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

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.
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