# Name-based package directories

The structure of this directory maps almost directly to top-level package attributes.
This is the recommended way to add new top-level packages to Nixpkgs [when possible](#limitations).

## Example

The top-level package `pkgs.some-package` may be declared by setting up this file structure:

```
pkgs
└── by-name
   ├── so
   ┊  ├── some-package
      ┊  └── package.nix

```

Where `some-package` is the package name and `so` is the lowercased 2-letter prefix of the package name.

The `package.nix` may look like this:

```nix
# A function taking an attribute set as an argument
{
  # Get access to top-level attributes for use as dependencies
  lib,
  stdenv,
  libbar,

  # Make this derivation configurable using `.override { enableBar = true }`
  enableBar ? false,
}:

# The return value must be a derivation
stdenv.mkDerivation {
  # ...
  buildInputs =
    lib.optional enableBar libbar;
}
```

You can also split up the package definition into more files in the same directory if necessary.

Once defined, the package can be built from the Nixpkgs root directory using:
```
nix-build -A some-package
```

See the [general package conventions](../README.md#conventions) for more information on package definitions.

### Changing implicit attribute defaults

The above expression is called using these arguments by default:
```nix
{
  lib = pkgs.lib;
  stdenv = pkgs.stdenv;
  libbar = pkgs.libbar;
}
```

But the package might need `pkgs.libbar_2` instead.
While the function could be changed to take `libbar_2` directly as an argument,
this would change the `.override` interface, breaking code like `.override { libbar = ...; }`.
So instead it is preferable to use the same generic parameter name `libbar`
and override its value in [`pkgs/top-level/all-packages.nix`](../top-level/all-packages.nix):

```nix
libfoo = callPackage ../by-name/so/some-package/package.nix {
  libbar = libbar_2;
};
```

## Limitations

There's some limitations as to which packages can be defined using this structure:

- Only packages defined using `pkgs.callPackage`.
  This excludes packages defined using `pkgs.python3Packages.callPackage ...`.

  Instead use the [category hierarchy](../README.md#category-hierarchy) for such attributes.

- Only top-level packages.
  This excludes packages for other package sets like `pkgs.pythonPackages.*`.

  Refer to the definition and documentation of the respective package set to figure out how such packages can be declared.

## Validation

CI performs [certain checks](../test/nixpkgs-check-by-name/README.md#validity-checks) on the `pkgs/by-name` structure.
This is done using the [`nixpkgs-check-by-name` tool](../test/nixpkgs-check-by-name).
The version of this tool used is the one that corresponds to the NixOS channel of the PR base branch.
See [here](../../.github/workflows/check-by-name.yml) for details.

The tool can be run locally using

```bash
nix-build -A tests.nixpkgs-check-by-name
result/bin/nixpkgs-check-by-name .
```
