# 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 {#sec-chicken-using}

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 {#sec-chicken-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 {#sec-chicken-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.

## Override Scope {#sec-chicken-override-scope}

The chicken package and its eggs, respectively, reside in a scope. This means,
the scope can be overridden to effect other packages in it.

This example shows how to use a local copy of `srfi-180` and have it affect
all the other eggs:

```nix
let
  myChickenPackages = pkgs.chickenPackages.overrideScope' (self: super: {
      # The chicken package itself can be overridden to effect the whole ecosystem.
      # chicken = super.chicken.overrideAttrs {
      #   src = ...
      # };

      chickenEggs = super.chickenEggs.overrideScope' (eggself: eggsuper: {
        srfi-180 = eggsuper.srfi-180.overrideAttrs {
          # path to a local copy of srfi-180
          src = ...
        };
      });
  });
in
# Here, `myChickenPackages.chickenEggs.json-rpc`, which depends on `srfi-180` will use
# the local copy of `srfi-180`.
# ...
```
