scheme - Replace first occurrence of symbol in (possibly nested) list -

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i replace first occurrence of symbol (say '-) symbol (say '+) inside list may contain lists. say,

'(((-))) turn '(((+)))

'((-) - b) '((+) - b)

here's another, different option: using mutable state find out when first replace has happened:

(define (replace-first)   (let ((found #f))     (define (replacer exp old new)       (cond ((null? exp) '())             ((not (pair? exp))              (cond ((and (eq? exp old) (not found))                     (set! found #t) new)                    (else exp)))             (else              (cons (replacer (car exp) old new)                    (replacer (cdr exp) old new)))))     replacer))  ((replace-first) '(((-))) '- '+) => '(((+)))  ((replace-first) '((-) - b) '- '+) => '((+) - b)  ((replace-first) '(+ 1 2) '+ '-) => '(- 1 2)  ((replace-first) '((+) 1 2) '+ '-) => '((-) 1 2)  ((replace-first) '(1 2 ((+)) 3 4) '+ '-) => '(1 2 ((-)) 3 4)  ((replace-first) '() '+ '-) => '()  ((replace-first) '(1 2 ((((((+ 3 (+ 4 5)))))))) '+ '-) => '(1 2 ((((((- 3 (+ 4 5))))))))

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