Overview

Using profunctor optics to focus modifications in Nix.

Motivation

Using nixpkgs often involves overriding deeply nested derivations. This often looks something like the following:

let

myoverlay = self: super:
  {
    foo = super.foo // {
      bar = super.foo.bar // {
        baz = super.foo.bar.baz.overrideAttrs
          { /* ... */ };
      };
    };
  };
  
in
{ /* ... */ }

We can avoid this ugly telescoping by writing a function to modify the innermost bit we actually care about, and simply focusing it past all the irrelevant layers of nesting. Profunctor optics are perfectly suited to this purpose.

With an appropriate lens, the code above can be refactored to:

let

modification = super:
  super.overrideAttrs { /* ... */ };

_foobarbaz = with optics fn; path [ "foo" "bar" "baz" ];

myoverlay = self: _foobarbaz modification;

in
{ /* ... */ }

Example

The default.nix file in this repo provides an illustration of the problem and solution. The ghc' variable contains an overlay for nixpkgs that modifies a deeply nested part of it. The ghc variable contains a lensy refactoring of the same overlay.