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1# CHICKEN {#sec-chicken}
2
3[CHICKEN](https://call-cc.org/) is a
4[R⁵RS](https://schemers.org/Documents/Standards/R5RS/HTML/)-compliant Scheme
5compiler. It includes an interactive mode and a custom package format, "eggs".
6
7## Using Eggs {#sec-chicken-using}
8
9Eggs described in nixpkgs are available inside the
10`chickenPackages.chickenEggs` attrset. Including an egg as a build input is
11done in the typical Nix fashion. For example, to include support for [SRFI
12189](https://srfi.schemers.org/srfi-189/srfi-189.html) in a derivation, one
13might write:
14
15```nix
16{
17 buildInputs = [
18 chicken
19 chickenPackages.chickenEggs.srfi-189
20 ];
21}
22```
23
24Both `chicken` and its eggs have a setup hook which configures the environment
25variables `CHICKEN_INCLUDE_PATH` and `CHICKEN_REPOSITORY_PATH`.
26
27## Updating Eggs {#sec-chicken-updating-eggs}
28
29nixpkgs only knows about a subset of all published eggs. It uses
30[egg2nix](https://github.com/the-kenny/egg2nix) to generate a
31package set from a list of eggs to include.
32
33The package set is regenerated by running the following shell commands:
34
35```
36$ nix-shell -p chickenPackages.egg2nix
37$ cd pkgs/development/compilers/chicken/5/
38$ egg2nix eggs.scm > eggs.nix
39```
40
41## Adding Eggs {#sec-chicken-adding-eggs}
42
43When we run `egg2nix`, we obtain one collection of eggs with
44mutually-compatible versions. This means that when we add new eggs, we may
45need to update existing eggs. To keep those separate, follow the procedure for
46updating eggs before including more eggs.
47
48To include more eggs, edit `pkgs/development/compilers/chicken/5/eggs.scm`.
49The first section of this file lists eggs which are required by `egg2nix`
50itself; all other eggs go into the second section. After editing, follow the
51procedure for updating eggs.
52
53## Override Scope {#sec-chicken-override-scope}
54
55The chicken package and its eggs, respectively, reside in a scope. This means,
56the scope can be overridden to affect other packages in it.
57
58This example shows how to use a local copy of `srfi-180` and have it affect
59all the other eggs:
60
61```nix
62let
63 myChickenPackages = pkgs.chickenPackages.overrideScope (
64 self: super: {
65 # The chicken package itself can be overridden to affect the whole ecosystem.
66 # chicken = super.chicken.overrideAttrs {
67 # src = ...
68 # };
69
70 chickenEggs = super.chickenEggs.overrideScope (
71 eggself: eggsuper: {
72 srfi-180 = eggsuper.srfi-180.overrideAttrs {
73 # path to a local copy of srfi-180
74 src = <...>;
75 };
76 }
77 );
78 }
79 );
80 # Here, `myChickenPackages.chickenEggs.json-rpc`, which depends on `srfi-180` will use
81 # the local copy of `srfi-180`.
82in
83<...>
84```