Recall the override design pattern from the nix pill 14. Instead of calling a function with parameters directly, we make the call (function + parameters) overridable.

We put the override function in the returned attribute set of the original function call.

Take for example graphviz. It has an input parameter xlibs. If it's null, then graphviz will build without X support.

$ nix-repl
nix-repl> :l <nixpkgs>
Added 4360 variables.
nix-repl> :b graphviz.override { xlibs = null; }

This will build graphviz without X support, it's as simple as that.

However let's say a package P depends on graphviz, how do we make P depend on the new graphviz without X support?

...you could do something like this:

pkgs = import <nixpkgs> {};
pkgs.graphviz = pkgs.graphviz.override { xlibs = null; };
build(pkgs.P)

Given pkgs.P depends on pkgs.graphviz, it's easy to build P with the replaced graphviz. On a pure functional language it's not that easy because you can assign to variables only once.

The fixed point with lazy evaluation is crippling but about necessary in a language like Nix. It lets us achieve something similar to what we'd do imperatively.

Follows the definition of fixed point in nixpkgs:

# Take a function and evaluate it with its own returned value.
fix = f: let result = f result; in result;

It's a function that accepts a function f, calls f result on the result just returned by f result and returns it. In other words it's f(f(f(....

At first sight, it's an infinite loop. With lazy evaluation it isn't, because the call is done only when needed.

nix-repl> fix = f: let result = f result; in result
nix-repl> pkgs = self: { a = 3; b = 4; c = self.a+self.b; }
nix-repl> fix pkgs
{ a = 3; b = 4; c = 7; }

Without the rec keyword, we were able to refer to a and b of the same set.

The trick is that c is not needed to be evaluated in the inner call, thus it doesn't go in an infinite loop.

Won't go further with the explanation here. A good post about fixed point and Nix can be found here.

nix-repl> overrides = { a = 1; b = 2; }
nix-repl> let newpkgs = pkgs (newpkgs // overrides); in newpkgs
{ a = 3; b = 4; c = 3; }
nix-repl> let newpkgs = pkgs (newpkgs // overrides); in newpkgs // overrides
{ a = 1; b = 2; c = 3; }

We've seen how to override attributes in a set such that they get recursively picked by dependant attributes. This approach can be used for derivations too, after all nixpkgs is a giant set of attributes that depend on each other.

To do this, nixpkgs offers config.packageOverrides. So nixpkgs returns a fixed point of the package set, and packageOverrides is used to inject the overrides.

Create a config.nix file like this somewhere:

{
  packageOverrides = pkgs: {
    graphviz = pkgs.graphviz.override { xlibs = null; };
  };
}

Now we can build e.g. asciidocFull and it will automatically use the overridden graphviz:

nix-repl> pkgs = import <nixpkgs> { config = import ./config.nix; }
nix-repl> :b pkgs.asciidocFull

Note how we pass the config with packageOverrides when importing nixpkgs. Then pkgs.asciidocFull is a derivation that has graphviz input (pkgs.asciidoc is the lighter version and doesn't use graphviz at all).

Since there's no version of asciidoc with graphviz without X support in the binary cache, Nix will recompile the needed stuff for you.

In the previous pill we already talked about this file. The above config.nix that we just wrote could be the content of ~/.nixpkgs/config.nix.

Instead of passing it explicitly whenever we import nixpkgs, it will be automatically imported by nixpkgs.

We've learned about a new design pattern: using fixed point for overriding packages in a package set.

Whereas in an imperative setting, like with other package managers, a library is installed replacing the old version and applications will use it, in Nix it's not that straight and simple. But it's more precise.

Nix applications will depend on specific versions of libraries, hence the reason why we have to recompile asciidoc to use the new graphviz library.

The newly built asciidoc will depend on the new graphviz, and old asciidoc will keep using the old graphviz undisturbed.

...we will stop diving nixpkgs for a moment and talk about store paths. How does Nix compute the path in the store where to place the result of builds? How to add files to the store for which we have an integrity hash?