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

Thanks!


On 6 March 2014 01:00, Alexander McLin <alex.mclin at gmail.com> wrote:

> Daniel, I think it'll be worthwhile to direct your attention to this blog
> entry which goes into more details about why mutable pairs were removed in
> the first place.
>
> http://blog.racket-lang.org/2007/11/getting-rid-of-set-car-and-set-cdr.html
>
>
> On Wed, Mar 5, 2014 at 4:15 PM, Jens Axel Søgaard <jensaxel at soegaard.net>wrote:
>
>> (require compatibility/mlist)
>> (mlist 1 2 3)
>>
>> See
>> http://docs.racket-lang.org/compatibility/mlists.html?q=mlist#%28def._%28%28lib._compatibility%2Fmlist..rkt%29._mlist%29%29
>>
>> 2014-03-05 22:06 GMT+01:00 Daniel Carrera <dcarrera at gmail.com>:
>> >
>> > 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.
>> >
>> > ____________________
>> >   Racket Users list:
>> >   http://lists.racket-lang.org/users
>> >
>>
>>
>>
>> --
>> --
>> Jens Axel Søgaard
>>
>> ____________________
>>   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.
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