I’d suggest one slight tweak to the design: To maintain compatibility with people’s current way of doing business, there should be a new command line / preference file switch that defaults to false and that, when true, causes the current OCaml version to be included in the name of the server command.

> On Jun 6, 2020, at 7:50 PM, Etienne URBAH <email address hidden> wrote:
> 
> On Ubuntu 20.04, I successfully downgraded Unison (text only) to the
> version compiled with OCaml 4.02 from https://www.urs-
> mueller.ch/de/computer/download/unison-2.48.4-linux-x86_64-text-
> static.tar.gz
> 
> Using the unison-jz snap installing Unison 2.51.2 compiled against OCaml 4.02.3 as described by https://unix.stackexchange.com/questions/583058/unison-and-version-compiler-conflicts/583377#583377 requires that :
> - The --devmode option of the snap command is NOT necessary,
> - The problem of the Include line in /etc/ssh/ssh_config is solved,
> - Binaries of Unison 2.51.2 compiled against OCaml 4.02.3 are also provided for :
>  - Mac OS-X, for example at https://www.urs-mueller.ch/de/computer/index.html
>  - Windows,  for example at https://www.irif.fr/~vouillon/unison/
> 
> On the long term, I would prefer that the Unison packages explicitly
> contain the OCaml version, and the default value of servercmd should
> contain the name of the server binary compiled with the same OCaml
> version.  For example :
> 
> - unison-OCaml-4.02 version 2.48.4 should contain
>  /usr/bin/unison-OCaml-4.02 --> /usr/bin/unison-OCaml-4.02-2.48.4
> 
> - unison-OCaml-4.02 version 2.51.2 should contain
>  /usr/bin/unison-OCaml-4.02 --> /usr/bin/unison-OCaml-4.02-2.51.2
> 
> - unison-OCaml-4.06 version 2.48.4 should contain
>  /usr/bin/unison-OCaml-4.06 --> /usr/bin/unison-OCaml-4.06-2.48.4
> 
> - unison-OCaml-4.06 version 2.51.2 should contain
>  /usr/bin/unison-OCaml-4.06 --> /usr/bin/unison-OCaml-4.06-2.51.2
> 
>  This would permit, on the server, to install Unison binaries compiled
> with ALL OCaml versions.
> 
> -- 
> You received this bug notification because you are subscribed to unison
> in Ubuntu.
> https://bugs.launchpad.net/bugs/1875475
> 
> Title:
>  Unison from Ubuntu 20.04 is incompatible with Unison from Ubuntu
>  18.04, Windows or Mac OS X
> 
> Status in unison package in Ubuntu:
>  Confirmed
> 
> Bug description:
>  Unison from Ubuntu 20.04 LTS (version unison=2.48.4-4ubuntu1) works
>  well with a client in exactly the same version, with exactly the same
>  OS.
> 
>  However, it does not work well with Unison 2.48.4-1ubuntu1 from Ubuntu
>  18.04, or with Unison on other OSes.
> 
>  Once the client has determined the files that were updated, when I click on 'Go', the synchronisation fails with a message like this one: 
>> Fatal error: 
>> Uncaught exception Failure("input_value: ill-formed message")
> 
>  Sometimes there is the mention "possibly because client and server
>  have been compiled with different versions of the Ocaml compiler."
> 
>  I have seen this issues in the following configurations
>  - 2.48.4-4ubuntu1 on Ubuntu 20.04 LTS (client) and 2.48.4-1ubuntu1 on Ubuntu 18.04 LTS (server) => Fails
>  - 2.48.? on Mac OS X (client) and 2.48.4-4ubuntu1 on Ubuntu 20.04 LTS (server) => Fails
>  - 2.48.3 on Windows 7 (client) and 2.48.4-4ubuntu1 on Ubuntu 20.04 LTS (server) => Fails
> 
>  Note that the Mac OS and Windows clients were working well with unison
>  2.48.4-1ubuntu1 (Ubuntu 18.04 LTS, server).
> 
>  Maybe my problem could be solved by reverting unison to the version
>  2.48.4-1ubuntu1 from Ubuntu 18.04 LTS. Is is possible to do so with
>  `apt install` on Ubuntu 20.04? Thanks!
> 
>  ProblemType: Bug
>  DistroRelease: Ubuntu 20.04
>  Package: unison 2.48.4-4ubuntu1
>  ProcVersionSignature: Ubuntu 5.4.0-26.30-generic 5.4.30
>  Uname: Linux 5.4.0-26-generic x86_64
>  ApportVersion: 2.20.11-0ubuntu27
>  Architecture: amd64
>  CasperMD5CheckResult: skip
>  CurrentDesktop: ubuntu:GNOME
>  Date: Mon Apr 27 19:18:59 2020
>  InstallationDate: Installed on 2020-04-16 (11 days ago)
>  InstallationMedia: Ubuntu 18.04.2 LTS "Bionic Beaver" - Release amd64 (20190210)
>  SourcePackage: unison
>  UpgradeStatus: Upgraded to focal on 2020-04-21 (6 days ago)
> 
> To manage notifications about this bug go to:
> https://bugs.launchpad.net/ubuntu/+source/unison/+bug/1875475/+subscriptions
>