From: Takehiko Abe
Subject: read-preserving-space and recursive-p
Date: Fri, 14 Apr 2006 13:55:29 +0000
Message-ID: <keke-1404062255290001@192.168.1.2>

HyperSpec says on read-preserving-whitespace:

;; read-preserving-whitespace is exactly like read when the recursive-p
;; argument to read-preserving-whitespace is true.

But later it provides an example with recursive-p set to t:

(defun slash-reader (stream char)
   (declare (ignore char))
   `(path . ,(loop for dir = (read-preserving-whitespace stream t nil t)
                   then (progn (read-char stream t nil t)
                               (read-preserving-whitespace stream t nil t))
                   collect dir
                   while (eql (peek-char nil stream nil nil t) #\/))))

(set-macro-character #\/ #'slash-reader)

And states:

;; Consider now calling read on this expression:
;; 
;;     (zyedh /usr/games/zork /usr/games/boggle)
;; 
;; [...] The entire example expression should therefore be read as
;; 
;; 
;;     (zyedh (path usr games zork) (path usr games boggle))
;; 

CMUCL/SBCL produces the identical result:

* (with-input-from-string (in "(zyedh /usr/games/zork /usr/games/boggle)")
             (read in))

(ZYEDH (PATH USR GAMES ZORK) (PATH USR GAMES BOGGLE))


However, in 23.1.3.2 The RECURSIVE-P argument, HyperSpec states:

;; 2. A recursive call does not alter whether the reading process
;; is to preserve whitespace[2] or not (as determined by whether
;; the outermost call was to read or read-preserving-whitespace).

According to this rule the above call to READ should produce:

   (ZYEDH (PATH USR GAMES ZORK USR GAMES BOGGLE))

instead.

Which is correct?

Thanks,
Takehiko

• Re: read-preserving-space and recursive-p Alan Manuel K. Gloria
  • Re: read-preserving-space and recursive-p Takehiko Abe

From: Alan Manuel K. Gloria
Subject: Re: read-preserving-space and recursive-p
Date: Sat, 15 Apr 2006 05:07:47 +0000
Message-ID: <1145077667.083323.80340@v46g2000cwv.googlegroups.com>

Hmm.  This indeed looks weird.

Both GCL 2.6.6 and CLISP 2.38 (on WinXP) get:
(ZYEDH (PATH USR GAMES ZORK USR GAMES BOGGLE))

However SBCL 0.9.9 (on WinXP) gets:
(ZYEDH (PATH USR GAMES ZORK) (PATH USR GAMES BOGGLE))

Looks like this is not a particularly well defined portion of CL.

From: Takehiko Abe
Subject: Re: read-preserving-space and recursive-p
Date: Sat, 15 Apr 2006 06:49:10 +0000
Message-ID: <keke-1504061549100001@192.168.1.2>

Alan Manuel K. Gloria:

> Hmm.  This indeed looks weird.
> 
> Both GCL 2.6.6 and CLISP 2.38 (on WinXP) get:
> (ZYEDH (PATH USR GAMES ZORK USR GAMES BOGGLE))
> 
> However SBCL 0.9.9 (on WinXP) gets:
> (ZYEDH (PATH USR GAMES ZORK) (PATH USR GAMES BOGGLE))
> 
> Looks like this is not a particularly well defined portion of CL.

I am inclined to think that SBCL is wrong here.
If we ignore the example, the standard is consistent.
And examples are not formally a part of the standard
(1.4.3).