[racket] About set-car! and set-cdr!

On 5 March 2014 19:54, Matthias Felleisen <matthias at ccs.neu.edu> wrote:

>
> Okay, now see mcons.
>


Neat... mcons, mcar, mcdr, mpair, set-mcar!, set-mcdr!

I notice that there is no mlist. Is there a shortcut similar to '(1 2 3) to
quote a list in a way that makes mutable pairs instead of regular pairs?

Cheers,
Daniel.



>
> On Mar 5, 2014, at 1:53 PM, Daniel Carrera wrote:
>
> I see.
>
> k is '(42 2 3) while l is '(1 2 3). This is what I expected to happen, but
> it is clearly not what was supposed to happen. I just tried the same
> example with Chicken, and for Chicken both k and l are equal to '(42 2 3).
>
> Thanks for the explanation.
>
> Cheers,
> Daniel.
>
> On 5 March 2014 19:24, Matthias Felleisen <matthias at ccs.neu.edu> wrote:
>
>>
>> Try
>>
>>  (define l (list 1 2 3))
>>  (define k l)
>>
>> Now what does (set-car! k 42) do? What should it do?
>>
>>
>> On Mar 5, 2014, at 1:23 PM, Daniel Carrera wrote:
>>
>> But isn't the final effect the same? The pair may be immutable, but I can
>> make a new pair and bind it to the old variable. The main difference that I
>> can see is that what I wrote is a macro, while I believe set-car! is
>> supposed to be a function. That could potentially break code.
>>
>> Cheers,
>> Daniel.
>>
>>
>> On 5 March 2014 19:18, Matthias Felleisen <matthias at ccs.neu.edu> wrote:
>>
>>>
>>> No, set! mutates variable bindings while set-car! mutates cons cells
>>> (the first slot of a data structure).
>>>
>>>
>>> On Mar 5, 2014, at 1:13 PM, Daniel Carrera wrote:
>>>
>>> > Hello,
>>> >
>>> > My understanding is that Racket intentionally does not provide
>>> set-car! and set-cdr! and that this is one of the ways in which Racket is
>>> not fully compatible with Scheme.
>>> >
>>> > Am I right to think that it is trivially easy to add these features to
>>> Racket? Specifically, I'm thinking of:
>>> >
>>> >
>>> > (define-syntax set-car!
>>> >   (syntax-rules ()
>>> >     ((_ l new_car) (set! l (cons new_car (cdr l))))))
>>> >
>>> > (define-syntax set-cdr!
>>> >   (syntax-rules ()
>>> >     ((_ l new_cdr) (set! l (cons (car l) new_cdr)))))
>>> >
>>> >
>>> > Or did I miss something?
>>> >
>>> > Cheers,
>>> > Daniel.
>>> > --
>>> > When an engineer says that something can't be done, it's a code phrase
>>> that means it's not fun to do.
>>> > ____________________
>>> >  Racket Users list:
>>> >  http://lists.racket-lang.org/users
>>>
>>>
>>
>>
>> --
>> When an engineer says that something can't be done, it's a code phrase
>> that means it's not fun to do.
>>
>>
>>
>
>
> --
> When an engineer says that something can't be done, it's a code phrase
> that means it's not fun to do.
>
>
>


-- 
When an engineer says that something can't be done, it's a code phrase that
means it's not fun to do.
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