The `build-asdf-system` function is documented
[here](#lisp-defining-packages-outside). Also, `packages.nix` is full of
examples of how to use it.

## Defining packages manually outside Nixpkgs {#lisp-defining-packages-outside}

Lisp derivations (`abcl`, `sbcl` etc.) also export the `buildASDFSystem`
function, which is similar to `build-asdf-system` from `packages.nix`, but is
part of the public API.

It takes the following arguments:

- `pname`: the package name
- `version`: the package version
- `src`: the package source
- `patches`: patches to apply to the source before build
- `nativeLibs`: native libraries used by CFFI and grovelling
- `javaLibs`: Java libraries for ABCL
- `lispLibs`: dependencies on other packages build with `buildASDFSystem`
- `systems`: list of systems to build

It can be used to define packages outside Nixpkgs, and, for example, add them
into the package scope with `withOverrides`.

### Including an external package in scope {#lisp-including-external-pkg-in-scope}

A package defined outside Nixpkgs using `buildASDFSystem` can be woven into the
Nixpkgs-provided scope like this:

```nix
let
  alexandria = sbcl.buildASDFSystem rec {
    pname = "alexandria";
    version = "1.4";
    src = fetchFromGitLab {
      domain = "gitlab.common-lisp.net";
      owner = "alexandria";
      repo = "alexandria";
      tag = "v${version}";
      hash = "sha256-1Hzxt65dZvgOFIljjjlSGgKYkj+YBLwJCACi5DZsKmQ=";
    };
  };
  sbcl' = sbcl.withOverrides (self: super: { inherit alexandria; });
in
sbcl'.pkgs.alexandria
```

## Overriding package attributes {#lisp-overriding-package-attributes}

Packages export the `overrideLispAttrs` function, which can be used to build a
new package with different parameters.

Example of overriding `alexandria`:

```nix
sbcl.pkgs.alexandria.overrideLispAttrs (oldAttrs: rec {
  version = "1.4";
  src = fetchFromGitLab {
    domain = "gitlab.common-lisp.net";
    owner = "alexandria";
    repo = "alexandria";
    tag = "v${version}";
    hash = "sha256-1Hzxt65dZvgOFIljjjlSGgKYkj+YBLwJCACi5DZsKmQ=";
  };
})
```

### Dealing with slashy systems {#lisp-dealing-with-slashy-systems}

Slashy (secondary) systems should not exist in their own packages! Instead, they
should be included in the parent package as an extra entry in the `systems`
argument to the `build-asdf-system`/`buildASDFSystem` functions.

The reason is that ASDF searches for a secondary system in the `.asd` of the
parent package. Thus, having them separate would cause either one of them not to