How it Works

flake-file integrates input definition as part of your Nix modules.

It provides a set of module options under the flake-file namespace that describe the contents of flake.nix.

When you run nix run .#write-flake, it evaluates those options and writes the file.

edit modules/*.nix     # declare flake-file options
  ↓
nix run .#write-flake  # extracts inputs and serializes
  ↓
flake.nix              # updated and locked

The Module System

Any Nix module can set flake-file.* options. Because this is the standard Nix module system, all the usual tools apply:

lib.mkDefault — set a value that can be overridden by other modules.
lib.mkForce — force a value regardless of what other modules set.
lib.mkIf — conditionally define an input.
Attribute merging — multiple files contribute to the same input set.

{ inputs, lib, ... }: {
  flake-file.inputs.nixpkgs.url = lib.mkDefault "github:NixOS/nixpkgs/nixpkgs-unstable";
  flake-file.inputs.nixpkgs-lib.follows = "nixpkgs";
}

The Option Schema

Options mirror the flake schema and extend it further:

Namespace	Purpose
`flake-file.description`	Flake description string
`flake-file.nixConfig`	Nix binary cache and substituter config
`flake-file.inputs.<name>.*`	Input declarations (url, follows, flake, ref, …)
`flake-file.outputs`	Literal Nix expression for the outputs function
`flake-file.write-hooks`	Commands run after writing
`flake-file.check-hooks`	Commands run during check
`flake-file.prune-lock.*`	Automatic lock flattening

See the Options Reference for the complete table.

Apps: write-flake and flake check

nix run .#write-flake collects inputs, serialises the merged options to valid Nix syntax, and writes flake.nix. It also runs any configured write-hooks.

nix flake check verifies that flake.nix is up to date with the current module state. If it is stale, the check fails. Add this to CI.

The outputs Function

flake-file.outputs is a literal Nix string that becomes the body of the outputs function in flake.nix. It defaults to:

inputs: import ./outputs.nix inputs

This keeps flake.nix as a pure dependency manifest. All Nix logic lives in outputs.nix. See the Outputs guide for details.

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