Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!bloom-beacon!mit-eddie!mit-amt!sokolov
From: sokolov@mit-amt (Michael Sokolov)
Newsgroups: comp.lang.lisp
Subject: Redefining read-eval-print loop.
Message-ID: <3185@mit-amt>
Date: 19 Oct 88 20:49:32 GMT
Reply-To: sokolov@media-lab.media.mit.edu (Michael Sokolov)
Organization: MIT Media Lab, Cambridge MA
Lines: 29

In article <589@dcl-csvax.comp.lancs.ac.uk> simon@comp.lancs.ac.uk (Simon Brooke) writes:

> And if we aren't allowed to redefine the way the top-level read - eval
>- print loop works, then the language certainly isn't as expressive: there is
>a large category of things it can't express.

	It is still possible to redefine the way the read-eval-print
loop works; you just have to wrap your own read-eval-print loop around
the lisp's, making sure to include an unwind-protect so that your loop
won't get aborted out of.  The only tricky thing is that you may have
to write an s-expression parser to do this just right, but without the
niceties: 

(defun repl ()
  (tagbody
    spot
    (format t "~%~A" *prompt*)
    (let (val)
      (unwind-protect
	  (loop
	      (setq val (eval (read)))
	      (format t "~S" val)
	      (format t "~%~A" *prompt*))
	  (go spot)))))


You can replace the call to eval in this function with a call to your
favorite evaluator! Once you evaluate "(repl)" it never returns.

MS