In the previous pill, we underlined the fact that the inputs pattern is great to decouple packages from the repository, in that we can pass manually the inputs to the derivation. The derivation declares its inputs, and the caller passes the arguments.

However as with usual programming languages, we declare parameter names, and then we have to pass arguments. We do the job twice. With package management, we often see common patterns. In the case of nixpkgs it's the following.

Some package derivation:

{ input1, input2, ... }:
...

Repository derivation:

rec {
  lib1 = import package1.nix { inherit input1 input2 ...; };
  program2 = import package1.nix { inherit inputX inputY lib1 ...; };
}

Where inputs may even be packages in the repository itself (note the rec keyword). The pattern here is clear, often inputs have the same name of the attributes in the repository itself. Our desire is to pass those inputs from the repository automatically, and in case be able to specify a particular argument (that is, override the automatically passed default argument).

To achieve this, we will define a callPackage function with the following synopsis:

{
  lib1 = callPackage package1.nix { };
  program2 = callPackage package2.nix { someoverride = overriddenDerivation; };
}

What should it do?

First of all, we need a way to introspect (reflection or whatever) at runtime the argument names of a function. That's because we want to automatically pass such arguments.

Then callPackage requires access to the whole packages set, because it needs to find the packages to pass automatically.

We start off simple with :

nix-repl> add = { a ? 3, b }: a+b
nix-repl> builtins.functionArgs add
{ a = true; b = false; }

Nix provides a builtin function to introspect the names of the arguments of a function. In addition, for each argument, it tells whether the argument has a default value or not. We don't really care about default values in our case. We are only interested in the argument names.

Now we need a set with all the values, let's call it values. And a way to intersect the attributes of values with the function arguments:

nix-repl> values = { a = 3; b = 5; c = 10; }
nix-repl> builtins.intersectAttrs values (builtins.functionArgs add)
{ a = true; b = false; }
nix-repl> builtins.intersectAttrs (builtins.functionArgs add) values
{ a = 3; b = 5; }

Perfect, note from the example above that the intersectAttrs returns a set whose names are the intersection, and the attribute values are taken from the second set.

We're done, we have a way to get argument names from a function, and match with an existing set of attributes. This is our simple implementation of callPackage:

nix-repl> callPackage = set: f: f (builtins.intersectAttrs (builtins.functionArgs f) set)
nix-repl> callPackage values add
8
nix-repl> with values; add { inherit a b; }
8

Clearing up the syntax:

In the code above, I've also shown that the callPackage call is equivalent to directly calling add a b.

We achieved what we wanted. Automatically call functions given a set of possible arguments. If an argument is not found in the set, that's nothing special. It's a function call with a missing parameter, and that's an error (unless the function has varargs ... as explained in the 5th pill).

Or not. We missed something. Being able to override some of the parameters. We may not want to always call functions with values taken from the big set. Then we add a further parameter, which takes a set of overrides:

nix-repl> callPackage = set: f: overrides: f ((builtins.intersectAttrs (builtins.functionArgs f) set) // overrides)
nix-repl> callPackage values add { }
8
nix-repl> callPackage values add { b = 12; }
15

Apart from the increasing number of parenthesis, it should be clear that we simply do a set union between the default arguments, and the overriding set.

Given our brand new tool, we can simplify the repository expression (default.nix).

Let me write it down first:

let
  nixpkgs = import <nixpkgs> {};
  allPkgs = nixpkgs // pkgs;
  callPackage = path: overrides:
    let f = import path;
    in f ((builtins.intersectAttrs (builtins.functionArgs f) allPkgs) // overrides);
  pkgs = with nixpkgs; {
    mkDerivation = import ./autotools.nix nixpkgs;
    hello = callPackage ./hello.nix { };
    graphviz = callPackage ./graphviz.nix { };
    graphvizCore = callPackage ./graphviz.nix { gdSupport = false; };
  };
in pkgs

Wow, there's a lot to say here:

Note how easy is to override arguments in the case of graphviz without gd. But most importantly, how easy it was to merge two repositories: nixpkgs and our pkgs!

The reader should notice a magic thing happening. We're defining pkgs in terms of callPackage, and callPackage in terms of pkgs. That magic is possible thanks to lazy evaluation.

The "callPackage" pattern has simplified a lot our repository. We're able to import packages that require some named arguments and call them automatically, given the set of all packages.

We've also introduced some useful builtin functions that allows us to introspect Nix functions and manipulate attributes. These builtin functions are not usually used when packaging software, rather to provide tools for packaging. That's why they are not documented in the nix manual.

Writing a repository in nix is an evolution of writing convenient functions for combining the packages. This demonstrates even more how nix is a generic tool to build and deploy something, and how suitable it is to create software repositories with your own conventions.

...we will talk about the "override" design pattern. The graphvizCore seems straightforward. It starts from graphviz.nix and builds it without gd. Now I want to give you another point of view: what if we instead wanted to start from pkgs.graphviz and disable gd?