Replace unsafeMergeVotingProcedures by mergeVotingProcedures

[smelc]

Changelog

- description: |
    Replace unsafeMergeVotingProcedures by mergeVotingProcedures, that handles incompatible votes and return an error
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - improvement    # QoL changes e.g. refactoring
  - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
  # - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

• This is the cardano-api part of Throw an error on identical action-ids in votes of one voter cardano-cli#681
• The return type for the error case is a bit different from the version in Throw an error on identical action-ids in votes of one voter cardano-cli#681. This version makes more sense for general-purpose callers and we can adapt the CLI's caller when we'll update.

How to trust this PR

See IntersectMBO/cardano-cli#681

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• Self-reviewed the diff

[Jimbo4350]

Why not use differenceWith since all we care about are conflicts? Then we can just do a union on the maps if there are none.

[smelc]

@Jimbo4350> but we need to report the conflicts. How would we do that with differenceWith, that is not within an error monad/Either-return type?

[Jimbo4350]

Here: ae88ae4

I think we should also property test this function.

[smelc]

I think we should also property test this function.

If you're fine with me spending time doing that, yes, 💯 This function has nice properties indeed.

Regarding ae88ae4, sorry I disagree: my version doesn't separate doing the job from checking conflicts, it's doing both at the same time; which is more intuitive, at the cost of very little duplication (only the symmetry of (Just _, Nothing) causes repetition). I rather keep my version for future readers and maintainers of the code (modulo introducing the type synonym which enhances readability, that is neat).

[smelc]

I added a newtype wrapping the error case and tracked adding properties tests in #499, so I'm enqueueing for merge.

[newhoggy]

differenceWith is only able to compare the keys.

If we have to GovActionIds that are equal according to Eq, is that considered a conflict?

[smelc]

@newhoggy> indeed we only compare the keys (GovActionId). So, if there are two equal GovActionId that map to the same VotingProcedures, a conflict will be reported; whereas it's not really a conflict, it's more of a duplication.