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Nicolas B. Pierron f5dfe78a1e Add overlays mechanism to Nixpkgs.
This patch add a new argument to Nixpkgs default expression named "overlays".

By default, the value of the argument is either taken from the environment variable `NIXPKGS_OVERLAYS`,
or from the directory `~/.nixpkgs/overlays/`.  If the environment variable does not name a valid directory
then this mechanism would fallback on the home directory.  If the home directory does not exists it will
fallback on an empty list of overlays.

The overlays directory should contain the list of extra Nixpkgs stages which would be used to extend the
content of Nixpkgs, with additional set of packages.  The overlays, i-e directory, files, symbolic links
are used in alphabetical order.

The simplest overlay which extends Nixpkgs with nothing looks like:

```nix
self: super: {
}
```

More refined overlays can use `super` as the basis for building new packages, and `self` as a way to query
the final result of the fix-point.

An example of overlay which extends Nixpkgs with a small set of packages can be found at:
  https://github.com/nbp/nixpkgs-mozilla/blob/nixpkgs-overlay/moz-overlay.nix

To use this file, checkout the repository and add a symbolic link to
the `moz-overlay.nix` file in `~/.nixpkgs/overlays` directory.
2017-01-16 01:17:33 +01:00
..
beam.xml Fix typo in beam.xml 2016-12-19 14:56:55 +00:00
bower.xml nixpkgs manual: Add documentation for bower2nix 2016-03-28 08:25:45 +01:00
coq.xml nixpkgs manual: split languages into separate files 2015-12-19 16:08:00 +01:00
go.xml buildGoPackage: deps.json -> deps.nix in NIXON 2016-09-16 00:04:55 +01:00
haskell.md fix documentation typo in doc/languages-frameworks/haskell.md 2016-11-08 17:42:27 +00:00
index.xml manual: add node to languages & frameworks 2016-09-20 20:38:24 +00:00
java.xml nixpkgs manual: split languages into separate files 2015-12-19 16:08:00 +01:00
lua.xml nixpkgs manual: split languages into separate files 2015-12-19 16:08:00 +01:00
perl.xml perl docs: update generated expression 2016-09-03 18:14:06 +02:00
python.md Add overlays mechanism to Nixpkgs. 2017-01-16 01:17:33 +01:00
qt.xml nixpkgs manual: split languages into separate files 2015-12-19 16:08:00 +01:00
ruby.xml ruby docs: improve the example 2016-03-20 13:33:20 +01:00
texlive.xml texlive manual: adds import of nixpkgs in nix-repl 2016-10-20 08:50:54 +02:00