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

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.
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