# CHICKEN {#sec-chicken}

[CHICKEN](https://call-cc.org/) is a
[R⁵RS](https://schemers.org/Documents/Standards/R5RS/HTML/)-compliant Scheme
compiler. It includes an interactive mode and a custom package format, "eggs".

## Using Eggs

Eggs described in nixpkgs are available inside the
`chickenPackages.chickenEggs` attrset. Including an egg as a build input is
done in the typical Nix fashion. For example, to include support for [SRFI
189](https://srfi.schemers.org/srfi-189/srfi-189.html) in a derivation, one
might write:

```nix
  buildInputs = [
    chicken
    chickenPackages.chickenEggs.srfi-189
  ];
```

Both `chicken` and its eggs have a setup hook which configures the environment
variables `CHICKEN_INCLUDE_PATH` and `CHICKEN_REPOSITORY_PATH`.

## Updating Eggs

nixpkgs only knows about a subset of all published eggs. It uses
[egg2nix](https://github.com/the-kenny/egg2nix) to generate a
package set from a list of eggs to include.

The package set is regenerated by running the following shell commands:

```
$ nix-shell -p chickenPackages.egg2nix
$ cd pkgs/development/compilers/chicken/5/
$ egg2nix eggs.scm > eggs.nix
```

## Adding Eggs

When we run `egg2nix`, we obtain one collection of eggs with
mutually-compatible versions. This means that when we add new eggs, we may
need to update existing eggs. To keep those separate, follow the procedure for
updating eggs before including more eggs.

To include more eggs, edit `pkgs/development/compilers/chicken/5/eggs.scm`.
The first section of this file lists eggs which are required by `egg2nix`
itself; all other eggs go into the second section. After editing, follow the
procedure for updating eggs.
