Eggs Tutorial

  1. Eggs Tutorial
  2. Introduction
  3. Programming your extension
    1. Code layout
  4. Additional files
    1. Documentation
      1. Sections
    2. The setup file
    3. The meta file
    4. Tests
      1. Testing your extension
  5. Managing and hosting eggs
  6. Caveats
    1. License
  7. Post check in notes


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).

We will assume your extension is called mpeg3. Replace occurences of that string throughout this file with the actual name of your extension.

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.

Additional files


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 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 ckeck out a copy of the subversion repository for wiki files:

 $ svn co


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.
Must briefly describe and state the purpose of the egg.
The egg authors and maintainers
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.
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).
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.
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.

The setup file

In order for chicken-install to install your extension, we recommend that you create a mpeg3.setup file with information about your egg. chicken-install will load this file. chicken-install adds some new procedures to the environment, which are documented in the "Extensions" section of the manual.

If your egg does not contain macros, your setup file should look similar to the following:

;; These two instructions will produce statically and dynamically linkable object files "mpeg3.o" and "" respectively.
(compile -s -O3 -d1 mpeg3.scm -j mpeg3)
(compile -s mpeg.import.scm -O3 -d0)

 ;; Name of your extension:
 ;; Files to install for your extension:
 '("" "")
 ;; Assoc list with properties for your extension:
 '((version "1.2")))

The first line will cause mpeg3.scm to be compiled into (a shared library for dynamic loading), which is installed by install-extension. The -j option will make sure that an import library is generated for your module(s) which, after compilation, provide module meta-information and exported syntax. Alternatively you can use -J without any arguments to generate import libraries for all defined modules.

The second line will compile the generated import library.

If your egg requires your code to be linked against a specific library or certain flags (eg. -l) to be passed to the C compiler, you should specify them here.

Note that version is represented by a string. That's because numbers can be tricky to represent versions. Integers are usually not enough to represent minor/major changes, and floating point numbers require you to determine the number of minor releases a priori, otherwise you'll inevitably end up with version 1.10, which is numerically equivalent to 1.1, and that would cause problems when chicken-install tries to match versions to determine dependencies.

The meta file

Finally, you will need to create mpeg3.meta, with information about your egg. This file is used by the process that releases and uploads new eggs. It should contain a single s-expr as follows:

; Your egg's license:
(license "BSD")

; Pick one from the list of categories (see below) for your egg and enter it
; here.
(category web)

; A list of eggs mpeg3 depends on.  If none, you can omit this declaration
; altogether. `depends' is an alias to `needs'.
; Notice that you should NOT put CHICKEN units (e.g., srfi-1, srfi-13
; and many others) in `needs' or in `depends'.
(needs sandbox syntax-case)

; A list of eggs required for TESTING ONLY.  See the `Tests' section.
; Just like `needs' and `depends', `test-depends' should NOT contain
; CHICKEN units.
(test-depends test)

(author "Your Name Goes Here")
(synopsis "A basic description of the purpose of the egg."))

For the category entry you can use any of the following:

Code generation
Algorithms and data-structures
Debugging tools
Documentation tools
Egg tools
Interfacing to other languages
Language extensions
Logic programming
Macros and meta-syntax
Mathematical libraries
Object-oriented programming
OS interface
Data formats and parsing
Sound related stuff
Command line tools
User interface toolkits
Web programming
XML processing
Concurrency and parallelism
Unsupported or redundant

Make sure you read Eggs Licensing!

More information about extension meta properties can be found here at the Metafile reference.


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 salmonella on it to try to catch some common mistakes in advance. 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.



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 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.