RFC003: Remove groups from product types
Status: Approved

The grammar production member_group seems to provide little value, any example using it seems better off with another structure. While writing the primer it became clear that no good example could be described that motivated the group production in a definitive way.

Document Status History

The current implementation allows for a grouping of fields in a structure such that annotations are attached to the group not to individual members.

StructureDef
    ::= "structure" Identifier StructuredBody?

StructuredBody
    ::= "is" Annotation* StructuredBodyInner "end"

StructuredBodyInner
    ::= ( MemberGroup | Member )+

MemberGroup
    ::= "group" Annotation* Member+ "end"

For example, this is an excerpt from the file examples/simple.sdm which grouped three fields in the invoice and labeled the group as rates applied. However, it’s not clear what we do with this when we transform the model; does the group represent a type? do attributes get copied to each field? is the group (and annotations) simply get discarded?

module invoice is
  entity Invoice is
    identity id -> integer
    customer -> customers:Customer
    vehicle -> vehicles:Vehicle
    rental_dates -> DateRange
    mileage -> OdometerRange

    group
      @dc:description = "Rates applied to the rental."@en
      day_rate -> finance:CurrencyRate
      mile_rate -> finance:CurrencyRate
      discount -> finance:DiscountPercentage
    end

  end
end

It is also hard to come up with examples for the documentation where a group of this kind is preferred to a new structure, as in the following. This would suggest that the group notion is not required as a core component of the language.

module invoice is
  entity Invoice is
    identity id -> integer
    customer -> customers:Customer
    vehicle -> vehicles:Vehicle
    rental_dates -> DateRange
    mileage -> OdometerRange

    rates -> RentalRates
  end

  structure RentalRates is
    @dc:description = "Rates applied to the rental."@en
    day_rate -> finance:CurrencyRate
    mile_rate -> finance:CurrencyRate
    discount -> finance:DiscountPercentage
  end
  
end

In the case where the grouping of fields was not intended to imply a nested structure it would be possible to add an annotation property that would affect code generation to flatten a structure into its parent.

module invoice is
  entity Invoice is
    identity id -> integer
    // ...

    rates -> RentalRates is
      @generator:flattened = true
    end
  end  
end

The following changes are required:

1. Remove the production member_group from the grammar.
2. Remove the production _structured_body_inner from the grammar.
3. Update structure_body and entity_body to include repeat1(member) and repeat(member) respectively.
4. Update queries for highlights, indents, and folds to remove member_group references.
5. Remove test case entity_with_groups.sdm.
6. Update test case entity_with_unknowns.sdm to remove group.
7. Update example simple.sdm to remove group.