implementation plan for delete directives

[aryairani]

For planning/estimation purposes, a discussion of how to implement scratch file delete/delete.term/delete.type directives

[mitchellwrosen]

Here is a rough plan:

• Add delete directives field to UnisonFile and TypecheckedUnisonFile data structures
  • data UnisonFile v a = UnisonFileId { ..., deleteTerms :: Set v, deleteTypes :: Set v, ... }
  • data TypecheckedUnisonFile v a = TypecheckedUnisonFileId { ..., deleteTerms :: Set v, deleteTypes :: Set v, ... }
• Parse delete directives in Unison.Syntax.FileParser.file
• Decide exactly how much work we want to do at "slurp" time, with respect to informing the user whether their update will succeed
  • Minimal work option: just say "ok, I'll delete these things" (even if they have dependents in either the namespace or the file itself, either of which would prevent deletion)
• Add delete directives field to SlurpResult
  • data SlurpResult = SlurpResult { ... deleteTerms :: Set Symbol, deleteTypes :: Set Symbol }
• Modify Typechecked output message to include info about deletes
• Process delete directives in update handler
  • Deleting a definition that doesn't exist in the codebase nor in the file
    • Ok, do nothing (no-op)
  • Deleting a definition that doesn't exist in the codebase but does exist in the file
    • Nonsensical file, complain and abort (can do this check at slurp time too)
      • Option: complain but don't abort (letting delete or update take precedence; probably delete)
  • Deleting a definition that does exist in the codebase
    • Proceed with update as normal, but find dependents of the deletes, too
    • Pretty-print in an environment that does include the requested deletes, but not for the only name of something (so it never gets printed as a hash, but will otherwise never use the deleted name if there are more aliases)
    • Parse in an environment that does include the requested deletes (so we get out-of-scope variable errors for dependents)

If I had to ballpark it, the above might take about 3 weeks, to account for all of that work, plus the unknown unknowns. Some of the other tickets (#5149, #5150, perhaps #5151) isn't exactly related to getting delete directives into the codebase, but would make them much more usable.