Replacing a symbol in a symbolic expression

Question

I wish to replace the first occurrence of a symbol within pairs. For example: take

(define n \'((a . b) . (a . d)))

and i define a method

Answer 1

The quickest way is to use a flag indicating whether the replacement has already been done, something along the lines of:

(define (context sxp sym)
  (define done #f)
  (let loop ((sxp sxp))
    (cond (done sxp)
          ((pair? sxp) (cons (loop (car sxp)) (loop (cdr sxp))))
          ((eq? sym sxp) (set! done #t) '())
          (else sxp))))

It's not very elegant to use set!, but the alternative would be to have the procedure return 2 values, and the resulting let-values code would be even worse in terms of readability IMO.

Also note that I didn't use atom? because it's not defined in standard Scheme; the usual way is to successively test null? then pair?, and handle the atom case in the else clause.