npt-japanese

% Function ABORT, CONTINUE, MUFFLE-WARNING, STORE-VALUE, USE-VALUE

Function ABORT, CONTINUE, MUFFLE-WARNING, STORE-VALUE, USE-VALUE

Function ABORT, CONTINUE, MUFFLE-WARNING, STORE-VALUE, USE-VALUE

構文

abort &optional condition => |
continue &optional condition => nil
muffle-warning &optional condition => |
store-value value &optional condition => nil
use-value value &optional condition => nil

引数と戻り値

value - オブジェクト
condition - コンディションオブジェクトか、nil

定義

もっとも最近確立された適用可能なrestartのうち、関数と同じ名前を持つものへ制御が遷移されます。つまり、abort関数は適用可能なabort restartを探し、 continue関数は適用可能なcontinue restartを探し、他も同様です。

もしそのようなrestartが存在しないときは、 continue, store-value, use-valueはnilを返却し、 abortとmuffle-warningは型control-errorのエラーを発生します。

もしconditionがnilではないときは、それらのrestartは、明にconditionと関連付けられているものか、あるいはどのコンディションにも関連付けられてないものかのどちらかのみが集められます。これは、排他されたrestartというのは、関連付けられたコンディションの集合が空ではなく、その要素に指定したconditionが含まれていないということです。もしconditionがnilのときは、全てのrestartが集められます。

例文

;;; ABORT restartの例

(defmacro abort-on-error (&body forms)
  `(handler-bind ((error #'abort))
     ,@forms)) =>  ABORT-ON-ERROR
(abort-on-error (+ 3 5)) =>  8
(abort-on-error (error "You lose."))
>>  Returned to Lisp Top Level.

;;; CONTINUE restartの例

(defun real-sqrt (n)
  (when (minusp n)
    (setq n (- n))
    (cerror "Return sqrt(~D) instead." "Tried to take sqrt(-~D)." n))
  (sqrt n))

(real-sqrt 4) =>  2
(real-sqrt -9)
>>  Error: Tried to take sqrt(-9).
>>  To continue, type :CONTINUE followed by an option number:
>>   1: Return sqrt(9) instead.
>>   2: Return to Lisp Toplevel.
>>  Debug> (continue)
>>  Return sqrt(9) instead.
=>  3
 
 (handler-bind ((error #'(lambda (c) (continue))))
   (real-sqrt -9)) =>  3

;;; MUFFLE-WARNING restartの例

(defun count-down (x)
  (do ((counter x (1- counter)))
      ((= counter 0) 'done)
    (when (= counter 1)
      (warn "Almost done"))
    (format t "~&~D~%" counter)))
=>  COUNT-DOWN
(count-down 3)
>>  3
>>  2
>>  Warning: Almost done
>>  1
=>  DONE
(defun ignore-warnings-while-counting (x)
  (handler-bind ((warning #'ignore-warning))
    (count-down x)))
=>  IGNORE-WARNINGS-WHILE-COUNTING
(defun ignore-warning (condition)
  (declare (ignore condition))
  (muffle-warning))
=>  IGNORE-WARNING
 (ignore-warnings-while-counting 3)
>>  3
>>  2
>>  1
=>  DONE

;;; STORE-VALUE restartとUSE-VALUE restartの例

(defun careful-symbol-value (symbol)
  (check-type symbol symbol)
  (restart-case (if (boundp symbol)
                    (return-from careful-symbol-value 
                                 (symbol-value symbol))
                    (error 'unbound-variable
                           :name symbol))
    (use-value (value)
      :report "Specify a value to use this time."
      value)
    (store-value (value)
      :report "Specify a value to store and use in the future."
      (setf (symbol-value symbol) value))))
(setq a 1234) =>  1234
(careful-symbol-value 'a) =>  1234
(makunbound 'a) =>  A
(careful-symbol-value 'a)
>>  Error: A is not bound.
>>  To continue, type :CONTINUE followed by an option number.
>>   1: Specify a value to use this time.
>>   2: Specify a value to store and use in the future.
>>   3: Return to Lisp Toplevel.
>>  Debug> (use-value 12)
=>  12
(careful-symbol-value 'a)
>>  Error: A is not bound.
>>  To continue, type :CONTINUE followed by an option number.
>>    1: Specify a value to use this time.
>>    2: Specify a value to store and use in the future.
>>    3: Return to Lisp Toplevel.
>>  Debug> (store-value 24)
=>  24
 (careful-symbol-value 'a)
=>  24

;;; USE-VALUE restartの例

(defun add-symbols-with-default (default &rest symbols)
  (handler-bind ((sys:unbound-symbol
                   #'(lambda (c)
                       (declare (ignore c)) 
                       (use-value default))))
    (apply #'+ (mapcar #'careful-symbol-value symbols))))
=>  ADD-SYMBOLS-WITH-DEFAULT
(setq x 1 y 2) =>  2
(add-symbols-with-default 3 'x 'y 'z) =>  6

副作用

適用可能なrestartが使用可能なときに制御の遷移が起こるかもしれませんし、あるいは（abort関数か muffle-warning関数の場合は）実行が停止するかもしれません。

影響

もし適用可能なabort restartが利用不可の状態でabort関数が実行されれたときか、適用可能なmuffle-warning restartが利用不可の状態でmuffle-warning関数が実行されれたときは、型control-errorのエラーが発生します。

参考

invoke-restart, 9.1.4.2. restart, 9.1.4.2.2. restartのインターフェイス, assert, ccase, cerror, check-type, ctypecase, use-value, warn

備考

(abort condition) ==  (invoke-restart 'abort)
(muffle-warning)  ==  (invoke-restart 'muffle-warning)
(continue)        ==  (let ((r (find-restart 'continue))) (if r (invoke-restart r)))
(use-value x) ==  (let ((r (find-restart 'use-value))) (if r (invoke-restart r x)))
(store-value x) ==  (let ((r (find-restart 'store-value))) (if r (invoke-restart r x)))

この仕様書でuse-value restartの提供に要求される関数定義はありません。

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