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LispKit Type

Library (lispkit type) provides a simple, lightweight type abstraction mechanism. It allows for creating new types at runtime that are disjoint from all existing types. The library provides two different types of APIs: a purely procedural API for type creation and management, as well as a declarative API. The procedural API does not have an explicit representation of types. The declarative API introduces extensible types which do have a runtime representation.

Usage of the procedural API

New types are created with function make-type. make-type accepts one argument, which is a type label. The type label is an arbitrary value that is only used for debugging purposes. Typically, symbols are used as type labels.

The following line introduces a new type for intervals:


(define-values (new-interval interval? interval-ref make-interval-subtype)
               (make-type 'interval))

(make-type 'interval) returns four functions:

  • new-interval is a procedure which takes one argument, the internal representation of the interval, and returns a new object of the new interval type
  • interval? is a type test predicate which accepts one argument and returns #t if the argument is of the new interval type, and #f otherwise.
  • interval-ref takes one object of the new interval type and returns its internal representation. interval-ref is the inverse operation of new-interval.
  • make-interval-subtype is a type generator (similar to make-type), a function that takes a type label and returns four functions representing a new subtype of the interval type.

Now it is possible to implement a constructor make-interval for intervals:


(define (make-interval lo hi)
  (if (and (real? lo) (real? hi) (<= lo hi))
      (new-interval (cons (inexact lo) (inexact hi)))
      (error "make-interval: illegal arguments" lo hi)))

make-interval first checks that the constructor arguments are valid and then calls new-interval to create a new interval object. Interval objects are represented via pairs whose car is the lower bound, and cdr is the upper bound. Nevertheless, pairs and interval objects are distinct values as the following code shows:


(define interval-obj (make-interval 1.0 9.5))
(define pair-obj (cons 1.0 9.5))

(interval? interval-obj)        ⟾ #t
(interval? pair-obj)            ⟾ #f
(equal? interval-obj pair-obj)  ⟾ #f

The type is displayed along with the representation in the textual representation of interval objects: #interval:(1.0 . 9.5).

Below are a few functions for interval objects. They all use interval-ref to extract the internal representation from an interval object and then operate on the internal representation.


(define (interval-length interval)
  (let ((bounds (interval-ref interval)))
    (- (cdr bounds) (car bounds))))

(define (interval-empty? interval)
  (zero? (interval-length interval)))

The following function calls show that interval-ref fails with a type error if its argument is not an interval object.


(interval-length interval-obj)  ⟾ 8.5
(interval-empty? '(1.0 . 1.0))  ⟾ 
    [error] not an instance of type interval: (1.0 . 1.0)

Usage of the declarative API

The procedural API provides the most flexible way to define a new type in LispKit. On the other hand, this approach comes with two problems:

  1. a lot of boilerplate needs to be written, and
  2. programmers need to be experienced to correctly encapsulate new data types and to provide means to extend them.

These problems are addressed by the declarative API of (lispkit type). At the core, this API defines a syntax define-type for declaring new types of data. define-type supports defining simple, encapsulated types as well as provides a means to make types extensible.

The syntax for defining a simple, non-extensible type has the following form:

(define-type name name?
    ((make-name x ...) expr ...)
    name-ref
    functions)

name is a symbol and defines the name of the new type. name? is a predicate for testing whether a given object is of type name. make-name defines a constructor which returns a value representing the data of the new type. name-ref is a function to unwrap values of type name. It is optional and normally not needed since functions can be declared such that the unwrapping happens implicitly. All functions defined via define-type take an object (usually called self) of the defined type as their first argument.

There are two forms to declare a function as part of define-type: one providing access to self directly, and one only providing access to the unwrapped data value:

((name-func self y ...) expr ...)

provides access directly to self (which is a value of type name), and

((name-func (repr) y ...) expr ...)

which provides access only to the unwrapped data repr.

With this new syntax, type interval from the section describing the procedural API, can now be re-written like this:


(define-type interval
  interval?
  ((make-interval lo hi)
    (if (and (real? lo) (real? hi) (<= lo hi))
        (cons (inexact lo) (inexact hi))
        (error "make-interval: illegal arguments" lo hi)))
  ((interval-length (bounds))
    (- (cdr bounds) (car bounds)))
  ((interval-empty? self)
    (zero? (interval-length self))))

interval is a standalone type which cannot be extended. define-type provides a simple means to make types extensible such that subtypes can be created reusing the base type definition. This is done with a small variation of the define-type syntax:

(define-type (name super) name?
    ((make-name x ...) expr ...)
    name-ref
    functions)

In this syntax, super refers to the type extended by name. All extensible types extend another extensible type and there is one supertype called object provided by library (lispkit type) as a primitive.

With this syntactic facility, interval can be easily re-defined to be extensible:


(define-type (interval object)
  interval?
  ((make-interval lo hi)
    (if (and (real? lo) (real? hi) (<= lo hi))
        (cons (inexact lo) (inexact hi))
        (error "make-interval: illegal arguments" lo hi)))
  ((interval-length (bounds))
    (- (cdr bounds) (car bounds)))
  ((interval-empty? self)
    (zero? (interval-length self))))

It is now possible to define a tagged-interval data structure which inherits all functions from interval and encapsulates a tag with the interval:


(define-type (tagged-interval interval)
  tagged-interval?
  ((make-tagged-interval lo hi tag)
    (values lo hi tag))
  ((interval-tag (bounds tag))
    tag))

tagged-interval is a subtype of interval; i.e. values of type tagged-interval are also considered to be of type interval. Thus, tagged-interval inherits all function definitions from interval and defines a new function interval-tag just for tagged-interval values. Here is some code explaining the usage of tagged-interval:


(define ti (make-tagged-interval 4.0 9.0 'inclusive))
(tagged-interval? ti)        ⟾ #t
(interval? ti)               ⟾ #t
(interval-length ti)         ⟾ 5.0
(interval-tag ti)            ⟾ inclusive
(interval-tag interval-obj)  ⟾ [error] not an instance of type tagged-interval: #interval:((1.0 . 9.5))

Constructors of extended types, such as make-tagged-interval return multiple values: all the parameters for a super-constructor call and one additional value (the last value) representing the data provided by the extended type. In the example above, make-tagged-interval returns three values: lo, hi, and tag. After the constructor make-tagged-interval is called, the super-constructor is invoked with arguments lo and hi. The result of make-tagged-interval is a tagged-interval object consisting of two state values contained in a list: one for the supertype interval (consisting of the bounds (lo . hi)) and one for the subtype tagged-interval (consisting of the tag). This can also be seen when displaying a tagged-interval value:


ti  ⟾ #tagged-interval:((4.0 . 9.0) inclusive)

This is also the reason why function interval-tag gets access to two unwrapped values, bounds and tag: one (bounds) corresponds to the value associated with type interval, and the other one (tag) corresponds to the value associated with type tagged-interval.

API

(make-type type-label)     [syntax]

Creates a new, unique type, and returns four procedures dealing with this new type:

  • The first procedure takes one argument returning a new object of the new type wrapping the argument
  • The second procedure is a type test predicate which accepts one argument and returns #t if the argument is of the new type, and #f otherwise.
  • The third procedure takes one object of the new type and returns its internal representation (what was passed to the first procedure).
  • The fourth procedure is a type generator (similar to make-type), a function that takes a type label and returns four functions representing a new subtype of the new type.

type-label is only used for debugging purposes. It is shown when an object's textual representation is used. In particular, calling the third procedure (the type de-referencing function) will result in an error message exposing the type label if the argument is of a different type than expected.

(define-type name name? ((make-name x ...) e ...) func ...)     [syntax]
(define-type name name? ((make-name x ...) e ...) ref func ...)

Defines a new standalone type name consisting of a type test predicate name?, a constructor make-name, and an optional function ref used to unwrap values of type name. ref is optional and normally not needed since functions func can be declared such that the unwrapping happens implicitly. All functions func defined via define-type take an object (usually called self) of the defined type as their first argument.

There are two ways to declare a function as part of define-type: one providing access to self directly, and one only providing access to the unwrapped data value:

  • ((name-func self y ...) expr ...) provides access directly to self (which is a value of type name), and
  • ((name-func (repr) y ...) expr ...) provides access only to the unwrapped data repr.

(define-type (name super) name? ((make-name x ...) e ...) func ...)     [syntax]
(define-type (name super) name? ((make-name x ...) e ...) ref func ...)

This variant of define-type defines a new extensible type name extending supertype super, which also needs to be an extensible type. A new extensible type name comes with a type test predicate name?, a constructor make-name, and an optional function ref used to unwrap values of type name. ref is optional and normally not needed since functions func can be declared such that the unwrapping happens implicitly. All functions func defined via define-type take an object (usually called self) of the defined type as their first argument.

There are two ways to declare a function as part of define-type: one providing access to self directly, and one providing access to the unwrapped data values (one for each type in the supertype chain):

  • ((name-func self y ...) expr ...) provides access directly to self (which is a value of type name), and
  • ((name-func (repr ...) y ...) expr ...) provides access only to the unwrapped data values repr.

Constructors of extended types return multiple values: all the parameters for a super-constructor call and one additional value (the last value) representing the data provided by the extended type.

object     [value]

The supertype of all extensible types defined via define-type.

(extensible-type? obj)     [procedure]

Returns #t if obj is a value representing an extensible type. For instance, (extensible-type? object) returns #t.