Structures and The Type System

type system

• visible
• extensible -- create new types at any time

TYPEP AND TYPE-OF

(typep 3 ’number) ⇒ t
(typep 3 ’integer) ⇒ t
(typep 3 ’float) ⇒ nil
(typep ’foo ’symbol) ⇒ t

(type-of 3.5) ⇒ short-float
(type-of ’(bat breath)) ⇒ cons
(type-of "Phooey") ⇒ (simple-string 6)

DEFINING STRUCTURES

(defstruct starship
(name nil)
(speed 0)
(condition ’green)
(shields ’down))

> (setf s2 ’#s(starship speed (warp 3)
condition red
shields up))
#S(STARSHIP NAME NIL
SPEED (WARP 3)
CONDITION RED
SHIELDS UP)

TYPE PREDICATES FOR STRUCTURES

(starship-p s2) ⇒ t
(typep s1 ’starship) ⇒ t
(type-of s2) ⇒ starship

ACCESSING AND MODIFYING STRUCTURES

When a new structure is defined, DEFSTRUCT creates accessor functions for each of its components

(starship-speed s2) ⇒ (warp 3)

(setf (starship-name s1) "Enterprise")

KEYWORD ARGUMENTS TO CONSTRUCTOR FUNCTIONS

> (setf s3 (make-starship :name "Reliant"
:shields ’damaged))

CHANGING STRUCTURE DEFINITIONS

Lisp Toolkit: DESCRIBE and INSPECT

> (describe ’fred)
FRED is an internal symbol in package USER.

> (describe s1)
#S(STARSHIP ...) is a structure of type STARSHIP.
NAME "Enterprise"
SPEED 1
CONDITION YELLOW
SHIELDS UP

EQUALITY OF STRUCTURES

The EQUAL function does not treat two distinctstructures as equal even if they have the same components

However, the EQUALP function will treat two structures as equal if they are of the same type and all their components are equal.

INHERITANCEFROM OTHER STRUCTURES

The fields of a STARSHIP structure include all the componentsof SHIP

(defstruct ship
(name nil)
(captain nil)
(crew-size nil))

(defstruct (starship (:include ship))
(weapons nil)
(shields nil))