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This page is part of the web mail archives of SRFI 13 from before July 7th, 2015. The new archives for SRFI 13 contain all messages, not just those from before July 7th, 2015.

> I've found string hacking to be a fundamental component of many different
> kinds of hacking I've done -- web servers and scripting in particular. So
> I'm very interested in developing a rich, carefully-thought-out library
> that will assist this kind of programming.

In my web server scripting I often have to escape strings into HTML or
LaTeX.  I understand some people have to escape strings to safely pass
to a Unix shell.  All of these are cases of escaping single characters
into multiple characters for the output string.  I think the string
library would be more useful with some efficient mechanism for
implementing such escapes.

Here's a sample implementation to help fuel discussion:

;; examples of use

(define html-escape (string-escaper '((#\< . "&lt;")
				      (#\> . "&gt;")
				      (#\& . "&amp;"))))

(define scheme-escape (string-escaper '((#\\ . "\\\\")
					(#\" . "\\\""))))

(define latex-escape (string-escaper '((#\\ . "\\\\")
				       (#\~ . "\\~")
				       (#\# . "\\#")
				       (#\$ . "\\$")
				       (#\% . "\\%")
				       (#\^ . "\\^")
				       (#\& . "\\&")
				       (#\{ . "\\{")
				       (#\} . "\\}")
				       (#\_ . "\\_"))))

;; example implementation

(define (string-escaper esc)
  (let ((spec (char-escape-spec esc)))
    (lambda (str) (string-escape str spec))))

(define (string-needs-escape? str esc)
  (let ((len (string-length str)))
    (let loop ((i 0))
      (if (= i len)
	  (let ((c (string-ref str i)))
	    (if (and (char>=? c (car esc))
		     (char<=? c (cadr esc)))
		(loop (+ 1 i))))))))

(define (string-escape str esc)
  (if (string-needs-escape? str esc)
	(let ((len (string-length str)))
	  (let loop ((i 0)
		     (li '()))
	    (if (= i len)
		(loop (+ 1 i)
		      (let ((c (string-ref str i)))
			(if (and (char>=? c (car esc))
				 (char<=? c (cadr esc)))
			    (let ((li2 (vector-ref
					(caddr esc)
					(- (char->integer c)
					   (char->integer (car esc))))))
			      (if li2
				  (append li2 li)
				  (cons c li)))
			    (cons c li)))))))))

(define (char-escape-spec speclist)
  (let ((minchar (caar speclist))
	(maxchar (caar speclist)))
    (let loop ((li (cdr speclist)))
      (if (not (null? li))
	    (let ((testchar (caar li)))
	      (if (char<? testchar minchar)
		  (set! minchar testchar))
	      (if (char>? testchar maxchar)
		  (set! maxchar testchar)))
	    (loop (cdr li)))))
     (let ((specv (make-vector (+ 1 (- (char->integer maxchar)
				       (char->integer minchar))) #f)))
      (map (lambda (specpair)
	     (vector-set! specv
			  (- (char->integer (car specpair))
			     (char->integer minchar))
			  (reverse (string->list (cdr specpair)))))