pkgs.callPackage

pkgs.callPackage is used to parameterize the construction of Nix Derivation. To understand its purpose, let's first consider how we would define a Nix package (also known as a Derivation) without using pkgs.callPackage.

1. Without pkgs.callPackage

We can define a Nix package using code like this:

pkgs.writeShellScriptBin "hello" ''echo "hello, ryan!"''

To verify this, you can use nix repl, and you'll see that the result is indeed a Derivation:

› nix repl -f '<nixpkgs>'
Welcome to Nix 2.13.5. Type :? for help.

Loading installable ''...
Added 19203 variables.

nix-repl> pkgs.writeShellScriptBin "hello" '' echo "hello, xxx!" ''
«derivation /nix/store/zhgar12vfhbajbchj36vbbl3mg6762s8-hello.drv»

While the definition of this Derivation is quite concise, most Derivations in nixpkgs are much more complex. In previous sections, we introduced and extensively used the import xxx.nix method to import Nix expressions from other Nix files, which can enhance code maintainability.

1. To enhance maintainability, you can store the definition of the Derivation in a separate file, e.g., hello.nix.
   1. However, the context within hello.nix itself doesn't include the pkgs variable, so you'll need to modify its content to pass pkgs as a parameter to hello.nix.
2. In places where you need to use this Derivation, you can use import ./hello.nix pkgs to import hello.nix and use pkgs as a parameter to execute the function defined within.

Let's continue to verify this using nix repl, and you'll see that the result is still a Derivation:

› cat hello.nix          
pkgs:
  pkgs.writeShellScriptBin "hello" '' echo "hello, xxx!" ''

› nix repl -f '<nixpkgs>'
Welcome to Nix 2.13.5. Type :? for help.

warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels' does not exist, ignoring
Loading installable ''...
Added 19203 variables.

nix-repl> import ./hello.nix pkgs
«derivation /nix/store/zhgar12vfhbajbchj36vbbl3mg6762s8-hello.drv»

2. Using pkgs.callPackage

In the previous example without pkgs.callPackage, we directly passed pkgs as a parameter to hello.nix. However, this approach has some drawbacks:

1. All other dependencies of the hello Derivation are tightly coupled with pkgs.
   1. If we need custom dependencies, we have to modify either pkgs or the content of hello.nix, which can be cumbersome.
2. In cases where hello.nix becomes complex, it's challenging to determine which Derivations from pkgs it relies on, making it difficult to analyze the dependencies between Derivations.

pkgs.callPackage, as a tool for parameterizing the construction of Derivations, addresses these issues. Let's take a look at its source code and comments nixpkgs/lib/customisation.nix#L101-L121:

  /* Call the package function in the file `fn` with the required
    arguments automatically.  The function is called with the
    arguments `args`, but any missing arguments are obtained from
    `autoArgs`.  This function is intended to be partially
    parameterised, e.g.,

      callPackage = callPackageWith pkgs;
      pkgs = {
        libfoo = callPackage ./foo.nix { };
        libbar = callPackage ./bar.nix { };
      };

    If the `libbar` function expects an argument named `libfoo`, it is
    automatically passed as an argument.  Overrides or missing
    arguments can be supplied in `args`, e.g.

      libbar = callPackage ./bar.nix {
        libfoo = null;
        enableX11 = true;
      };
  */
  callPackageWith = autoArgs: fn: args:
    let
      f = if lib.isFunction fn then fn else import fn;
      fargs = lib.functionArgs f;

      # All arguments that will be passed to the function
      # This includes automatic ones and ones passed explicitly
      allArgs = builtins.intersectAttrs fargs autoArgs // args;    

    # ......

In essence, pkgs.callPackage is used as pkgs.callPackage fn args, where the place holder fn is a Nix file or function, and args is an attribute set. Here's how it works:

1. pkgs.callPackge fn args first checks if fn is a function or a file. If it's a file, it imports the function defined within.
   1. After this step, you have a function, typically with parameters like lib, stdenv, fetchurl, and possibly some custom parameters.
2. Next, pkgs.callPackge fn args merges args with the pkgs attribute set. If there are conflicts, the parameters in args will override those in pkgs.
3. Then, pkgs.callPackge fn args extracts the parameters of the fn function from the merged attribute set and uses them to execute the function.
4. The result of the function execution is a Derivation, which is a Nix package.

What can a Nix file or function, used as an argument to pkgs.callPackge, look like? You can examine examples we've used before in Nixpkgs's Advanced Usage - Introduction: hello.nix, fcitx5-rime.nix, vscode/with-extensions.nix, and firefox/common.nix. All of them can be imported using pkgs.callPackage.

For instance, if you've defined a custom NixOS kernel configuration in kernel.nix and made the development branch name and kernel source code configurable:

{
  lib,
  stdenv,
  linuxManualConfig,

  src,
  boardName,
  ...
}:
(linuxManualConfig {
  version = "5.10.113-thead-1520";
  modDirVersion = "5.10.113";

  inherit src lib stdenv;

  # file path to the generated kernel config file(the `.config` generated by make menuconfig)
  #
  # here is a special usage to generate a file path from a string
  configfile = ./. + "${boardName}_config";

  allowImportFromDerivation = true;
})

You can use pkgs.callPackage ./hello.nix {} in any Nix module to import and use it, replacing any of its parameters as needed:

{ lib, pkgs, pkgsKernel, kernel-src, ... }:

{
  # ......

  boot = {
    # ......
    kernelPackages = pkgs.linuxPackagesFor (pkgs.callPackage ./pkgs/kernel {
        src = kernel-src;  # kernel source is passed as a `specialArgs` and injected into this module.
        boardName = "licheepi4a";  # the board name, used to generate the kernel config file path.
    });

  # ......
}

As shown above, by using pkgs.callPackage, you can pass different src and boardName to the function defined in kernel.nix, to generate different kernel packages. This allows you to adapt the same kernel.nix to different kernel source code and development boards.

The advantages of pkgs.callPackage are:

1. Derivation definitions are parameterized, and all dependencies of the Derivation are the function parameters in its definition. This makes it easy to analyze dependencies between Derivations.
2. All dependencies and other custom parameters of the Derivation can be easily replaced by using the second parameter of pkgs.callPackage, greatly enhancing Derivation reusability.
3. While achieving the above two functionalities, it does not increase code complexity, as all dependencies in pkgs can be automatically injected.

So it's always recommended to use pkgs.callPackage to define Derivations.

References

• Chapter 13. Callpackage Design Pattern - Nix Pills
• callPackage, a tool for the lazy - The Summer of Nix
• Document what callPackage does and its preconditions - Nixpkgs Issues