Prevent property duplication within a ruleset.

[nsaunders]

Description

#6 resolved an issue with declaration ordering where the rendered order would not usually correspond to input order. This was achieved by using a writer instead of a record to collect declarations. A side effect of moving away from the record model is that a property can now appear more than once within a ruleset; but this should probably (continue to) yield a type error, hence this PR.

Design considerations

The discard function associated with the Rule.do syntax currently builds a tuple-list of properties as they are added to a ruleset. This is useful for ensuring that:

• @font-face rules contain the minimum required font-family and src descriptors; and
• all properties appearing within a @keyframe block are animatable.

The general idea was to use the same list of properties to prevent duplicates, but I couldn't think of a way to check for the absence of a property. On the other hand, the Prim.Row.Lacks type class does exactly this, given a row type of course. Therefore, the majority of the changes here involve migrating the list of properties from a tuple-list structure to a row type.

Future plans

N/A

References

N/A

Code change checklist

• Any new or updated functionality includes corresponding unit test coverage.
• I have verified code formatting, run the unit tests, and checked for any changes in the examples.
• I have added an entry to the Unreleased section of the CHANGELOG.