Subject: Re: The usual silly subject elided
From: Erik Naggum <>
Date: 1999/10/29
Newsgroups: comp.lang.lisp
Message-ID: <>

* Rahul Jain
| I know the utility of the s-exp form, but it's a real pain to have to
| convert a mathematical equation to it. Maybe it's just because I'm only
| used to in- and postfix notation. Eventually I'll master prefix notation
| and be the master of all notation (except for any really strange ones
| that I've never head of....) :P

(defun trivial-infix-reader (input-stream char)
  (declare (ignore char))
  (let ((list (read-delimited-list #\] input-stream t)))
    (cond ((null (cddr list)) list)	;trivial: 1- and 2-lists 
	  ((null (cdddr list))		;swap first and second of 3-lists
	   (list (second list) (first list) (third list)))
	  (t (list* (second list) (first list)
		    (loop with operator = (second list)
			  for pair on (cdr list) by #'cddr
			  unless (cdr pair) do
			  (error "infix expression is not well-formed")
			  unless (eq operator (first pair)) do
			  (error "infix operator is not properly redundant")
			  collect (second pair)))))))

  when this definition is available, you may evaluate

(set-macro-character #\[ 'trivial-infix-reader)

(set-syntax-from-char #\] #\))

  and type things like [2 + 3], [sqrt pi], [10 log 2], [x < y < z], and
  have them all come out right.

  for a more powerful infix reader that does precedence and such, there is
  some stuff available in the Common Lisp archives, but I suggest you stick
  to the above simplicity so you don't get dragged into maintaining a mini-
  language which nobody will benefit from, including yourself some ways
  down the road.