[Chicken-users] scheme student perplexed by failures ...
hi been working through 'yet another scheme tutorial' [http://www.shido.info/lisp/idx_scm_e.html], and have come up against something i cannot figure out. here is the answer: (define (my-reverse ls) ;(my-reverse-rec ls ())) (my-reverse-rec ls '())) (define (my-reverse-rec ls0 ls1) (if (null? ls0) ls1 (my-reverse-rec (cdr ls0) (cons (car ls0) ls1 works great- reverses the list. and here is my propsed solution: define js-reverse (lambda (ls) ((reverse-rec ls '() (define js-reverse-rec (lambda (asc dsc) ((if (null? asc) dsc ;(js-reverse-rec (cdr asc) (list dsc (car asc))) (js-reverse-rec (cdr asc) (cons (car asc) dsc)) pretty much identical [i left in my initial (incorrect) use of 'list'] but my code errors out! #;38 (js-reverse '(1 2)) Error: call of non-procedure: (dsc) Call history: syntax (quote (1 2)) syntax (##core#quote (1 2)) eval(js-reverse (quote (1 2))) eval[js-reverse] ((reverse-rec ls (quote ( eval[js-reverse] (reverse-rec ls (quote ())) eval[reverse-rec] (null? asc) eval[reverse-rec] (reverse-rec (cdr asc) (cons (car asc) dsc)) eval[reverse-rec] (cdr asc) eval[reverse-rec] (cons (car asc) dsc) eval[reverse-rec] (car asc) eval[reverse-rec] (null? asc) eval[reverse-rec] (reverse-rec (cdr asc) (cons (car asc) dsc)) eval[reverse-rec] (cdr asc) eval[reverse-rec] (cons (car asc) dsc) eval[reverse-rec] (car asc) eval[reverse-rec] (null? asc) -- i have tried putting the dsc in parens, calling 'quote' all to no avail. i hope it's something idiotic that i am just missing. any help is welcome. -- \js [http://or8.net/~johns/] : complete obscure contrariness ___ Chicken-users mailing list Chicken-users@nongnu.org https://lists.nongnu.org/mailman/listinfo/chicken-users
Re: [Chicken-users] scheme student perplexed by failures ...
On Sat, 11 Aug 2012, john saylor wrote: [...] and here is my propsed solution: define js-reverse (lambda (ls) ((reverse-rec ls '() [...] Hello John, your code above lacks an opening paranthesis before the define and it has an extraneous pair of parentheses around the call to reverse-rec. [...] (define js-reverse-rec (lambda (asc dsc) ((if (null? asc) dsc ;(js-reverse-rec (cdr asc) (list dsc (car asc))) (js-reverse-rec (cdr asc) (cons (car asc) dsc)) [...] Again, there is an extraneous pair of parentheses around the use of the if syntax. In Scheme, parentheses are relevant syntax that indicate uses of syntax or calls to procedures. The expression ((reverse-rec ls '())) means Call reverse-rec with the arguments ls and '(), then call the result with no arguments. Since the results of reverse-rec and of the conditional in js-reverse-rec are not procedures but lists, these extra calls you programmed cause the type errors you see. Ciao, Thomas -- When C++ is your hammer, every problem looks like your thumb. smime.p7s Description: S/MIME Cryptographic Signature ___ Chicken-users mailing list Chicken-users@nongnu.org https://lists.nongnu.org/mailman/listinfo/chicken-users
