defstruct name-and-options [documentation] {!slot-description}{*}
=> structure-name
name-and-options ::=structure-name | (structure-name [[!options]])
options ::=!conc-name-option | {!constructor-option}{* |} !copier-option | !include-option | !initial-offset-option | !named-option | !predicate-option | !printer-option | !type-option
conc-name-option ::=:conc-name | (:conc-name) | (:conc-name conc-name)
constructor-option ::=:constructor | (:constructor) | (:constructor constructor-name) | (:constructor constructor-name constructor-arglist)
copier-option ::=:copier | (:copier) | (:copier copier-name)
predicate-option ::=:predicate | (:predicate) | (:predicate predicate-name)
include-option ::=(:include included-structure-name {!slot-description}{*)}
printer-option ::=!print-object-option | !print-function-option
print-object-option ::=(:print-object printer-name) | (:print-object)
print-function-option ::=(:print-function printer-name) | (:print-function)
type-option ::=(:type type)
named-option ::=:named
initial-offset-option ::=(:initial-offset initial-offset)
slot-description ::=slot-name | (slot-name [slot-initform [[!slot-option]]])
slot-option ::=:type slot-type | :read-only slot-read-only-p
conc-name---a string designator.
constructor-arglist---a boa lambda list.
constructor-name---a symbol.
copier-name---a symbol.
included-structure-name---an already-defined structure name.
Note that a derived type is not permissible, even if it would expand into a structure name.
initial-offset---a non-negative integer.
predicate-name---a symbol.
printer-name---a function name or a lambda expression.
slot-name---a symbol.
slot-initform---a form.
slot-read-only-p---a generalized boolean.
structure-name---a symbol.
type---one of the type specifiers list, vector, or (vector size), or some other type specifier defined by the implementation to be appropriate.
documentation---a string; not evaluated.
defstruct defines a structured type, named structure-type, with named slots as specified by the slot-options.
defstruct defines readers for the slots and arranges for setf to work properly on such reader functions. Also, unless overridden, it defines a predicate named name-p, defines a constructor function named make-constructor-name, and defines a copier function named copy-constructor-name. All names of automatically created functions might automatically be declared inline (at the discretion of the implementation).
If documentation is supplied, it is attached to structure-name as a documentation string of kind structure,
and unless :type is used, the documentation is also attached to structure-name as a documentation string of kind type and as a documentation string to the class object for the class named structure-name.
defstruct defines a constructor function that is used to create instances of the structure created by defstruct. The default name is make-structure-name. A different name can be supplied by giving the name as the argument to the constructor option. nil indicates that no constructor function will be created.
After a new structure type has been defined, instances of that type normally can be created by using the constructor function for the type. A call to a constructor function is of the following form:
(constructor-function-name
slot-keyword-1 form-1
slot-keyword-2 form-2
...)
The arguments to the constructor function are all keyword arguments. Each slot keyword argument must be a keyword whose name corresponds to the name of a structure slot. All the keywords and forms are evaluated. If a slot is not initialized in this way, it is initialized by evaluating slot-initform in the slot description
at the time the constructor function is called.
If no slot-initform is supplied, the consequences are undefined if an attempt is later made to read the slot's value before a value is explicitly assigned.
Each slot-initform supplied for a defstruct component, when used by the constructor function for an otherwise unsupplied component, is re-evaluated on every call to the constructor function.
The slot-initform is not evaluated unless it is needed in the creation of a particular structure instance. If it is never needed, there can be no type-mismatch error, even if the type of the slot is specified; no warning should be issued in this case.
For example, in the following sequence, only the last call is an error.
(defstruct person (name 007 :type string)) (make-person :name "James") (make-person)
It is as if the slot-initforms were used as initialization forms for the keyword parameters of the constructor function.
The symbols which name the slots must not be used by the implementation as the names for the lambda variables in the constructor function, since one or more of those symbols might have been proclaimed special or might be defined as the name of a constant variable. The slot default init forms are evaluated in the lexical environment in which the defstruct form itself appears and in the dynamic environment in which the call to the constructor function appears.
For example, if the form (gensym) were used as an initialization form, either in the constructor-function call or as the default initialization form in defstruct, then every call to the constructor function would call gensym once to generate a new symbol.
Each slot-description in defstruct can specify zero or more slot-options.
A slot-option consists of a pair of a keyword and a value (which is not a form to be evaluated, but the value itself). For example:
(defstruct ship (x-position 0.0 :type short-float) (y-position 0.0 :type short-float) (x-velocity 0.0 :type short-float) (y-velocity 0.0 :type short-float) (mass *default-ship-mass* :type short-float :read-only t))
This specifies that each slot always contains a short float, and that the last slot cannot be altered once a ship is constructed.
The available slot-options are:
The following keyword options are available for use with defstruct. A defstruct option can be either a keyword or a list of a keyword and arguments for that keyword; specifying the keyword by itself is equivalent to specifying a list consisting of the keyword and no arguments. The syntax for defstruct options differs from the pair syntax used for slot-options. No part of any of these options is evaluated.
(defstruct (door (:conc-name dr-)) knob-color width material) => DOOR (setq my-door (make-door :knob-color 'red :width 5.0)) => #S(DOOR :KNOB-COLOR RED :WIDTH 5.0 :MATERIAL NIL) (dr-width my-door) => 5.0 (setf (dr-width my-door) 43.7) => 43.7 (dr-width my-door) => 43.7Whether or not the :conc-name option is explicitly supplied, the following rule governs name conflicts of generated reader (or accessor) names: For any structure type S_1 having a reader function named R for a slot named X_1 that is inherited by another structure type S_2 that would have a reader function with the same name R for a slot named X_2, no definition for R is generated by the definition of S_2; instead, the definition of R is inherited from the definition of S_1. (In such a case, if X_1 and X_2 are different slots, the implementation might signal a style warning.)
(copier-name x) = (copy-structure (the structure-name x))
(defstruct person name age sex)To make a new structure to represent an astronaut that has the attributes of name, age, and sex, and functions that operate on person structures, astronaut is defined with :include as follows:
(defstruct (astronaut (:include person)
(:conc-name astro-))
helmet-size
(favorite-beverage 'tang))
:include causes the structure being defined
to have the same slots as the included structure.
This is done in such a way
that the reader functions for the included
structure also work on the structure being defined.
In this example, an
astronaut therefore has five slots: the three defined in
person and the two defined in astronaut
itself. The reader functions defined by the person structure
can be applied to instances of the astronaut structure, and they
work correctly.
Moreover, astronaut has its own reader functions for
components defined by the person structure.
The following examples illustrate the
use of astronaut structures:
(setq x (make-astronaut :name 'buzz
:age 45.
:sex t
:helmet-size 17.5))
(person-name x) => BUZZ
(astro-name x) => BUZZ
(astro-favorite-beverage x) => TANG
(reduce #'+ astros :key #'person-age) ; obtains the total of the ages
; of the possibly empty
; sequence of astros
The difference between the reader functions person-name and astro-name
is that person-name can be correctly applied to any person,
including an astronaut, while astro-name can be correctly
applied only to an astronaut. An implementation might
check for incorrect use of reader functions.
At most one :include can be supplied in a single defstruct.
The argument to :include is required and must be the
name of some previously defined structure. If the structure being
defined has no :type option, then the included structure must
also have had no :type option supplied for it.
If the structure being defined has a :type option,
then the included structure must have been declared with a :type
option specifying the same representation type.
If no :type option is involved, then
the structure name of the including structure definition
becomes the name of a data type, and therefore
a valid type specifier recognizable by typep; it becomes
a subtype of the included structure.
In the above example,
astronaut is a subtype of person; hence
(typep (make-astronaut) 'person) => trueindicating that all operations on persons also work on astronauts. The structure using :include can specify default values or slot-options for the included slots different from those the included structure specifies, by giving the :include option as:
(:include included-structure-name {slot-description}{*})
Each slot-description must have a slot-name
that is the same
as that of some slot in the included structure.
If a slot-description has no slot-initform,
then in the new structure the slot has no initial value.
Otherwise its initial value form is replaced by
the slot-initform in the slot-description.
A normally writable slot can be made read-only.
If a slot is read-only in the included structure, then it
must also be so in the including structure.
If a type is supplied for a slot, it must be
a subtype of
the
type specified in the included structure.
For example, if the
default age for an astronaut is 45, then
(defstruct (astronaut (:include person (age 45)))
helmet-size
(favorite-beverage 'tang))
If :include is used with the :type
option, then the effect is first to skip over as many representation
elements as needed to represent the included structure, then to
skip over any additional elements supplied by the :initial-offset
option, and then to begin allocation of elements from that point.
For example:
(defstruct (binop (:type list) :named (:initial-offset 2))
(operator '? :type symbol)
operand-1
operand-2) => BINOP
(defstruct (annotated-binop (:type list)
(:initial-offset 3)
(:include binop))
commutative associative identity) => ANNOTATED-BINOP
(make-annotated-binop :operator '*
:operand-1 'x
:operand-2 5
:commutative t
:associative t
:identity 1)
=> (NIL NIL BINOP * X 5 NIL NIL NIL T T 1)
The first two nil elements stem from the :initial-offset of 2
in the definition of binop. The next four elements contain the
structure name and three slots for binop. The next three nil elements
stem from the :initial-offset of 3 in the definition of
annotated-binop. The last three list elements contain the additional
slots for an annotated-binop.
(defstruct (binop (:type list) (:initial-offset 2)) (operator '? :type symbol) operand-1 operand-2) => BINOPwould result in the following behavior for make-binop:
(make-binop :operator '+ :operand-1 'x :operand-2 5) => (NIL NIL + X 5) (make-binop :operand-2 4 :operator '*) => (NIL NIL * NIL 4)The selector functions binop-operator, binop-operand-1, and binop-operand-2 would be essentially equivalent to third, fourth, and fifth, respectively. Similarly, the form
(defstruct (binop (:type list) :named (:initial-offset 2)) (operator '? :type symbol) operand-1 operand-2) => BINOPwould result in the following behavior for make-binop:
(make-binop :operator '+ :operand-1 'x :operand-2 5) => (NIL NIL BINOP + X 5) (make-binop :operand-2 4 :operator '*) => (NIL NIL BINOP * NIL 4)The first two nil elements stem from the :initial-offset of 2 in the definition of binop. The next four elements contain the structure name and three slots for binop.
(defstruct (binop (:type list)) (operator '? :type symbol) operand-1 operand-2) => BINOPThis defines a constructor function make-binop and three selector functions, namely binop-operator, binop-operand-1, and binop-operand-2. (It does not, however, define a predicate binop-p, for reasons explained below.) The effect of make-binop is simply to construct a list of length three:
(make-binop :operator '+ :operand-1 'x :operand-2 5) => (+ X 5) (make-binop :operand-2 4 :operator '*) => (* NIL 4)It is just like the function list except that it takes keyword arguments and performs slot defaulting appropriate to the binop conceptual data type. Similarly, the selector functions binop-operator, binop-operand-1, and binop-operand-2 are essentially equivalent to car, cadr, and caddr, respectively. They might not be completely equivalent because, for example, an implementation would be justified in adding error-checking code to ensure that the argument to each selector function is a length-3 list. binop is a conceptual data type in that it is not made a part of the Common Lisp type system. typep does not recognize binop as a type specifier, and type-of returns list when given a binop structure. There is no way to distinguish a data structure constructed by make-binop from any other list that happens to have the correct structure. There is not any way to recover the structure name binop from a structure created by make-binop. This can only be done if the structure is named. A named structure has the property that, given an instance of the structure, the structure name (that names the type) can be reliably recovered. For structures defined with no :type option, the structure name actually becomes part of the Common Lisp data-type system. type-of, when applied to such a structure, returns the structure name as the type of the object; typep recognizes the structure name as a valid type specifier. For structures defined with a :type option, type-of returns a type specifier such as list or (vector t), depending on the type supplied to the :type option. The structure name does not become a valid type specifier. However, if the :named option is also supplied, then the first component of the structure (as created by a defstruct constructor function) always contains the structure name. This allows the structure name to be recovered from an instance of the structure and allows a reasonable predicate for the conceptual type to be defined: the automatically defined name-p predicate for the structure operates by first checking that its argument is of the proper type (list, (vector t), or whatever) and then checking whether the first component contains the appropriate type name. Consider the binop example shown above, modified only to include the :named option:
(defstruct (binop (:type list) :named) (operator '? :type symbol) operand-1 operand-2) => BINOPAs before, this defines a constructor function make-binop and three selector functions binop-operator, binop-operand-1, and binop-operand-2. It also defines a predicate binop-p. The effect of make-binop is now to construct a list of length four:
(make-binop :operator '+ :operand-1 'x :operand-2 5) => (BINOP + X 5) (make-binop :operand-2 4 :operator '*) => (BINOP * NIL 4)The structure has the same layout as before except that the structure name binop is included as the first list element. The selector functions binop-operator, binop-operand-1, and binop-operand-2 are essentially equivalent to cadr, caddr, and cadddr, respectively. The predicate binop-p is more or less equivalent to this definition:
(defun binop-p (x) (and (consp x) (eq (car x) 'binop))) => BINOP-PThe name binop is still not a valid type specifier recognizable to typep, but at least there is a way of distinguishing binop structures from other similarly defined structures.
(defmethod print-object ((object structure-name) stream) (funcall (function printer-name) object stream <<current-print-depth>>))where the <<current-print-depth>> represents the printer's belief of how deep it is currently printing. It is implementation-dependent whether <<current-print-depth>> is always 0 and *print-level*, if non-nil, is re-bound to successively smaller values as printing descends recursively, or whether current-print-depth varies in value as printing descends recursively and *print-level* remains constant during the same traversal. If the :print-object option is used, then when a structure of type structure-name is to be printed, the designated printer function is called on two arguments:
(defmethod print-object ((object structure-name) stream) (funcall (function printer-name) object stream))If no :type option is supplied, and if either a :print-function or a :print-object option is supplied, and if no printer-name is supplied, then a print-object method specialized for structure-name is generated that calls a function that implements the default printing behavior for structures using #S notation; see section Printing Structures. If neither a :print-function nor a :print-object option is supplied, then defstruct does not generate a print-object method specialized for structure-name and some default behavior is inherited either from a structure named in an :include option or from the default behavior for printing structures; see the function print-object and section Printing Structures. When *print-circle* is true, a user-defined print function can print objects to the supplied stream using write, prin1, princ, or format and expect circularities to be detected and printed using the #n# syntax. This applies to methods on print-object in addition to :print-function options. If a user-defined print function prints to a stream other than the one that was supplied, then circularity detection starts over for that stream. See the variable *print-circle*.
(defstruct (quux (:type list) :named) x y)should make a constructor that builds a list exactly like the one that list produces, with quux as its car. If this type is defined:
(deftype quux () '(satisfies quux-p))then this form
(typep (make-quux) 'quux)should return precisely what this one does
(typep (list 'quux nil nil) 'quux)If :type is not supplied, the structure is represented as an object of type structure-object. defstruct without a :type option defines a class with the structure name as its name. The metaclass of structure instances is structure-class.
The consequences of redefining a defstruct structure are undefined.
In the case where no defstruct options have been supplied, the following functions are automatically defined to operate on instances of the new structure:
If a defstruct form appears as a top level form, the compiler must make the structure type name recognized as a valid type name in subsequent declarations (as for deftype) and make the structure slot readers known to setf. In addition, the compiler must save enough information about the structure type so that further defstruct definitions can use :include in a subsequent deftype in the same file to refer to the structure type name. The functions which defstruct generates are not defined in the compile time environment, although the compiler may save enough information about the functions to code subsequent calls inline. The #S reader macro might or might not recognize the newly defined structure type name at compile time.
An example of a structure definition follows:
(defstruct ship x-position y-position x-velocity y-velocity mass)
This declares that every ship is an object with five named components. The evaluation of this form does the following:
(setq ship2 (make-ship))sets ship2 to a newly created ship object. One can supply the initial values of any desired component in the call to make-ship by using keyword arguments in this way:
(setq ship2 (make-ship :mass *default-ship-mass*
:x-position 0
:y-position 0))
This constructs a new ship and initializes three of its components.
This function is called the "constructor function"
because it constructs a new structure.
setf can be used to alter the components of a ship:
(setf (ship-x-position ship2) 100)
This alters the x-position of ship2 to be 100. This works because defstruct behaves as if it generates an appropriate defsetf for each access function.
;;;
;;; Example 1
;;; define town structure type
;;; area, watertowers, firetrucks, population, elevation are its components
;;;
(defstruct town
area
watertowers
(firetrucks 1 :type fixnum) ;an initialized slot
population
(elevation 5128 :read-only t)) ;a slot that can't be changed
=> TOWN
;create a town instance
(setq town1 (make-town :area 0 :watertowers 0)) => #S(TOWN...)
;town's predicate recognizes the new instance
(town-p town1) => true
;new town's area is as specified by make-town
(town-area town1) => 0
;new town's elevation has initial value
(town-elevation town1) => 5128
;setf recognizes reader function
(setf (town-population town1) 99) => 99
(town-population town1) => 99
;copier function makes a copy of town1
(setq town2 (copy-town town1)) => #S(TOWN...)
(= (town-population town1) (town-population town2)) => true
;since elevation is a read-only slot, its value can be set only
;when the structure is created
(setq town3 (make-town :area 0 :watertowers 3 :elevation 1200))
=> #S(TOWN...)
;;;
;;; Example 2
;;; define clown structure type
;;; this structure uses a nonstandard prefix
;;;
(defstruct (clown (:conc-name bozo-))
(nose-color 'red)
frizzy-hair-p polkadots) => CLOWN
(setq funny-clown (make-clown)) => #S(CLOWN)
;use non-default reader name
(bozo-nose-color funny-clown) => RED
(defstruct (klown (:constructor make-up-klown) ;similar def using other
(:copier clone-klown) ;customizing keywords
(:predicate is-a-bozo-p))
nose-color frizzy-hair-p polkadots) => klown
;custom constructor now exists
(fboundp 'make-up-klown) => true
;;;
;;; Example 3
;;; define a vehicle structure type
;;; then define a truck structure type that includes
;;; the vehicle structure
;;;
(defstruct vehicle name year (diesel t :read-only t)) => VEHICLE
(defstruct (truck (:include vehicle (year 79)))
load-limit
(axles 6)) => TRUCK
(setq x (make-truck :name 'mac :diesel t :load-limit 17))
=> #S(TRUCK...)
;vehicle readers work on trucks
(vehicle-name x)
=> MAC
;default taken from :include clause
(vehicle-year x)
=> 79
(defstruct (pickup (:include truck)) ;pickup type includes truck
camper long-bed four-wheel-drive) => PICKUP
(setq x (make-pickup :name 'king :long-bed t)) => #S(PICKUP...)
;:include default inherited
(pickup-year x) => 79
;;;
;;; Example 4
;;; use of BOA constructors
;;;
(defstruct (dfs-boa ;BOA constructors
(:constructor make-dfs-boa (a b c))
(:constructor create-dfs-boa
(a &optional b (c 'cc) &rest d &aux e (f 'ff))))
a b c d e f) => DFS-BOA
;a, b, and c set by position, and the rest are uninitialized
(setq x (make-dfs-boa 1 2 3)) => #(DFS-BOA...)
(dfs-boa-a x) => 1
;a and b set, c and f defaulted
(setq x (create-dfs-boa 1 2)) => #(DFS-BOA...)
(dfs-boa-b x) => 2
(eq (dfs-boa-c x) 'cc) => true
;a, b, and c set, and the rest are collected into d
(setq x (create-dfs-boa 1 2 3 4 5 6)) => #(DFS-BOA...)
(dfs-boa-d x) => (4 5 6)
If any two slot names (whether present directly or inherited by the :include option) are the same under string=, defstruct should signal an error of type program-error.
The consequences are undefined if the included-structure-name does not name a structure type.
@xref{documentation; (setf documentation)} , section print-object [Standard Generic Function] , @xref{setf; psetf} , section subtypep [Function] , section type-of [Function] , section typep [Function] , section Compilation
The printer-name should observe the values of such printer-control variables as *print-escape*.
The restriction against issuing a warning for type mismatches between a slot-initform and the corresponding slot's :type option is necessary because a slot-initform must be specified in order to specify slot options; in some cases, no suitable default may exist.
The mechanism by which defstruct arranges for slot accessors to be usable with setf is implementation-dependent; for example, it may use setf functions, setf expanders, or some other implementation-dependent mechanism known to that implementation's code for setf.
Go to the first, previous, next, last section, table of contents.