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From: Patricia Ferreira <pferreira@example.com>
Newsgroups: pt.comp.programacao
Subject: Re: sobre loops em common lisp
Date: Tue, 05 Mar 2024 20:05:22 -0300
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Patricia Ferreira <pferreira@example.com> writes:

[...]

> Eis meu segundo.  Gere um nome (de arquivo) aleatório e tente usá-lo.
> Se calhar de alguém já tê-lo escolhido, tente de novo (com outro nome).
> Feito isso, retorne-nos esse nome sortudo e pra que a gente possa
> renomeá-lo pro seu nome final---accounts.lisp.
>
> (defun write-accounts ()
>   (let ((name
>           (loop 
>             (let* ((tmp (random-string 10))
>                    (name (format nil "~a.tmp" tmp)))
>               (when 
>                   (ignore-errors 
>                    (with-open-file 
>                        (s name
>                           :direction :output
>                           :if-exists :error
>                           :if-does-not-exist :create)
>                      (write *accounts* :stream s)))
>                 (return name))))))
>     (rename-file name "accounts.lisp")
>     *accounts*))
>
> Tecnicamente temos um problema com esse procedimento.  Se o rename-file
> não obtiver sucesso, nem vamos saber.

Não sei o que fazer se o rename não ocorrer, mas ficamos a versão final:

(defun write-accounts ()
  (let ((name
          (loop 
            (let* ((tmp (random-string 10))
                   (name (format nil "~a.tmp" tmp)))
              (when 
                  (ignore-errors 
                   (with-open-file 
                       (s name
                          :direction :output
                          :if-exists :error
                          :if-does-not-exist :create)
                     (write *accounts* :stream s)))
                (return name))))))
    (if (ignore-errors (rename-file name "accounts.lisp"))
        *accounts*
        (format t "could not rename ~a to account.lisp" name))))