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## 11 Structures

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.

Macro: cl-defstruct name slots…

  The `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 name age sex)

  defines a struct type called `person` that 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 forms, for example:

      (cl-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`, and 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 that 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. See Declarations.

      (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 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.

  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.

      (cl-defstruct person
           (name nil :read-only t)
           age
           (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.)

      (cl-defstruct (person (:constructor create-person)
                            (:type list)
                            :named)
           name age sex)

  The following structure options are recognized.

  `:conc-name`

    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.

  `:constructor`

    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. See 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 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 `&aux` arguments that 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.

        (cl-defstruct
            (person
             (: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.

  `:copier`

    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`.)

  `:predicate`

    The argument is an alternate name for the predicate that 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, `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.

  `:include`

    This option implements a very limited form of C++-style inheritance. The argument is the name of another structure type previously created with `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 name (age 0) sex)
                ⇒ person
        (cl-defstruct (astronaut (:include person (age 45)))
             helmet-size
             (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.

  `:print-function`

    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 `:print-function`.

  `:type`

    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.

  `:named`

    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`.

        (cl-defstruct (person1) name age sex)
        (cl-defstruct (person2 (:type list) :named) name age sex)
        (cl-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, `cl-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 proceeding.

  `:initial-offset`

    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 `cl-defstruct` facility of this package is entirely compatible with that of Common Lisp.

The `cl-defstruct` package also provides a few structure introspection functions.

Function: cl-struct-sequence-type struct-type

  This function returns the underlying data structure for `struct-type`, which is a symbol. It returns `vector` or `list`, or `nil` if `struct-type` is not actually a structure.

Function: cl-struct-slot-info struct-type

  This function returns a list of slot descriptors for structure `struct-type`. Each entry in the list is `(name . opts)`, where `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 `:initial-offset`.

Function: cl-struct-slot-offset struct-type slot-name

  Return the offset of slot `slot-name` in `struct-type`. The 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 `struct-type` does not contain `slot-name`.

Function: cl-struct-slot-value struct-type slot-name inst

  Return the value of slot `slot-name` in `inst` of `struct-type`. `struct` and `slot-name` are symbols. `inst` is a structure instance. This routine is also a `setf` place. Can signal the same errors as `cl-struct-slot-offset`.

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