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Re: How the backquote and the comma really work?
From: |
Marcin Borkowski |
Subject: |
Re: How the backquote and the comma really work? |
Date: |
Fri, 10 Jul 2015 13:36:01 +0200 |
On 2015-06-25, at 20:39, Marcin Borkowski <mbork@mbork.pl> wrote:
> On 2015-06-25, at 20:22, Michael Heerdegen <michael_heerdegen@web.de> wrote:
>
>> Marcin Borkowski <mbork@mbork.pl> writes:
>>
>>> Seeing a simplistic (though working in typical/correct cases) version
>>> might be rather illuminating, no?
>>
>> Yes. Want to give it a try?
>
> Sure. I'll get back here with some code (notice: it might make some
> time, from a week to a few months - it's not the only thing I have to
> do;-)) to discuss.
OK, so -- as I said -- I'm back. I don't have my metacircular
interpreter (yet), and I want to make it rather a simplistic one (no
assignments, for instance -- just evaluating functions, conditionals and
(maybe) while loops), but I concentrated on the reader to start with.
So here's my humble attempt at the reader itself. It does nothing with
ticks, backticks and commas -- AFAIUC, it shouldn't be done at this
level anyway -- it just translates them to special forms (quote ...),
(quasi-quote ...) and (unquote ...). Do I get it correctly that it's
the eval function which should handle these?
--8<---------------cut here---------------start------------->8---
;; A simple metacircular interpreter for (a subset of) Emacs Lisp
(require 'anaphora) ; we'll use acase
(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."
(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))
(if (eq next :eob)
(error "Unexpected EOB while reading a list --
mci/read-list-contents")
(push next list)
(setq next (mci/next-token))))
(nreverse list)))
--8<---------------cut here---------------end--------------->8---
I'd be thankful for any input, either on correctness of the above code,
or on its elegance and `lispy-ness', or on ways to make it better for
novices to understand.
TIA for your help! (And I'm feeling a bit guilty that I ask a fair
share of simple questions -- but my mission here is to try to understand
this stuff as well as I can, and then write about it, so that it will be
easier for others to `get it' -- so that makes my conscience easier;-).)
Best,
--
Marcin Borkowski
http://octd.wmi.amu.edu.pl/en/Marcin_Borkowski
Faculty of Mathematics and Computer Science
Adam Mickiewicz University
- Re: How the backquote and the comma really work?,
Marcin Borkowski <=
- Re: How the backquote and the comma really work?, Michael Heerdegen, 2015/07/12
- Re: How the backquote and the comma really work?, Marcin Borkowski, 2015/07/12
- Re: How the backquote and the comma really work?, Marcin Borkowski, 2015/07/12
- Re: How the backquote and the comma really work?, Marcin Borkowski, 2015/07/14
- Re: How the backquote and the comma really work?, Emanuel Berg, 2015/07/14
- Re: How the backquote and the comma really work?, Michael Heerdegen, 2015/07/21
- Re: How the backquote and the comma really work?, Michael Heerdegen, 2015/07/24
- Re: How the backquote and the comma really work?, Michael Heerdegen, 2015/07/21
- Re: How the backquote and the comma really work?, Michael Heerdegen, 2015/07/21