8.6 Composing overlays

Why would anyone pass a list of 15 overlays to nixpkgs? Because overlays compose. This short lesson shows how separate overlays for production packages and debug instrumentation combine into a testing overlay, and how a whole list of overlays collapses into one that behaves like a single final: prev: function.

In this video

• One overlay for your product packages, a second that patches them with debug symbols and extra logging
• A testing overlay that is nothing but the two combined, so integration tests run against the instrumented build
• Swapping the plain overlay against the combined one to find out whether your debug changes break the tests
• Merging many overlays into one function, so consumers never see how many are behind it
• Where to read on: lib/fixed-points.nix in the nixpkgs repository

After this lesson

You can package customizations as exchangeable building blocks, combine them per environment, and hand users a single overlay that hides the composition.

Builds on: Overlay basics (8.2)