scheme - Changing a function into CPS style -


we asked write procedure when given list replace first occurrence of given element , first, catch write in cps style. unable turn cps style written procedure given success-cont , fail-cont..

if willing give try appreciate :]

the procedure have (graciously provided answers here):

(define (replace-one list old new)   (cond ((pair? list)          (let ((next (replace-one (car list) old new)))            (cons next                   (if (equal? next (car list))            ; changed?                      (replace-one (cdr list) old new)    ;   no,  recurse on rest                      (cdr list)))))                      ;   yes, done          ((eq? list old) new)          (else list)))

edited

a big @willness pointing out , fixing bug, lurking in original code. here's corrected implementation based on code (with stepwise derivation), commented , made idiomatic racket:

(define (replace-one lst b)   (let loop ([lst lst]                ; input list              [f #f]                   ; have made first replacement?              [k (lambda (ls f) ls)])  ; continue results: list , flag     (cond        (f                              ; replaced already:          (k lst f))                    ; continue without changing       ((empty? lst)                   ; empty list case         (k lst f))                    ; go on empty lst , flag       ((not (pair? lst))              ; - none replaced yet - atom?         (if (eq? lst a)               ; atom being searched?             (k b #t)                  ; replace, continue updated flag             (k lst f)))               ; no match, continue       (else                           ; list?         (loop (first lst)             ; process `car` of `lst`           f                           ; according flag's value, ,           (lambda (x f)               ; accept resulting list , flag, ,             (loop (rest lst)          ; process `cdr` of `lst`               f                       ; according new value of flag,                (lambda (y f)           ; getting results that, ,                 (if f                 ; - if replacement made -                   (k                  ; continuing new list, built                     (cons x y)        ; results of processing 2 branches,                     f)                ; , new flag, or                    (k lst f))))))))))  ; old list if nothing changed

notice single success continuation used (called k in code above) accepts two resulting values: list , flag. initial continuation returns final resulting list, , discards final flag value. return flag, indication of whether replacement have been made @ or not. used internally preserve of original list structure possible, usual persistent data types (as seen in answer).

finally, test code:

; fixed, wasn't working correctly (replace-one '((((1 2) 3 4) a) 6) 'a 'b) => '((((1 2) 3 4) b) 6)  (replace-one '(((-))) '- '+) => '(((+)))  (replace-one '((-) - b) '- '+) => '((+) - b)  (replace-one '(+ 1 2) '+ '-) => '(- 1 2)  (replace-one '((+) 1 2) '+ '-) => '((-) 1 2)  (replace-one '(1 2 ((+)) 3 4) '+ '-) => '(1 2 ((-)) 3 4)  (replace-one '() '+ '-) => '()  (replace-one '(1 2 ((((((+ 3 (+ 4 5)))))))) '+ '-) => '(1 2 ((((((- 3 (+ 4 5))))))))

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