Scheme 48 Manual -- Finite record types

Scheme 48 Manual | Contents | In Chapter: Libraries
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Finite record types

The structure finite-types has two macros for defining `finite' record types. These 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-enumerated-type tag type-name
  predicate-name
  vector-of-instances-name
  name-accessor
  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 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.

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

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

Finite types are enumerations that allow the user to add additional fields in the type. The syntax for defining a finite type is:

(define-finite-type tag type-name
  (field-tag ...)
  predicate-name
  vector-of-instances-name
  name-accessor
  index-accessor
  (field-tag accessor-name [modifier-name])
  ...
  ((instance-name field-value ...)
   ...))

The additional fields are specified exactly as with define-record-type. The field arguments to the constructor are listed after the type-name; these do not include the name and index fields. The form ends with the names and the initial field values for the instances of the type. The instances are constructed by applying the (unnamed) constructor to these initial field values. The name must be first and the remaining values must match the field-tags in the constructor's argument list.

(define-finite-type color :color
  (red green blue)
  color?
  colors
  color-name
  color-index
  (red   color-red)
  (green color-green)
  (blue  color-blue)
  ((black    0   0   0)
   (white  255 255 255)
   (purple 160  32 240)
   (maroon 176  48  96)))

(color-name (color black))         -> black
(color-name (vector-ref colors 1)) -> white
(color-index (color purple))       -> 2
(color-red (color maroon))         -> 176

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