing to other languages
graphics
Graphics
io
Input/Output
lang-exts
Language extensions
logic
Logic programming
macros
Macros and meta-syntax
math
Mathematical libraries
misc
Miscellaneous
net
Networking
oop
Object-oriented programming
os
OS interface
parsing
Data formats and parsing
sound
Sound related stuff
testing
Unit-testing
tools
Command line tools
ui
User interface toolkits
web
Web programming
xml
XML processing
hell
Concurrency and parallelism
uncategorized
Uncategorized
obsolete
Unsupported or redundant

Licensing

Please refer to Eggs Licensing.

Tests

chicken-install can automatically run a test suite on a freshly installed egg, if the egg directory contains a directory named tests, which should include a Scheme source file named run.scm. When chicken-install is invoked with the -test option, then this file will be executed (with test being the current working directory). It is recommended to add a test suite to your extension, as it allows some sort of automated testing of installed extensions. If "run.scm" never calls "error" during the execution then the test is considered successful.

If your tests depend on extra eggs, do not use needs/depends to require them. Use test-depends instead, so they'll only be required when chicken-install is used with the -test option.

For proper integration to the test infrastructure, run.scm should exit 0 (zero) when all tests pass or any other number in case some test fails. If you are using the test egg, a call to test-exit makes run.scm exit properly.

With regard to the test infrastructure, notice that only the latest released egg versions are tested.

Testing your extension

Before publishing your egg, it is recommended to run test-new-egg on it to try to catch some common mistakes in advance.

Assuming the release of your egg is ready (i.e., the code is available in the source repository and a tag to represent the egg version has been created), you can just run:

 $ chicken-install test-new-egg # in case you don't have test-new-egg installed
 $ test-new-egg <egg-name> <URL of the .release-info file>

test-new-egg will make sure that releases of your egg can be fetched and installed.

After your egg has been published (i.e., the .release-info file is known to work fine), the next releases can be tested with salmonella. Here's how you can do that:

 $ chicken-install salmonella # in case you don't have salmonella installed
 $ cd egg-dir # the directory where your egg code is stored
 $ salmonella

Salmonella will report to the standard output a summary of the tests it performs. It also generates a detailed log file containing all the data generated along the tests execution. You can use salmonella-log-viewer to see details about the whole test procedure:

 $ salmonella-log-viewer salmonella.log

See the salmonella documentation for more information on how to test eggs.

If your egg installs an executable file and tests call that file, take a look at this caveat.

Managing and hosting eggs

CHICKEN currently supports distributing eggs hosted on remote repositories (in the past, we had a central Subversion repository which is now deprecated for new eggs). So, you can host your egg code on popular repository providers like github, bitbucket etc, or even using your own server.

See the Releasing your egg document for information on how to host egg code and manage releases.

Caveats

License

This may seem a bit annoying but since we have had this a couple of times on our mailing list, please make sure that your egg's license is compatible with the licenses of its dependencies.

Post check in notes

Once your code hits the eggs repository, it'll be daily tested by salmonella to check if it can be installed by chicken-install. See http://tests.call-cc.org for more information.

Strive to keep the documentation for your eggs in good shape and up to date. It highly influences the quality of your eggs.