Your reasoning is correct. What's the performance hit?
On Nov 9, 2010, at 9:13 PM, John Clements wrote: > Here's a simple macro that prevents same-argument recursive calls (including > non-tail ones). It's illustrated by a student function that made a teeny > mistake which would result in looping forever, but (using this macro) instead > signals an error. To the best of my limited knowledge, beginner is purely > functional aside from the use of the test functions, right? So any recursive > call with eq? args is guaranteed to loop forever, right? > > Hmm... well, I suppose there's (random)... > > It seems to me that a macro like this one (or even complete memoization) > could potentially reduce student frustration, and would not prevent students > from writing the programs that they wanted to. > > No? > > John Clements > > > > #lang racket > > (require rackunit) > > (define-syntax (define/noloop stx) > (syntax-case stx () > [(_ (funname arg ...) body ...) > #`(define funname > (let () > (define fresh-key (gensym 'funname)) > (define (funname arg ...) > (let* ([most-recent-args (continuation-mark-set-first #f > fresh-key)] > [these-args (list arg ...)]) > (when (and most-recent-args > (= (length these-args) > (length most-recent-args)) > (andmap eq? these-args most-recent-args)) > (error 'funname "recursive call with identical arguments ~s > is guaranteed to run forever." these-args)) > (with-continuation-mark fresh-key these-args > body > ...))) > funname))])) > > ;; NOW I'M A STUDENT: > > ;; only-long-strings : (listof string) -> (listof string) > ;; return a list containing the strings longer than 2 chars > (define/noloop (only-long-strings l) > (cond [(empty? l) empty] > [else (cond [(< 2 (string-length (first l))) > (cons (first l) > (only-long-strings (rest l)))] > [else (only-long-strings l)])])) > > > (check-equal? (only-long-strings (cons "abc" (cons "de" (cons "fgh" empty)))) > (cons "abc" (cons "fgh" > empty)))_________________________________________________ > For list-related administrative tasks: > http://lists.racket-lang.org/listinfo/dev _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/dev
