[R6RS] Enumerations proposal pre-draft

[Michael Sperber]

... is attached.  It's adapted from the Scheme 48 manual.  The
`define-enum-type' specified here differs slightly from Scheme 48's
`define-enumerated-type' (<name-constructor> was added, which I need
quite often).  The enum-set stuff is identical to what's in Scheme 48.
It would need a bit of fleshing out (condition types, for instance),
but the basics are there.  I also have a reference implementation
done.

-- 
Cheers =8-} Mike
Friede, Völkerverständigung und überhaupt blabla

-------------- next part --------------
Enumeration Types
=================

Enumeration types are record types for which there are a fixed number
of instances, all of which are created at the same time as the record
type itself.

The syntax for defining an enumerated type is:

(define-enum-type <tag> <type-name>
  <predicate-name>
  <vector-of-instances-name>
  <name-accessor> <name-constructor>
  <index-accessor>
  (<instance-name> ...))

This defines a new record type, bound to <type-name>, with as many
 instances as there are <instance-name>'s.
<Vector-of-instances-name> is bound to a vector containing the instances
 of the type in the same order as the <instance-name> list.
<Tag> is bound to a macro that when given an <instance-name> expands
 into an expression that returns the corresponding instance.
The name lookup is done at macro-expansion time.
<Predicate-name> is a predicate for the new type.
<Name-accessor> and <index-accessor> are accessors for the
 name and index (in <vector-of-instances>) of instances of the type.
<Name-constructor> is a procedure that accepts a symbol naming an
instance, and returns the corresponding instance.  If it is applied
to a symbol that does not name an instance, the procedure bound to
<name-constructor> returns #f.

Example:

(define-enum-type color :color
  color?
  colors
  color-name name->color
  color-index
  (black white purple maroon))

(color-name (vector-ref colors 0)) => black
(color-name (color white))         => white
(color-name (name->color 'white))  => white
(color-index (color purple))       => 2

Enumeration-Set Types
=====================

Enumeration-set types work naturally with the enumeration types, but
are not tied to them.  The syntax for defining such a type is:

(define-enum-set-type <id> <type-name> <predicate> <constructor>
   <element-syntax> <element-predicate> <all-elements> <element-index-ref>)

This defines <id> to be syntax for constructing sets, <type-name> to
be a value representing the type, <predicate> to be a predicate for
those sets, and <constructor> a procedure for constructing one from a
list.

<Element-syntax> must be the name of a macro for constructing set
elements from names (akin to the <tag> argument to
`define-enum-type').  <Element-predicate> must be a predicate for the
element type, <all-elements> a vector of all values of the element
type, and <element-index-ref> must return the index of an element
within the <all-elements> vector.

(enum-set->list enum-set) => list
(enum-set-member? enum-set enumerand) => boolean
(enum-set=? enum-set enum-set) => boolean
(enum-set-union enum-set enum-set) => enum-set
(enum-set-intersection enum-set enum-set => enum-set
(enum-set-negation enum-set) => enum-set

`Enum-set->list' converts a set into a list of its elements.
`Enum-set-member?' tests for membership.  `Enum-set=?' tests two sets
of equal type for equality.  (If its arguments are not of the same
type, `enum-set=?' raises an exception.)  `Enum-set-union' computes
the union of two sets of equal type, `enum-set-intersection' computes
the intersection, and `enum-set-negation' computes the complement of a
set.

Here is an example.  Given the above enumerated type `color', we can
define sets of colors:

(define-enum-set-type color-set :color-set
                      color-set?
                      make-color-set
  color color? colors color-index)

> (map color-name (enum-set->list (color-set black white)))
(black white)
> (map color-name (enum-set->list (enum-set-negation (color-set black white))))
(purple maroon)
> (enum-set-member? (color-set black white) (color white))
#t