swipl-nix - manage swipl versions using Nix

SWI-Prolog (swipl for short) is a prolog environment that has had a lot of versions over the years. It is useful to be able to quickly fetch and run one such version, for example for testing code against different versions.

This project allows you to do so using Nix. It also allows Nix code to depend on specific versions of swipl easily.

Basic usage

Commands below assume a Nix install with flakes enabled. Instructions for this can be found on the NixOS wiki.

None of these commands require that you clone this repository. In fact, there should never be a need to do so manually, unless you want to contribute to the way things are packaged here.

Run the latest stable

nix run github:matko/swipl-nix

or more explicitely,

nix run github:matko/swipl-nix#latest

Run the latest devel

nix run github:matko/swipl-nix#latest-devel

or

nix run github:matko/swipl-nix#devel

Run a specific version

nix run github:matko/swipl-nix#9_2_3

nix run github:matko/swipl-nix#9_2

nix run github:matko/swipl-nix#9

nix run github:matko/swipl-nix#9-devel

Versions can be specified either completely (9_2_3 above) or incompletely, in which case the most recent version with that prefix is used.

When only a major version is specified, the version is completed to the most recent stable version with that prefix. If the latest devel version is required instead, add -devel at the end.

Alternative: swivm

swipl-nix requires you to first install Nix, after which Nix does most of the heavy lifting for version management. If you are not already a Nix/NixOS user and your goal is just to get a specific swipl version installed as soon as possible, swivm is probably the quicker and easier option for you.

So why Nix?

As a non-Nix user you might be wondering why you'd use this project at all. The real advantage comes into play when you need to build code that depends on a specific version of swipl, maybe even a version that carries some custom patches. It can be a pain to ensure that this version is the one in scope for a particular project that needs it. It becomes even more of a pain when you need different versions of swipl for different projects. If you use any sort of third-party native libraries, your chances of things just working on different versions of SWI-Prolog become pretty bleak.

Nix solves this sort of problem by allowing the construction of build and runtime environments that are specific to projects. It allows you to write prolog and native code in an environment which will be the same regardless of what machine it is on.

Minimum version

Currently, only versions from 8.4.0 upwards are supported. This is the earliest version that builds cleanly with the unmodified package definition in nixpkgs.

I would like to support older versions, but this will require modifying the package definition to fix the build for older versions. Specifically, it appears that openssl's RSA_SSLV23_PADDING macro was removed in newer versions of openssl, and older swipl versions depend on it.

Structure

This project is usable as a flake or as an overlay.

Flake

As a flake, this project exposes a package set for easy running using the basic examples above, where each package name is the version number.

Additionally, this flake provides an overlay under the overlays.default property, which provides the same overlay as described below.

Overlay

This project can also be used without flakes. overlay.nix defines an overlay which can be used with nixpkgs to get a package set under swipl-nix. For example, to get version 8.5.3, you'd use pkgs.swipl-nix."8_5_3".

For more information on overlays, consult the NixOS wiki.

Project internals

For any external source (in our case, SWI-Prolog and its modules), nix requires that we know in advance what the hash will be which it will assign after doing its import. In order to provide a package set and overlay, we will likewise need to know in advance what these values are.

Nixpkgs provides a nice tool for doing this: nix-prefetch-git. This tool will download a git repo, initialize all submodules, then compute a hash out of all fetched items. This repository contains code for automating this process for all version tags in the swipl and swipl-devel repositories.

Since the swipl repository makes heavy use of submodules, and doing a fresh clone for every single version is prohibitively expensive, the automation additionally ensures that all these repositories are fetched only once.

refreshing the package set

To refresh the exposed package set, do the following:

nix develop .
./refresh.sh

If you are using direnv, the development environment will activate automatically when entering the directory, and you won't have to do nix develop . manually.

refresh.sh is a simple script which does the following:

1. check out all relevant SWI-Prolog packages in a temporary directory.
2. create tags.json with the computed hashes for each version tag
3. create alias.json with aliases for each incomplete version
4. delete the temporary directory

You are then supposed to check in the two generated json files.

nix package set generation

Both flake.nix and overlay.nix make use of mkPackages to generate a package set.

mkPackages.nix parses tags.json and alias.json to generate a package set. For the actual derivations, package.nix uses the version of swi-prolog provided by nixpkgs, but overrides the version and source appropriately.

Since the generated package set simply exposes slightly modified nixpkgs swi-prolog derivations, you can use the same overrides with these as you could with the version provided by nixpkgs. You could also use an overlay to replace swi-prolog with a specific version throughout nixpkgs, or override the version used in specific derivations.