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

[Benjamin Greenman]

>
> but although I changed else expresion, Racket sometimes returns same
> warning(but now with #f instead of #).


After you call `select-room`, you'll want to check that the result is an
mlist (instead of #f or #) before calling mcdr.

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

[Joan Martin]

Sorry, now with attachment. 

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wumpus 5.rkt
Description: Binary data

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

[Joan Martin]

Thank you very much,
but although I changed else expresion, Racket sometimes returns same 
warning(but now with #f instead of #).

But when I click Run again (sometimes twice or more) it returns what I want. 
That´s the strangest thing.

Could it be an error in Racket settings?

I attach my code, maybe I miss someting else.(please ignore auxiliary 
procedures, warning appears while running (set-traps) and of course set-symbol! 
)

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

[Gustavo Massaccesi]

The most literal translation is

#lang racket
(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)))
(void)  ;<-- explicit "else" branch

You can replace (void) with whatever you want to signal that nothing
is found, like 'not-found, but the more idiomatic versions is to
return #f.

(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)))
#f  ;<-- explicit "else" branch

It's not possible to return "nothing", you must return something, like
(void) or #f or 'not-found. When you are going to ignore the result,
it's better to return (void), but if you want to signal that something
is not found it's better to return #f, or raise an error. (Actually,
you can return "nothing", but let's hide that under the carpet for few
months, because it doesn't play nice with most racket code.)

But now set-symbol has to check that traps is not #f:

(define set-symbol!
  (lambda (symbol number_of_room)
(let ((traps (mcdr(select-room number_of_room environment
  (if traps
 (set-mcdr! traps (mcons symbol (mcdr traps)))
 (void ;<-- explicit "else" branch

Here, it's better to return (void) because you are going to ignore the
result, and set-mcdr! returns also (void). The default printer ignore
voids results and prints "nothing", so (void) will be better here.

(I renamed set-symbol as set-symbol!, because the name of most
functions that mutate their argument have bang. It's not a rule, only
a style recommendation.)

A more idiomatic version use when, unless, and and or, but I'm triying
to make the minimal changes to your code.

Gustavo



On Sun, Apr 10, 2016 at 11:30 AM, Joan Martin  wrote:
> Thank for your answer,
> but how should I change the procedure and ged rid of Racket warning?
> I cant figure it out.
>
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[racket-users] Re: Problem with # and mutable lists

[Joan Martin]

Thank for your answer,
but how should I change the procedure and ged rid of Racket warning?
I cant figure it out.

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

[Joan Martin]

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: #;;(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

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. 


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