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Message from discussion Method specialization style question
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Cliff Crawford  
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 More options Mar 27 2004, 3:34 pm
Newsgroups: comp.lang.lisp
From: Cliff Crawford <cj...@cornell.edu>
Date: Sat, 27 Mar 2004 20:34:47 GMT
Local: Sat, Mar 27 2004 3:34 pm
Subject: Re: Method specialization style question
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On 2004-03-26, Peter Seibel <pe...@javamonkey.com> wrote:
|
|  Anyone have any stylistic preference for one or the other? (I guess
|  the practical difference is just trading a type-error in for a
|  no-applicable-method error if withdraw is ever called with a
|  non-numeric second argument.)

A third possibility would be to use CHECK-TYPE:

(defmethod withdraw ((account account) amount)
  (check-type amount number)
  (if (>= (balance account) amount)
    (decf (balance amount) amount)
    (error "Insufficient funds.")))

Now if you call WITHDRAW with a non-numeric AMOUNT, the debugger will
give you an opportunity to change it (via the STORE-VALUE restart).

--
 Cliff Crawford             ***             cj...@cornell.edu

"The perfection of art is to conceal art."      -- Quintilian


 
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