Re: [racket-users] Problem with # and mutable lists

[Benjamin Greenman]

On Sun, Apr 10, 2016 at 7:29 AM, Joan Martin  wrote:

> 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: #;;(returned by select-room ?)
>

This problem may be because your first if-statement in select-room is
missing an else branch.



>
> In Racket documentation I found : " # is returned by most forms and
> procedures that have a side-effect and no useful result."
>

These functions return # because the useful result is usually the
argument you passed the procedure in the first place. The example below
uses set-mcar! to update a list -- the useful result is the original list,
not the result of set-mcar!

#lang racket/base

(define lst (mcons 1 (mcons 2 '(

(displayln lst)
;; (1 2)

(set-mcar! lst 3)

(displayln lst)
;; (3 2)

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Re: [racket-users] Problem with # and mutable lists

[Pierpaolo Bernardi]

On Sun, Apr 10, 2016 at 1:29 PM, Joan Martin  wrote:
> 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)) )

In Racket this doesn't even compile. So I assume you must be using
some other programming language.

Anyway, what does this function return when (null? l) ?

If you had used Racket, it would have shown you immediately where's the problem.

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[racket-users] Problem with # and mutable lists

[Joan Martin]

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: #;;(returned by select-room ?)

In Racket documentation I found : " # 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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