[net.lang.lisp] A Handy Infix Character Macro for Franz LISP

davidson (02/01/83)

I.  Introduction

The latest version of Franz Lisp contains the ability to declare single
character infix operators to read.  I have written some code to create
an infix operator, default / (pronounced pipe), which provides a handy 
notation for nested monadic functions analogous to the pipe syntax in
the UNIX shell, or to record field selection in Pascal and C.  Pipe was
previously available only within my function defining macro `to'.

II.  Definition By Example

Example one, printing a person's mother's maiden name:

    (print person/mother/father/name/last/car)

which the reader expands to

    (print (car (last (name (father (mother (person)))))))

This first example can also be written as

    (print person/mother/father/name/last/1)

since pipe translates integer functions into appropriate selection functions.

Example two, a slow way to reverse a list:

    (def reverse
	 (lambda (l)
		 (if l
		     (append1 l/cdr/reverse l/1) ) ) )

which the reader expands to

    (def reverse
	 (lambda (l)
		 (if l
		     (append1 (reverse (cdr l)) (car l)) ) ) )

III.  Limitations

Pipe cannot be employed at the top level, nor when it begins
a list, i.e.,

    (l/cdr)

expands to

    ((cdr l))

Also, a read error will occur when pipe is the last symbol in a list:

    (this that /)

However, it is perfectly fine to have pipe do double duty as a regular
lisp function:

    (/ this that)

expands to itself.

IV.  Choice Of Pipe Character

The default pipe character is `/'.  If you wish another character to be
used, set the value of pipeop-char to be that character.

You may be concerned about the use of `/', since slash is frequently
desired as part of the spelling of atom names.  I have found, however,
that if I use strings for filenames instead of quoted atoms (a better
practice anyway), that I no longer want slashes in atom names.

V.  The Code

Note:  If you are on sdcsvax, ignore the rest of this message and look
at the message I've posted to `lisp'.

File pipeop.l:

(declare (localf pipeop-n))

(declare (special pipeop-char))

(cond ((not (boundp 'pipeop-char)) (setq pipeop-char '\/)))

(def pipeop
     (lambda (q)
	     (cond ((null q) (tconc nil pipeop-char))
		   ((null (car q)) (tconc nil pipeop-char))
		   (t (rplaca (cdr q)
			      ((lambda (f x)
				       (cond ((numberp f) (pipeop-n f x))
					     (t (list f x)) ) )
			       (read) (cadr q) ) )
		      q ) ) ) )

(def pipeop-n
     (lambda (n x)
	     (cond ((eq n 1) (list 'car x))
		   ((eq n 2) (list 'cadr x))
		   ((eq n 3) (list 'caddr x))
		   (t (list 'nthelem n x)) ) ) )

(setsyntax pipeop-char 'vinfix-macro 'pipeop)