Jim Meehan Comments

[Dick Gabriel <RPG@SAIL.STANFORD.EDU>]

Jim Meehan asked me to forward a remark he was unable to successfully
mail to Common-Lisp.
			-rpg-

ps. Oh yeah, by the way, here it is:

Date: Thu, 18 Sep 86 13:24:42 edt
From: James R. Meehan <csi!meehan@UUCP>
To: common-lisp@su-ai.arpa
Subject: Ignoring the DOTIMES variable

CLtL doesn't say whether DOTIMES actually "uses" the loop-variable, as
opposed to its value, and this ambiguity causes problems with portable
code.  If DOTIMES does use the loop-variable, then

  (DOTIMES (I 10) (DECLARE (IGNORE I)) (FOO))

can cause the compiler to issue a warning, but if DOTIMES doesn't use
the loop-variable, then

  (DOTIMES (I 10) (FOO)) 

can also cause the compiler to issue a warning.

Personally, I think DOTIMES shouldn't use the variable, so that

  (DOTIMES (I 10) (DECLARE (IGNORE I)) (FOO))

would be the correct style. That is, it should behave as if it were
implemented this way, more or less:

  (DEFMACRO DOTIMES ((VAR END &OPTIONAL FINAL) &BODY BODY)
    (LET ((I (GENSYM)) (STOP (GENSYM)))
      `(DO ((,I 0 (1+ ,I))
            (,STOP ,END))
           ((>= ,I ,STOP) (LET ((,VAR ,I)) ,FINAL))
         (LET ((,VAR ,I)) ,@BODY))))

[This may need some additional work for copying declarations in the
"final" code, via the proposed PARSE-BODY or whatever.]

I've seen several implementations where DOTIMES actually uses the
loop-variable, thus permitting horrors like using SETQ to change the
value of the loop-variable and therefore control the iterations.  The
implementation above would prevent that.