dev meeting 20230809

Present: @rgrinberg @kit-ty-kate @Leonidas-from-XIV

available field

• There is a PR for it, but do we want it?
• Do we want full support for all OPAM fields in the generator?
  • Dune can't do anything with available
  • So it can't do anything than just pass it on in the generator
  • People already use Dune as OPAM file generator
  • available in OPAM is used as a filter before being fed into the solver
• If Dune can't use available, why does it support maintainers?
  • Can be used in dune subst
  • But the explanation is admittedly a bit post-hoc
• How else to solve it?
  • Traverse source tree and collect enabled_if
  • But what if people forget to add package to their stanzas?
  • What if we went the other way and set enabled_if from available
  • Do we have the same variables
    • no, but we could start with a subset
    • we most likely won't have support for user variables
  • It's gonna be tough
    • The filter languages of OPAM and Dune don't map very well
    • The enabled_if filter would need to be extended
• Users expect Dune to write OPAM files
• It doesn't make sense to support every opam field
  • What goes into dune-project vs dune-workspace
  • What about other fields like uninstall
    • Packages can use the field to remove config files
    • How is this possible with Dune, given it generates an .install file
      • A binary in the package could generate the config file, then the uninstall would remove it. So it is not Dune/OPAM creating it.
• Why are OPAM files generated at all?
  • No good answer, but it is nice that there is a common useful subset for OPAM and Dune
  • The idea was that OPAM would be able to read the dune-project but so far this hasn't happened
  • Nix doesn't use the dune data but OPAM data
  • In the future it could use the info from the lock dir
  • At least this way users only need to know Dune sexp syntax and not OPAM syntax on top of it
• Would available/enabled_if support be a helpful addition to opam-repository maintainers?
  • Kate says it would be because some packages are missing available and maintainers need to add it by hand.
• Conclusions
  • We agree that it is useful to have the metadata in dune-project checked within the build to verify that it somehow makes sense. A bad example is depends which is there but can also be completely wrong.