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From: Marcin Borkowski <mbork@mbork.pl>
To: help-gnu-emacs@gnu.org
Subject: Re: How the backquote and the comma really work?
Date: Sun, 12 Jul 2015 22:33:37 +0200	[thread overview]
Message-ID: <87k2u5azfi.fsf@mbork.pl> (raw)
In-Reply-To: <87mvz1b16h.fsf@mbork.pl>


On 2015-07-12, at 21:55, Marcin Borkowski <mbork@mbork.pl> wrote:

> On 2015-07-12, at 17:54, Michael Heerdegen <michael_heerdegen@web.de> wrote:
>
>> There is a problem though when the read expression is nested.  I tried
>> to `mci/read' this string for example:
>>
>>   "(defun fac (x) (if (< 2 x) 1 (* x (fac (1- x)))))"
>>
>> and got
>>
>>   (defun fac :open-paren x)
>>
>> as result.  If you Edebug your functions, you can see what goes wrong.
>> Please tell me if you need more hints...
>
> Good catch, thanks.  No need for edebug, I guess – I haven’t looked at
> my code, but I guess I know the problem already.  Stupid me.

So, what about this?  It seems to work.  OTOH, I think it's not the most
elegant thing possible, since there is some code duplication: mci/read
has this: (:open-paren (mci/read-list-contents)) in a (a)case statement,
and mci/read-list-contents has this: (:open-paren (setq next
(mci/read-list-contents))).  Something tells my mathematical mind that
there probably exists a cleaner approach.

--8<---------------cut here---------------start------------->8---
(require 'anaphora)			; we'll need acase

;; Reader

(defun mci/next-token ()
  "Get the next token from the current buffer at point position.
A token can be: an integer, a symbol, a parenthesis, a comma,
a backquote or a quote.  Return a number (in case of an integer),
a symbol (in case of a symbol), or one of the symbols: :open-paren,
:close-paren, :quote, :quasi-quote, :unquote, :eob.  (Of course, if
someone is devious enough to include one of these symbols in the
expression being read, he'll get what he deserves: a chaos.)"
  (skip-chars-forward " \t\n")
  (cond ((eq (char-after) ?\()
	 (forward-char)
	 :open-paren)
	((eq (char-after) ?\))
	 (forward-char)
	 :close-paren)
	((eq (char-after) ?\')
	 (forward-char)
	 :quote)
	((eq (char-after) ?\`)
	 (forward-char)
	 :quasi-quote)
	((eq (char-after) ?\,)
	 (forward-char)
	 :unquote)
	((looking-at "\\([-+]?[[:digit:]]+\\)[ \t\n)]")
	 (skip-chars-forward "[:digit:]")
	 (string-to-number (match-string 1)))
	((looking-at "[^ \t\n)]+")
	 (goto-char (match-end 0))
	 (intern (match-string-no-properties 0)))
	((eobp)
	 :eob)))

(defun mci/read ()
  "Read one Elisp expression from the buffer at point."
  (acase (mci/next-token)
    (:open-paren (mci/read-list-contents))
    (:close-paren
     (error "Unexpected closing paren at line %d encountered -- mci/read"
	    (line-number-at-pos)))
    (:quote (list 'quote (mci/read)))
    (:quasi-quote (list 'quasi-quote (mci/read)))
    (:unquote (list 'unquote (mci/read)))
    (:eob nil)
    (t it)))

(defun mci/read-list-contents ()
  "Read list contents (until the closing paren), gobble the
closing paren."
  (let ((next (mci/next-token))
	list)
    (while (not (eq next :close-paren))
      (case next
	(:open-paren (setq next (mci/read-list-contents)))
	(:eob (error "Unexpected EOB while reading a list -- mci/read-list-contents"))
	(t (push next list)
	   (setq next (mci/next-token)))))
    (nreverse list)))
--8<---------------cut here---------------end--------------->8---

Best,

-- 
Marcin Borkowski
http://octd.wmi.amu.edu.pl/en/Marcin_Borkowski
Faculty of Mathematics and Computer Science
Adam Mickiewicz University



  reply	other threads:[~2015-07-12 20:33 UTC|newest]

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-06-25 17:09 How the backquote and the comma really work? Marcin Borkowski
2015-06-25 17:33 ` Michael Heerdegen
2015-06-25 18:06   ` Marcin Borkowski
2015-06-25 18:22     ` Michael Heerdegen
2015-06-25 18:39       ` Marcin Borkowski
2015-06-25 18:44         ` Marcin Borkowski
2015-06-25 19:06           ` Michael Heerdegen
2015-07-10 11:36         ` Marcin Borkowski
2015-07-12 15:54           ` Michael Heerdegen
2015-07-12 19:55             ` Marcin Borkowski
2015-07-12 20:33               ` Marcin Borkowski [this message]
2015-07-14 18:17                 ` Marcin Borkowski
2015-07-14 22:08                   ` Emanuel Berg
2015-07-21 22:08                   ` Michael Heerdegen
2015-07-24 13:01                     ` Michael Heerdegen
2015-08-11 11:41                       ` Marcin Borkowski
2015-08-12 15:29                         ` Michael Heerdegen
     [not found]                         ` <mailman.8207.1439393377.904.help-gnu-emacs@gnu.org>
2015-08-12 16:30                           ` Pascal J. Bourguignon
2015-08-23  8:30                             ` Marcin Borkowski
     [not found]                             ` <mailman.110.1440318650.11330.help-gnu-emacs@gnu.org>
2015-08-23 16:46                               ` Pascal J. Bourguignon
2015-07-21 21:54                 ` Michael Heerdegen
2015-08-11 10:15                   ` Marcin Borkowski
2015-08-11 17:20                     ` Thorsten Jolitz
2015-08-12 15:01                       ` Michael Heerdegen
2015-07-21 21:50               ` Michael Heerdegen
2015-06-25 18:10 ` Drew Adams
2015-06-25 18:40   ` Michael Heerdegen
2015-06-25 18:53     ` Marcin Borkowski
2015-06-25 19:39       ` Michael Heerdegen
2015-06-25 20:05         ` Drew Adams
2015-06-25 20:18           ` Marcin Borkowski
2015-06-25 20:37             ` Drew Adams
2015-06-25 23:55     ` Robert Thorpe
     [not found]     ` <mailman.5697.1435276533.904.help-gnu-emacs@gnu.org>
2015-06-26  1:41       ` Rusi
2015-06-26 14:24         ` Michael Heerdegen
     [not found]         ` <mailman.5716.1435328741.904.help-gnu-emacs@gnu.org>
2015-06-26 14:35           ` Rusi
2015-06-26 14:51             ` Michael Heerdegen
2015-06-25 18:46   ` Marcin Borkowski
2015-06-26  7:31 ` tomas
2015-06-26 13:48   ` Drew Adams
2015-06-26 14:06     ` tomas
2015-06-26 15:06 ` Emanuel Berg
2015-07-12 17:38 ` Vaidheeswaran C
     [not found] <mailman.5657.1435252169.904.help-gnu-emacs@gnu.org>
2015-06-30 16:27 ` sokobania.01

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