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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 type.

   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.

 - Special Form: defstruct NAME SLOTS...
     The `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,

          (defstruct person name age sex)

     defines a struct type called `person' which contains three slots.
     Given a `person' object P, you can access those slots by calling
     `(person-name P)', `(person-age P)', and `(person-sex P)'.  You
     can also change these slots by using `setf' on any of these place

          (incf (person-age birthday-boy))

     You can create a new `person' by calling `make-person', which
     takes keyword arguments `:name', `:age', and `: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 `nil'.)

     Given a `person', `(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 looks
     like a `person', false otherwise.  (Again, in Common Lisp this
     predicate would be exact; in Emacs Lisp the best it can do is
     verify that X is a vector of the correct length which starts with
     the correct tag symbol.)

     Accessors like `person-name' normally check their arguments
     (effectively using `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 `person'").  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.  *Note

          (setq dave (make-person :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-name dave) (person-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 "struct options"; each SLOT is
     either a slot symbol or a list of the form `(SLOT-NAME
     which is evaluated any time an instance of the structure type is
     created without specifying that slot's value.

     Common Lisp defines several slot options, but the only one
     implemented in this package is `:read-only'.  A non-`nil' value
     for this option means the slot should not be `setf'-able; the
     slot's value is determined when the object is created and does not
     change afterward.

          (defstruct person
            (name nil :read-only t)
            (sex 'unknown))

     Any slot options other than `:read-only' are 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.)

          (defstruct (person (:constructor create-person)
                             (:type list)
            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 `(:conc-name p-)' would change this prefix to `p-'.
          Specifying `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 is `make-NAME', e.g., `make-person'.  The above
          example changes this to `create-person'.  Specifying `nil' 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.  Note: Program
          Structure, for a description of the format of Common Lisp
          argument lists.  All options, such as `&rest' and `&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, `&optional' and `&key' arguments which
          don't specify defaults take their defaults from the slot
          descriptor.  It is legal to include arguments which don't
          correspond to slot names; these are useful if they are
          referred to in the defaults for optional, keyword, or `&aux'
          arguments which *do* correspond to slots.

          You can specify any number of full-format `:constructor'
          options on a structure.  The default constructor is still
          generated as well unless you disable it with a simple-format
          `:constructor' option.

                 (:constructor nil)   ; no default constructor
                 (:constructor new-person (name sex &optional (age 0)))
                 (:constructor new-hound (&key (name "Rover")
                                               (dog-years 0)
                                          &aux (age (* 7 dog-years))
                                               (sex 'canine))))
                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 are `"Jane"', 0, and
          `female', respectively.

          The second constructor takes two keyword arguments, `:name',
          which initializes the `name' slot and defaults to `"Rover"',
          and `:dog-years', which does not itself correspond to a slot
          but which is used to initialize the `age' slot.  The `sex'
          slot is forced to the symbol `canine' with no syntax for
          overriding it.

          The argument is an alternate name for the copier function for
          this type.  The default is `copy-NAME'.  `nil' means not to
          generate a copier function.  (In this implementation, all
          copier functions are simply synonyms for `copy-sequence'.)

          The argument is an alternate name for the predicate which
          recognizes objects of this type.  The default is `NAME-p'.
          `nil' means not to generate a predicate function.  (If the
          `:type' option is used without the `:named' option, no
          predicate is ever generated.)

          In true Common Lisp, `typep' is always able to recognize a
          structure object even if `:predicate' was used.  In this
          package, `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++-style
          inheritance.  The argument is the name of another structure
          type previously created with `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:

               (defstruct person name (age 0) sex)
                    => person
               (defstruct (astronaut (:include person (age 45)))
                 (favorite-beverage 'tang))
                    => astronaut
               (setq joe (make-person :name "Joe"))
                    => [cl-struct-person "Joe" 0 nil]
               (setq buzz (make-astronaut :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-name buzz)
                    => "Buzz"
               (astronaut-name joe)
                    => error: "astronaut-name accessing a non-astronaut"

          Thus, if `astronaut' is a specialization of `person', then
          every `astronaut' is also a `person' (but not the other way
          around).  Every `astronaut' includes all the slots of a
          `person', plus extra slots that are specific to astronauts.
          Operations that work on people (like `person-name') work on
          astronauts just like other people.

          In full Common Lisp, this option allows you to specify a
          function which 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 `:print-function'.

          The argument should be one of the symbols `vector' or `list'.
          This tells which underlying Lisp data type should be used to
          implement the new structure type.  Vectors are used by
          default, but `(:type list)' will cause structure objects to
          be stored as lists instead.

          The vector representation for structure objects has the
          advantage that all structure slots can be accessed quickly,
          although creating vectors is 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 features.

          The default, if you don't specify `:type' explicitly, is to
          use named vectors.  Therefore, `:named' is only useful in
          conjunction with `:type'.

               (defstruct (person1) name age sex)
               (defstruct (person2 (:type list) :named) name age sex)
               (defstruct (person3 (:type list)) name age sex)
               (setq p1 (make-person1))
                    => [cl-struct-person1 nil nil nil]
               (setq p2 (make-person2))
                    => (person2 nil nil nil)
               (setq p3 (make-person3))
                    => (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, `defstruct' is not
          able to make a useful predicate for recognizing them.  Also,
          accessors like `person3-name' will be generated but they will
          not be able to do any type checking.  The `person3-name'
          function, for example, will simply be a synonym for `car' in
          this case.  By contrast, `person2-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
          with `nil' by the constructors and ignored otherwise.  If the
          type `:include's another type, then `:initial-offset'
          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 `defstruct' facility of this package is
entirely compatible with that of Common Lisp.

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