这一节后半段需要CLOS的知识, 所以为了不产生疑惑，在读到define-condition的时候先去看看CLOS
Ignoring all errors, returning nil
有时候你知道函数会失败，并且你想忽略这个错误，可以使用ignore-errors
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(ignore-errors
(/ 3 0))
; in: IGNORE-ERRORS (/ 3 0)
; (/ 3 0)
;
; caught STYLE-WARNING:
; Lisp error during constant folding:
; arithmetic error DIVISION-BY-ZERO signalled
; Operation was (/ 3 0).
;
; compilation unit finished
; caught 1 STYLE-WARNING condition
NIL
#<DIVISION-BY-ZERO {1008FF5F13}>
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3/0 是一个错误，我们收到了警告。但是函数会正常返回nil，函数执行的状态被返回，失败了。
Catching any condition (handler-case)
ignore-errors是建立在handler-case上的。我们可以通过捕获err重写上面的例子,但是我们可以返回我们想返回的东西
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(handler-case (/ 3 0)
(error (c)
(format t "We caught a condition.~&")
(values 0 c)))
; in: HANDLER-CASE (/ 3 0)
; (/ 3 0)
;
; caught STYLE-WARNING:
; Lisp error during constant folding:
; Condition DIVISION-BY-ZERO was signalled.
;
; compilation unit finished
; caught 1 STYLE-WARNING condition
We caught a condition.
0
#<DIVISION-BY-ZERO {1004846AE3}>
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可以看到我们返回了0 和 c
handler-case的通用模板是
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(handler-case (code that errors out)
(condition-type (the-condition) ;; <-- optional argument
(code))
(another-condition (the-condition)
...))
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我们也可以用t 来捕获条件
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(handler-case
(progn
(format t "This won`t work...~%")
(/3 0))
(t (c)
(format t "Got an exception: ~a~%" c)
(values 0 c)))
;; …
;; This won't work…
;; Got an exception: arithmetic error DIVISION-BY-ZERO signalled
;; Operation was (/ 3 0).
;; 0
;; #<DIVISION-BY-ZERO {100608F0F3}>
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捕获一个特殊的条件
我们可以指定哪些条件需要处理
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(handler-case (/ 3 0)
(division-by-zero (c)
(format t "Caught division by zero: ~a~%" c)))
;; …
;; Caught division by zero: arithmetic error DIVISION-BY-ZERO signalled
;; Operation was (/ 3 0).
;; NIL
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这个和try/catch形式非常像，但是我们可以做更多的事情
handler-case vs handler-bind
handler-case和其他语言的try/catch非常像
handler-bind 用在哪些当错误出现时我们需要绝对控制的情况下。他允许我们交互的使用debugger和编程化的restart,
defining and make conditions
我们可以使用define-condition 定义condtions，使用make-condition 初始化他们
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(define-condition my-division-by-zero (error)
())
(make-condition 'my-division-by-zero)
;; #<MY-DIVISION-BY-ZERO {1005A5FE43}>
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定义condition时，最好给出更多的信息
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(define-condition my-division-by-zero (error)
((dividend :initarg :dividend
:initform nil
:reader dividend)) ;; <-- we'll get the dividend with (dividend condition). See the CLOS tutorial if needed.
(:documentation "Custom error when we encounter a division by zero.")) ;; good practice ;)
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