Today we are releasing a new feature we’ve been working on the last couple of weeks.

# Generating Nix expressions

As a developer you often bump a dependency version or add a new dependency.

Every time your package files change, you need to regenerate the Nix expressions that describe how the project is built.

There are two ways to regenerate Nix expressions in that case:

1. Outside the Nix domain, possibly with an automated script and commands like bundix, cabal2nix, yarn2nix. This quickly grows from a nuisance to a maintenance headache as your git repository grows in size due to generated artifacts. It requires special care when diffing, merging, etc.

2. Let Nix generate Nix expressions during the build. Sounds simple, but it’s quite subtle.

As of today Hercules natively supports option (2), let’s dig into the subtleties.

# Evaluation and realization

The Nix language describes how software is built, which happens in two phases.

The first phase is called evaluation:

Evaluation takes a Nix expression and results into a dependency tree of derivations.

A derivation is a set of instructions how to build software.

The second phase is called realization:

Realizing a derivation is the process of building. The builder is usually a shell script, although any executable can be specified.

Since a derivation describes all the necessary inputs, the result is guaranteed to be deterministic.

# Derivations

This begs the question, why have intermediate representation (derivations)? There are a couple of reasons:

• Evaluation can include significant computation. It can range from a couple of seconds, to typically minutes, or even an hour for huge projects. We want to evaluate only once and then distribute derivations to multiple machines for speedup and realize them as we traverse the graph of dependencies.

• Evaluation can produce derivations that are built on different platforms or require some specific hardware. By copying the derivations to these machines, we don’t need to worry about running evaluation on those specific machines.

• In case of a build failure, it allows the machine to retry immediately instead of re-evaluating again.

All in all, derivation files save us computation compared to evaluating more than once.

# Interleaving evaluation and realization

Sometimes it’s worth mixing the two phases.

A build produces Nix expressions that we now would like to evaluate, but we’re already in the realization phase, so we have:

1. Evaluate to get the derivation that will output a Nix file
2. Realize that derivation
3. Continue evaluating by importing the derivation containing the Nix file
4. Realize the final derivation set

This is called Import-From-Derivation or shortly, IFD.

# A minimal example

let
pkgs = import <nixpkgs> {};
getHello = pkgs.runCommand "get-hello.nix" {} ''
# Call any command here to generate an expression. A simple example:
echo 'pkgs: pkgs.hello' > \$out
'';
in import getHello pkgs


In the last line we’re importing from getHello, which is a Nix derivation that we need to build before evaluation can continue to use pkgs: pkgs.hello Nix expression in the output.

Given a Haskell project with a Cabal file (Haskell’s package manager), drop the following default.nix into root of your repository:

let
pkgs = import (import ./nix/sources.nix).nixpkgs {};
{ index-state = "2019-08-26T00:00:00Z"; src = pkgs.lib.cleanSource ./.;};

plan-pkgs = import plan;
pkg-def-extras = [];
modules = [];
};
in pkgSet.config.hsPkgs.mypackage.components.all


Once you replace mypackage with the name from your Cabal file, your whole dependency tree is deterministic by pinning the package index to a timestamp using index-state and hash of your local folder using ./..

Haskell.nix will generate all expressions how to build each package on the fly via import from derivation.

# Native support in CI

Using different platforms (typically Linux and macOS) during IFD is one of the reasons why upstream forbids IFD, since their evaluator is running on Linux and it can’t build for macOS.

Our CI dispatches all builds during IFD back to our scheduler, so it’s able to dispatch those builds to either specific platform or specific hardware.

IFD support is seamless. There’s nothing extra to configure.

In case of build errors during evaluation UI will show you all the details including build log:

In order to use IFD support you will need to upgrade to hercules-ci-agent-0.4.0.

# Future work

Some Nix tools already embrace IFD, such as haskell.nix, yarn2nix (Node.js), pnpm2nix (Node.js) and opam2nix (OCaml).

We encourage more language tools to take advantage of this feature.

Currently Nix evaluation is single threaded and IFD evaluation is blocking until the builds are done. We have some ideas to make IFD concurrent.

We believe this is a huge step forward to simplify day-to-day Nix development.

## What we do

Automated hosted infrastructure for Nix, reliable and reproducible developer tooling, to speed up adoption and lower integration cost. We offer Continuous Integration and Binary Caches.