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

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
>
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