THIS IS NOT A PROPOSAL. It's just a dumping ground for some stuff I don't want to lose track of. There will be a proper proposal later.
Based on unique objects, not symbols.
Each enum object has four properties: enum type, name (a symbol), ordinal (a non-negative exact integer), value (anything). Enum objects are unique across enum types. Names and ordinals are unique within an enum type. Accessors are enum-type, enum-name, enum-ordinal, enum-value.
An enum type contains a map from names to enum objects and a vector indexed by the ordinal. We need enum-ref (uses the vector) and enum-name->enum (uses the map) as fundamental procedures on enum types. Simple composition gives us enum-name->ordinal, enum-ordinal->name, enum-ordinal->value, and enum->name->value. There is no way to work backward from an enum value because it need not be unique.
Possibly kill enum values in favor of just using enum maps? A single value is very handy for C definitions, etc.
Make-enumeration-type accepts either names or (name value) lists. Ordinals are assigned in order. Define-enumeration-type binds an identifier to the enumeration type and an identifier per enum (same as the name) to each enum.
From SetsCowan, not yet consistent with the above.
Except as noted below, the procedures for creating and manipulating enumeration sets are the same as those for sets, except that set is replaced by enum-set in their names. Wherever a newly allocated enumeration set is returned, it has the same enumeration type as the source sets. It is an error to operate on enumeration sets of different types.
This design is founded on R6RS enumerations, but with the addition of reified enumeration types along the lines suggested by R6RS Formal Comment #262. The prefix enum is used in all cases instead of using both enum and enumeration as R6RS does.
(enum-type-symbols enum-type)
Return a newly allocated list of the symbols in enum-type in the original order.
(enum-type-index enum-type symbol)
Return an exact integer corresponding to the position of symbol in the original list that created enum-type, or #f if it was not one of those symbols. The R6RS equivalent is the procedure returned by enum-type-indexer when applied to an enum-set belonging to enum-type.
(make-enum-set enum-type)
Returns a newly allocated enumeration set. The possible elements of the set are the symbols in enum-type. The set is empty. The approximate R6RS equivalents are enum-set-constructor and make-enumeration.
(make-universal-enum-set enum-type)
Returns a newly allocated enumeration set. The possible elements of the set are the symbols in enum-type. The set contains all possible elements. The approximate R6RS equivalent is enum-set-universe.
(enum-set enum-type element ...)
Returns a newly allocated enumeration set. The possible elements of the set are the symbols in enum-type. The set is initialized to contain the elements. There is no R6RS equivalent.
(list->enum-set enum-type list)
Returns a newly allocated enumeration set. The possible elements of the set are the symbols in enum-type. The set is initialized to contain the elements of list. There is no R6RS equivalent.
(enum-set-complement enum-set)
Returns a newly allocated enumeration set that is the complement of enum-set. This procedure is also in R6RS.
(enum-set-projection enum-set enum-type)
Returns a newly allocated enumeration set of type enum-type. Its elements are the symbols belonging to enum-set, ignoring any symbols which are not in enum-type. This procedure is also in R6RS, but uses a second enum-set in place of enum-type.
Based on vectors indexed by the enum's ordinal. API is reduced form of HashTablesCowan.