define-condition (ANSI and GNU Common Lisp Document)

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#### 9.2.29 define-condition [Macro]

[Editorial Note by KMP: This syntax stuff is still very confused and needs lots of work.]

`define-condition` name ({parent-type}*) ({!slot-spec}*) {option}*
⇒ name

slot-spec ::=slot-name | (slot-name !slot-option)

slot-option ::=[[ {`:reader` symbol}* | {`:writer` !function-name}* | {`:accessor` symbol}* | {`:allocation` !allocation-type} | {`:initarg` symbol}* | {`:initform` form} | {`:type` type-specifier} ]]

option ::=[[ (`:default-initargs` `.` initarg-list) | (`:documentation` string) | (`:report` report-name) ]]

function-name ::={symbol | `(setf symbol)`}

allocation-type ::=`:instance` | `:class`

report-name ::=string | symbol | lambda expression

#### Arguments and Values::

name—a symbol.

parent-type—a symbol naming a condition type. If no parent-types are supplied, the parent-types default to `(condition)`.

default-initargs—a list of keyword/value pairs.

[Editorial Note by KMP: This is all mixed up as to which is a slot option and which is a main option. I’ll sort that out. Also, some of this is implied by the bnf and needn’t be stated explicitly.]

Slot-spec – the name of a slot or a list consisting of the slot-name followed by zero or more slot-options.

Slot-name – a slot name (a symbol), the list of a slot name, or the list of slot name/slot form pairs.

Option – Any of the following:

`:reader`

  `:reader` can be supplied more than once for a given slot and cannot be nil.

`:writer`

  `:writer` can be supplied more than once for a given slot and must name a generic function.

`:accessor`

  `:accessor` can be supplied more than once for a given slot and cannot be nil.

`:allocation`

  `:allocation` can be supplied once at most for a given slot. The default if `:allocation` is not supplied is `:instance`.

`:initarg`

  `:initarg` can be supplied more than once for a given slot.

`:initform`

  `:initform` can be supplied once at most for a given slot.

`:type`

  `:type` can be supplied once at most for a given slot.

`:documentation`

  `:documentation` can be supplied once at most for a given slot.

`:report`

  `:report` can be supplied once at most.

#### Description::

define-condition defines a new condition type called name, which is a subtype of

the type or types named by parent-type. Each parent-type argument specifies a direct supertype of the new condition. The new condition inherits slots and methods from each of its direct supertypes, and so on.

If a slot name/slot form pair is supplied, the slot form is a form that can be evaluated by make-condition to produce a default value when an explicit value is not provided. If no slot form is supplied, the contents of the slot is initialized in an implementation-dependent way.

If the type being defined and some other type from which it inherits have a slot by the same name, only one slot is allocated in the condition, but the supplied slot form overrides any slot form that might otherwise have been inherited from a parent-type. If no slot form is supplied, the inherited slot form (if any) is still visible.

Accessors are created according to the same rules as used by defclass.

A description of slot-options follows:

`:reader`

  The `:reader` slot option specifies that an unqualified method is to be defined on the generic function named by the argument to `:reader` to read the value of the given slot.

`*`

  The `:initform` slot option is used to provide a default initial value form to be used in the initialization of the slot. This form is evaluated every time it is used to initialize the slot. The lexical environment in which this form is evaluated is the lexical environment in which the define-condition form was evaluated. Note that the lexical environment refers both to variables and to functions. For local slots, the dynamic environment is the dynamic environment in which make-condition was called; for shared slots, the dynamic environment is the dynamic environment in which the define-condition form was evaluated.

  [Reviewer Note by Barmar: Issue CLOS-CONDITIONS doesn’t say this.] No implementation is permitted to extend the syntax of define-condition to allow `(slot-name form)` as an abbreviation for `(slot-name :initform form)`.

`:initarg`

  The `:initarg` slot option declares an initialization argument named by its symbol argument and specifies that this initialization argument initializes the given slot. If the initialization argument has a value in the call to initialize-instance, the value is stored into the given slot, and the slot’s `:initform` slot option, if any, is not evaluated. If none of the initialization arguments specified for a given slot has a value, the slot is initialized according to the `:initform` slot option, if specified.

`:type`

  The `:type` slot option specifies that the contents of the slot is always of the specified type. It effectively declares the result type of the reader generic function when applied to an object of this condition type. The consequences of attempting to store in a slot a value that does not satisfy the type of the slot is undefined.

`:default-initargs`

  [Editorial Note by KMP: This is an option, not a slot option.]

  This option is treated the same as it would be defclass.

`:documentation`

  [Editorial Note by KMP: This is both an option and a slot option.]

  The `:documentation` slot option provides a documentation string for the slot.

`:report`

  [Editorial Note by KMP: This is an option, not a slot option.]

  Condition reporting is mediated through the print-object method for the condition type in question, with *print-escape* always being nil. Specifying `(:report report-name)` in the definition of a condition type `C` is equivalent to:

       (defmethod print-object ((x c) stream)
         (if *print-escape* (call-next-method) (report-name x stream)))

  If the value supplied by the argument to `:report` (report-name) is a symbol or a lambda expression, it must be acceptable to function. `(function report-name)` is evaluated in the current lexical environment. It should return a function of two arguments, a condition and a stream, that prints on the stream a description of the condition. This function is called whenever the condition is printed while *print-escape* is nil.

  If report-name is a string, it is a shorthand for

       (lambda (condition stream)
         (declare (ignore condition))
         (write-string report-name stream))

  This option is processed after the new condition type has been defined, so use of the slot accessors within the `:report` function is permitted. If this option is not supplied, information about how to report this type of condition is inherited from the parent-type.

The consequences are unspecifed if an attempt is made to read a slot that has not been explicitly initialized and that has not been given a default value.

The consequences are unspecified if an attempt is made to assign the slots by using setf.

If a define-condition form appears as a top level form, the compiler must make name recognizable as a valid type name, and it must be possible to reference the condition type as the parent-type of another condition type in a subsequent define-condition form in the file being compiled.

#### Examples::

The following form defines a condition of type `peg/hole-mismatch` which inherits from a condition type called `blocks-world-error`:

    (define-condition peg/hole-mismatch
                      (blocks-world-error)
                      ((peg-shape  :initarg :peg-shape
                                   :reader peg/hole-mismatch-peg-shape)
                       (hole-shape :initarg :hole-shape
                                   :reader peg/hole-mismatch-hole-shape))
      (:report (lambda (condition stream)
                 (format stream "A ~A peg cannot go in a ~A hole."
                         (peg/hole-mismatch-peg-shape  condition)
                         (peg/hole-mismatch-hole-shape condition)))))

The new type has slots `peg-shape` and `hole-shape`, so make-condition accepts `:peg-shape` and `:hole-shape` keywords. The readers `peg/hole-mismatch-peg-shape` and `peg/hole-mismatch-hole-shape` apply to objects of this type, as illustrated in the `:report` information.

The following form defines a condition type named `machine-error` which inherits from error:

    (define-condition machine-error
                      (error)
                      ((machine-name :initarg :machine-name
                                     :reader machine-error-machine-name))
      (:report (lambda (condition stream)
                 (format stream "There is a problem with ~A."
                         (machine-error-machine-name condition)))))

Building on this definition, a new error condition can be defined which is a subtype of `machine-error` for use when machines are not available:

    (define-condition machine-not-available-error (machine-error) ()
      (:report (lambda (condition stream)
                 (format stream "The machine ~A is not available."
                         (machine-error-machine-name condition)))))

This defines a still more specific condition, built upon `machine-not-available-error`, which provides a slot initialization form for `machine-name` but which does not provide any new slots or report information. It just gives the `machine-name` slot a default initialization:

    (define-condition my-favorite-machine-not-available-error
                      (machine-not-available-error)
      ((machine-name :initform "mc.lcs.mit.edu")))

Note that since no `:report` clause was given, the information inherited from `machine-not-available-error` is used to report this type of condition.

     (define-condition ate-too-much (error)
         ((person :initarg :person :reader ate-too-much-person)
          (weight :initarg :weight :reader ate-too-much-weight)
          (kind-of-food :initarg :kind-of-food
                        :reader :ate-too-much-kind-of-food)))
    ⇒  ATE-TOO-MUCH
     (define-condition ate-too-much-ice-cream (ate-too-much)
       ((kind-of-food :initform 'ice-cream)
        (flavor       :initarg :flavor
                      :reader ate-too-much-ice-cream-flavor
                      :initform 'vanilla ))
       (:report (lambda (condition stream)
                  (format stream "~A ate too much ~A ice-cream"
                          (ate-too-much-person condition)
                          (ate-too-much-ice-cream-flavor condition)))))
    ⇒  ATE-TOO-MUCH-ICE-CREAM
     (make-condition 'ate-too-much-ice-cream
                     :person 'fred
                     :weight 300
                     :flavor 'chocolate)
    ⇒  #<ATE-TOO-MUCH-ICE-CREAM 32236101>
     (format t "~A" *)
     |>  FRED ate too much CHOCOLATE ice-cream
    ⇒  NIL

#### See Also::

make-condition , defclass , Condition System Concepts

Next: make-condition, Previous: ignore-errors, Up: Conditions Dictionary