Dne neděle 10. dubna 2016 13:29:43 UTC+2 Joan Martin napsal(a): > Hello, > I try to write a programme (a game), where I use lots of mutable lists (named > environment).Something like this: > (define environment > (mcons (mcons (mcons 1 (mcons (mcons 2(mcons 5(mcons 8 '())))'() )) > (mcons (mcons 2 (mcons (mcons 1(mcons 3(mcons 10 '()))) '() )) > ...... > '() )) > It should represent a 2D dekadedron. > And for easy manipulation I wrote a procedure: select-room and set-symbol. > > (define select-room (lambda (number rooms) > (let loop ((l (mcar rooms))) > (if (not (null? l)) > (if (= (mcar(mcar l)) number) > (mcar l) > (loop (mcdr l)) ))))) > (define set-symbol > (lambda (symbol number_of_room) > (let ((traps (mcdr(select-room number_of_room environment)) )) > (set-mcdr! traps (mcons symbol (mcdr traps)) )))) > > I use it without problems : (select-room 2 environment) in this case. > But when I use it in another procedure (for example: set-symbol), sometimes > Racket returns this: > > mcdr: contract violation ;;(mcdr in let in set-symbol) > expected: mpair? > given: #<void> ;;(returned by select-room ?) > > In Racket documentation I found : " #<void> is returned by most forms and > procedures that have a side-effect and no useful result." > > But for me it REALLY has useful result. It´s really strange it appears only > sometimes. And select-room returns a mutable list, so I´m really confused. > > I would be glad for any help. > > Thanks > Joan
I learn Scheme and compiling in Racket with language Pretty Big isn´t problem. Select-room should be iterative procedure - it goes through list. (null? l) tells it is at the end of the list and it returns nothing, because I don´t have second else expresion in IF. -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
