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Impure Incremental Builds

This example demonstrates how to use RepX in an "impure" mode to facilitate rapid development cycles (incremental compilation) by bypassing Nix's strict sandboxing.

Location: examples/impure-incremental

The Problem​

Standard Nix builds are hermetic: source code is copied into the Nix store, and builds happen in a sandbox. This guarantees reproducibility but kills the edit-compile-test cycle for languages like C++ or Rust because make/cargo cannot reuse artifacts from previous builds. Every change triggers a full rebuild.

The Solution​

We can define a derivation that:

  1. Disables the sandbox (__noChroot = true).
  2. Reads the current working directory (PWD) to access source files directly.
  3. Runs the build tool (e.g., make) in place, allowing it to use existing .o files.

Implementation​

flake.nix​

We define two packages: labPure (for production) and labImpure (for development).

labImpure = (import ./nix/lab.nix) {
  pkgs = pkgsImpure; # Uses the impure overlay
  inherit repx-lib;
};

Impure Overlay (nix/pkgs/impure-make-pkg.nix)​

The impure package definition accesses the host filesystem.

{ stdenv }:
let
  # Get the absolute path to the 'src' directory on the host
  localDevPath = builtins.getEnv "PWD" + "/src";
in
stdenv.mkDerivation {
  name = "make-pkg";
  
  # Disable the sandbox
  __noChroot = true;
  
  # Ensure the derivation hash changes on every invocation 
  # (otherwise Nix will cache the result and not rebuild)
  version = builtins.toString builtins.currentTime;

  buildPhase = ''
    echo "Building in ${localDevPath}"
    cd "${localDevPath}"
    
    # Run make directly in the source directory
    make
  '';

  installPhase = ''
    mkdir -p $out/bin
    cp "${localDevPath}/mybinary" $out/bin/make-pkg
  '';
}

Running the Example​

To run this example, you must use the --impure flag with Nix.

# Enter the directory
cd examples/impure-incremental

# Run the impure lab app
nix run --impure .

Or build manually:

nix build .#lab-impure --impure
repx run build --lab ./result

Warning​

warning

Impure builds are NOT reproducible. They depend on the state of your local filesystem, environment variables, and unversioned file changes.

Use this pattern strictly for local development loops. When you are ready to publish or run experiments, switch back to the pure pipeline to ensure your results can be verified by others.