The Common Lisp structure mechanism provides a general way
to define data types similar to C’s
struct types. A
structure is a Lisp object containing some number of slots,
each of which can hold any Lisp data object. Functions are
provided for accessing and setting the slots, creating or copying
structure objects, and recognizing objects of a particular structure
In true Common Lisp, each structure type is a new type distinct from all existing Lisp types. Since the underlying Emacs Lisp system provides no way to create new distinct types, this package implements structures as vectors (or lists upon request) with a special “tag” symbol to identify them.
cl-defstruct form defines a new structure type called
name, with the specified slots. (The slots
may begin with a string which documents the structure type.)
In the simplest case, name and each of the slots
are symbols. For example,
(cl-defstruct person first-name age sex)
defines a struct type called
person that contains three slots.
person object p, you can access those slots by
(person-sex p). You can also change these
slots by using
setf on any of these place forms, for example:
(cl-incf (person-age birthday-boy))
You can create a new
person by calling
which takes keyword arguments
:sex to specify the initial values of these slots in the
new object. (Omitting any of these arguments leaves the corresponding
slot “undefined”, according to the Common Lisp standard; in Emacs
Lisp, such uninitialized slots are filled with
(copy-person p) makes a new
object of the same type whose slots are
eq to those of p.
Given any Lisp object x,
(person-p x) returns
true if x is a
person, and false otherwise.
person-first-name normally check their arguments
person-p) and signal an error if the
argument is the wrong type. This check is affected by
(optimize (safety …)) declarations. Safety level 1,
the default, uses a somewhat optimized check that will detect all
incorrect arguments, but may use an uninformative error message
(e.g., “expected a vector” instead of “expected a
Safety level 0 omits all checks except as provided by the underlying
aref call; safety levels 2 and 3 do rigorous checking that will
always print a descriptive error message for incorrect inputs.
(setq dave (make-person :first-name "Dave" :sex 'male)) ⇒ [cl-struct-person "Dave" nil male] (setq other (copy-person dave)) ⇒ [cl-struct-person "Dave" nil male] (eq dave other) ⇒ nil (eq (person-first-name dave) (person-first-name other)) ⇒ t (person-p dave) ⇒ t (person-p [1 2 3 4]) ⇒ nil (person-p "Bogus") ⇒ nil (person-p '[cl-struct-person counterfeit person object]) ⇒ t
In general, name is either a name symbol or a list of a name symbol followed by any number of structure options; each slot is either a slot symbol or a list of the form ‘(slot-name default-value slot-options…)’. The default-value is a Lisp form that is evaluated any time an instance of the structure type is created without specifying that slot’s value.
(cl-defstruct person (first-name nil :read-only t) age (sex 'unknown))
slot-options is a list of keyword-value pairs, where the following keywords can be used:
nil value means the slot should not be
the slot’s value is determined when the object is created and does
not change afterward.
The expected type of the values held in this slot.
A documentation string describing the slot.
Other slot options are currently ignored.
For obscure historical reasons, structure options take a different form than slot options. A structure option is either a keyword symbol, or a list beginning with a keyword symbol possibly followed by arguments. (By contrast, slot options are key-value pairs not enclosed in lists.)
(cl-defstruct (person (:constructor create-person) (:type list) :named) first-name age sex)
The following structure options are recognized.
The argument is a symbol whose print name is used as the prefix for
the names of slot accessor functions. The default is the name of
the struct type followed by a hyphen. The option
would change this prefix to
nil as an
argument means no prefix, so that the slot names themselves are used
to name the accessor functions.
In the simple case, this option takes one argument which is an
alternate name to use for the constructor function. The default
make-person. The above
example changes this to
as an argument means that no standard constructor should be
generated at all.
In the full form of this option, the constructor name is followed
by an arbitrary argument list. See Program Structure, for a
description of the format of Common Lisp argument lists. All
options, such as
&key, are supported.
The argument names should match the slot names; each slot is
initialized from the corresponding argument. Slots whose names
do not appear in the argument list are initialized based on the
default-value in their slot descriptor. Also,
&key arguments that don’t specify defaults take their
defaults from the slot descriptor. It is valid to include arguments
that don’t correspond to slot names; these are useful if they are
referred to in the defaults for optional, keyword, or
arguments that do correspond to slots.
You can specify any number of full-format
options on a structure. The default constructor is still generated
as well unless you disable it with a simple-format
(cl-defstruct (person (:constructor nil) ; no default constructor (:constructor new-person (first-name sex &optional (age 0))) (:constructor new-hound (&key (first-name "Rover") (dog-years 0) &aux (age (* 7 dog-years)) (sex 'canine)))) first-name age sex)
The first constructor here takes its arguments positionally rather
than by keyword. (In official Common Lisp terminology, constructors
that work By Order of Arguments instead of by keyword are called
“BOA constructors”. No, I’m not making this up.) For example,
(new-person "Jane" 'female) generates a person whose slots
"Jane", 0, and
The second constructor takes two keyword arguments,
which initializes the
name slot and defaults to
:dog-years, which does not itself correspond to a slot
but which is used to initialize the
age slot. The
slot is forced to the symbol
canine with no syntax for
The argument is an alternate name for the copier function for
this type. The default is
means not to generate a copier function. (In this implementation,
all copier functions are simply synonyms for
The argument is an alternate name for the predicate that recognizes
objects of this type. The default is
means not to generate a predicate function. (If the
option is used without the
:named option, no predicate is
In true Common Lisp,
typep is always able to recognize a
structure object even if
:predicate was used. In this
cl-typep simply looks for a function called
typename-p, so it will work for structure types
only if they used the default predicate name.
This option implements a very limited form of C
The argument is the name of another structure type previously
cl-defstruct. The effect is to cause the new
structure type to inherit all of the included structure’s slots
(plus, of course, any new slots described by this struct’s slot
descriptors). The new structure is considered a “specialization”
of the included one. In fact, the predicate and slot accessors
for the included type will also accept objects of the new type.
If there are extra arguments to the
:include option after
the included-structure name, these options are treated as replacement
slot descriptors for slots in the included structure, possibly with
modified default values. Borrowing an example from Steele:
(cl-defstruct person first-name (age 0) sex) ⇒ person (cl-defstruct (astronaut (:include person (age 45))) helmet-size (favorite-beverage 'tang)) ⇒ astronaut (setq joe (make-person :first-name "Joe")) ⇒ [cl-struct-person "Joe" 0 nil] (setq buzz (make-astronaut :first-name "Buzz")) ⇒ [cl-struct-astronaut "Buzz" 45 nil nil tang] (list (person-p joe) (person-p buzz)) ⇒ (t t) (list (astronaut-p joe) (astronaut-p buzz)) ⇒ (nil t) (person-first-name buzz) ⇒ "Buzz" (astronaut-first-name joe) ⇒ error: "astronaut-first-name accessing a non-astronaut"
astronaut is a specialization of
astronaut is also a
person (but not the
other way around). Every
astronaut includes all the slots
person, plus extra slots that are specific to
astronauts. Operations that work on people (like
work on astronauts just like other people.
If this option is present, this structure’s functions will not be inlined, even functions that normally would.
In full Common Lisp, this option allows you to specify a function
that is called to print an instance of the structure type. The
Emacs Lisp system offers no hooks into the Lisp printer which would
allow for such a feature, so this package simply ignores
The argument should be one of the symbols
list. This tells which underlying Lisp data type should be
used to implement the new structure type. Records are used by
(:type vector) will cause structure objects to be
stored as vectors and
(:type list) lists instead.
The record and vector representations for structure objects have the advantage that all structure slots can be accessed quickly, although creating them are a bit slower in Emacs Lisp. Lists are easier to create, but take a relatively long time accessing the later slots.
This option, which takes no arguments, causes a characteristic “tag”
symbol to be stored at the front of the structure object. Using
:type without also using
:named will result in a
structure type stored as plain vectors or lists with no identifying
The default, if you don’t specify
:type explicitly, is to use
records, which are always tagged. Therefore,
:named is only
useful in conjunction with
(cl-defstruct (person1) first-name age sex) (cl-defstruct (person2 (:type list) :named) first-name age sex) (cl-defstruct (person3 (:type list)) first-name age sex) (cl-defstruct (person4 (:type vector)) first-name age sex) (setq p1 (make-person1)) ⇒ #s(person1 nil nil nil) (setq p2 (make-person2)) ⇒ (person2 nil nil nil) (setq p3 (make-person3)) ⇒ (nil nil nil) (setq p4 (make-person4)) ⇒ [nil nil nil] (person1-p p1) ⇒ t (person2-p p2) ⇒ t (person3-p p3) ⇒ error: function person3-p undefined
Since unnamed structures don’t have tags,
cl-defstruct is not
able to make a useful predicate for recognizing them. Also,
person3-first-name will be generated but they
will not be able to do any type checking. The
function, for example, will simply be a synonym for
this case. By contrast,
person2-first-name is able to verify
that its argument is indeed a
person2 object before
The argument must be a nonnegative integer. It specifies a
number of slots to be left “empty” at the front of the
structure. If the structure is named, the tag appears at the
specified position in the list or vector; otherwise, the first
slot appears at that position. Earlier positions are filled
nil by the constructors and ignored otherwise. If
:includes another type, then
specifies a number of slots to be skipped between the last slot
of the included type and the first new slot.
Except as noted, the
cl-defstruct facility of this package is
entirely compatible with that of Common Lisp.
cl-defstruct package also provides a few structure
This function returns the underlying data structure for
struct-type, which is a symbol. It returns
not actually a structure.
This function returns a list of slot descriptors for structure
struct-type. Each entry in the list is
(name . opts),
name is the name of the slot and
opts is the list
of slot options given to
defstruct. Dummy entries represent
the slots used for the struct name and that are skipped to implement
Return the offset of slot
returned zero-based slot index is relative to the start of the
structure data type and is adjusted for any structure name and
:initial-offset slots. Signal error if struct
Return the value of slot
slot-name are symbols.
inst is a structure instance. This routine is also a
setf place. Can signal the same errors as