Infix expressions in Lisp

mexp.lisp

;; M-expression to S-expression macro converter
;; Author: Dmytro Sirenko
;; License: BEERWARE
;; 
;; Examples of usage:
;;      (@ 2+2)            => 4
;;      (@ (98 + 11 + 17)/sqrt(9))  => 42.0

(defvar bin-ops '(+ - * /) )

(defun process-branches-for (ops seq process)
  "returns nil if no op ; seq must be list"
  (if ops
      (let* ((op (car ops))
         (pos (position op seq)))
    (if pos
        (list op
          (funcall process (subseq seq 0 pos))
          (funcall process (subseq seq (1+ pos))))
        (process-branches-for (cdr ops) seq process)))))

(defun m-to-s (expr)
  "returns s-expression made from m-expression"
  (cond
    ((atom expr) expr)
    ((process-branches-for bin-ops expr #'m-to-s))
    ((cdr expr) (mapcar #'m-to-s expr))
    (t (m-to-s (car expr)))))

(defun listify (expr-str &optional (res ""))
  "returns m-exp made from string expr-str"
  (if (zerop (length expr-str))
      (read-from-string res)
      (let ((c (elt expr-str 0)))
    (listify (subseq expr-str 1)
         (if (position c (remove #\Space (princ-to-string bin-ops)))
             (concatenate 'string res " " (string c) " ")
             (concatenate 'string res (string c)))))))

(defun mexp (expr)
  (m-to-s (listify (princ-to-string expr))))

(defmacro @ (&body body)
  (mexp body))