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.
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.
The procedure enum-set-value is just the identity function, so it is not provided.
(make-enum-type symbol-list)
Returns a newly allocated enumeration type suitable for constructing enumeration sets whose members are the symbols in symbol-list. These symbols are said to be in the enumeration type. In R6RS the function of this procedure is provided as part of make-enumeration.
(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.