flake-inputs

A helper to use flakes from stable Nix.

Based on original work in

• edolstra/flake-compat
• nix-community/flake-compat
• nix-community/dream2nix

since none of those seem to be actively maintained.

This library also exposes:

• fetchTree: Polyfill for the experimental builtins.fetchTree
• getFlake: Polyfill for the experimental builtins.getFlake
• datetime-from-timestamp: A converter from Unix timestamps to Gregorian date and time, ported from Howard Hinnant's chrono-Compatible Low-Level Date Algorithms
• pad: The legendary pad function

Use cases

• Separately managing development dependencies to reduce closure size for flake consumers
• Gradually migrating away from flakes
• Offering a first-class experience for users of stable Nix

Using sources from flake.lock in default.nix

In default.nix obtain the flake-inputs library and use sources flake.lock:

# default.nix
let
  inherit (import (fetchTarball "https://github.com/fricklerhandwerk/flake-inputs/tarball/4.0"))
    import-flake
    ;
  inherit (import-flake {
    src = ./.;
  }) inputs;
in
{
  system ? builtins.currentSystem,
  config ? { },
  overlays ? [ ],
  ...
}@args:
let
  pkgs = import inputs.nixpkgs ({ inherit system config overlays; } // args);
in
{
  packages = {
    inherit (pkgs) cowsay lolcat;
  };
}

In flake.nix use attributes from default.nix for outputs:

# flake.nix
{
  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
  inputs.flake-utils.url = "github:numtide/flake-utils";

  outputs =
    { flake-utils, ... }@inputs:
    flake-utils.lib.eachDefaultSystem (
      system:
      import ./default.nix { inherit system; };
    );
}

Keep using the inputs attribute to specify sources, and nix flake update to pull the latest versions.

Importing flakes

Obtain flake-inputs and some arbitrary project that is very inconvenient to evaluate from stable Nix:

nix-shell -p npins --run "
npins init --bare
npins add github fricklerhandwerk flake-inputs --at 4.0
npins add github nixos nix --branch 2.29-maintenance
"

In default.nix import the flake with load-flake from the flake-inputs library:

# default.nix
let
  sources = import ./npins;
in
{
  flake-inputs ? sources.flake-inputs,
  nix ? sources.nix,
}:
let
  inherit (import flake-inputs) load-flake;
in
(load-flake nix).packages.${builtins.currentSystem}.default

Realise the derivation from a substituter to demonstrate that it works as intended:

nix-build --no-out-link