User confusion when updating after system compiler upgrade.

[amirmc]

Not sure if this is the correct repo to post this on. Just some feedback :)

I just updated to 4.03 via homebrew and then tried an opam update.
Normally my experience with this has been pretty straightforward but today I got the following (comments follow).

$ opam update
Your system compiler has been changed. Do you want to upgrade your OPAM installation ? [Y/n] y

=-=- Upgrading core -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=  🐫
[WARNING] bin_prot.112.35.00 is not available because your system doesn't comply with ocaml-version >= "4.02.1" & ocaml-version < "4.03". Skipping.
[WARNING] camlp4.4.02+system is not available because your system doesn't comply with preinstalled & ocaml-version >= "4.02" & ocaml-version < "4.03".
          Skipping.
[WARNING] core.112.35.01 is not available because your system doesn't comply with ocaml-version >= "4.02.1" & ocaml-version < "4.03". Skipping.
[WARNING] core_kernel.112.35.00 is not available because your system doesn't comply with ocaml-version >= "4.02.1" & ocaml-version < "4.03". Skipping.
[WARNING] enumerate.111.08.00 is not available because your system doesn't comply with ocaml-version >= "4.00.1" & ocaml-version < "4.03". Skipping.
[WARNING] fieldslib.109.20.03 is not available because your system doesn't comply with ocaml-version >= "4.00.0" & ocaml-version < "4.03". Skipping.
[WARNING] herelib.112.35.00 is not available because your system doesn't comply with ocaml-version >= "4.02.1" & ocaml-version < "4.03". Skipping.
[WARNING] pa_ounit.112.35.00 is not available because your system doesn't comply with ocaml-version >= "4.02.1" & ocaml-version < "4.03". Skipping.
[WARNING] pa_structural_sexp.112.35.00 is not available because your system doesn't comply with ocaml-version >= "4.02.1" & ocaml-version < "4.03". Skipping.
[WARNING] pa_test.112.24.00 is not available because your system doesn't comply with ocaml-version >= "4.02.1" & ocaml-version < "4.03". Skipping.
[WARNING] sexplib.112.35.00 is not available because your system doesn't comply with ocaml-version >= "4.02.1" & ocaml-version < "4.03". Skipping.
[WARNING] type_conv.112.01.02 is not available because your system doesn't comply with ocaml-version >= "4.02.0" & ocaml-version < "4.03". Skipping.
[WARNING] typerep.112.35.00 is not available because your system doesn't comply with ocaml-version >= "4.02.1" & ocaml-version < "4.03". Skipping.
[WARNING] variantslib.109.15.03 is not available because your system doesn't comply with ocaml-version >= "4.00.0" & ocaml-version < "4.03". Skipping.
The following dependencies couldn't be met:
  - comparelib -> camlp4
Your request can't be satisfied:
  - camlp4 is not available because your system doesn't comply with ocaml-version >= "4.04".

No solution found, exiting
The former package state can be restored with opam switch import "/Users/amir/.opam/backup/state-20160413122338.export" --switch core

Some notes and thoughts:

• I get why the above happened (packages that don't support 4.03).
• Most of my switches are aliases of the system switch. I have about 7 of them.
• It wasn't immediately obvious which of those switches had the issue above.
• Even though it failed on the core switch, it didn't do anything with the other switches. Printing a note about how far it got and what wasn't done might be nice (similarly to what happens with packages).
• I didn't bother trying to restore the packages and now the switch is empty. The base packages are gone.

All of the above is easy for me to figure out but I thought I'd leave some comments here. If there's nothing to discuss, then I'll close the issue in a couple of days.

[AltGr]

IIRC, the behaviour for opam 1.2.2 is to check for a new compiler on any command (i.e. this is not specific to the update you specified), and for the current switch. The other switches may trigger a similar reinstall once you switch to them.

Then, what is triggered is an equivalent of opam switch reinstall, itself a wrapper around opam switch import, which actually triggered the errors above.

Now, in 2.0~alpha, with the removed comp files (and associated notion of a system compiler), most of this has been changed. It remains a difficult case to handle, though:

• packages now have a mechanism to indicate a dependency on the hash of system files, and will be filed for reinstall if that changes. The "system compiler package" polls the system and registers one such hook.
• switch import could still be improved to better handle partially impossible situations (Best effort opam switch import #2450)
• 2.0~alpha still relies on a ocaml-version variable and the available field; having actual dependencies on a given ocaml version would give a smoother experience in this case, but is not that easy. (See discussion at Next: ocaml-version not updated with ocaml's version #2537)