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
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