[racket] struct-copy with sub-types

Ok, yes, on second thought, I guess it is fine. With your fix,

(struct-copy sub (sub 1 2 3) [c 4])   ==> (sub 1 2 4)
(struct-copy sub (sub 1 2 3) [b 4])   ==> (sub 1 4 3)
(struct-copy base (sub 1 2 3) [b 4]) ===> (base 1 4)

is that right? So that sentence applies to the third situation.

I guess when the second one didn't work, I assumed somehow that "id"
could only be the struct type where the field was actually defined. So
it was a bug. Sorry for the confusion,

--- nadeem


On Fri, Mar 25, 2011 at 11:08 AM, Robby Findler
<robby at eecs.northwestern.edu> wrote:
> The phrase "immediate instance of id" is meant to suggest that it
> doesn't have the extended fields.
>
> What would you have written?
>
> Robby
>
> On Fri, Mar 25, 2011 at 10:04 AM, Nadeem Abdul Hamid <nadeem at acm.org> wrote:
>> In that case, the documentation should probably be updated clearly to
>> note this different behavior -- the last sentence of the current:
>> http://docs.racket-lang.org/reference/struct-copy.html
>> says:
>>
>> "
>> (struct-copy id struct-expr [field-id expr] ...)
>> ....
>> The result of struct-expr can be an instance of a sub-type of id, but
>> the resulting copy is an immediate instance of id (not the sub-type).
>> "
>>
>> Which is why I didn't interpret the behavior as a bug, but a
>> limitation (albeit weird) of struct-copy.
>>
>> --- nadeem
>>
>>
>> On Fri, Mar 25, 2011 at 10:49 AM, Robby Findler
>> <robby at eecs.northwestern.edu> wrote:
>>> Hi, and sorry for the delay. I've pushed a fix for this, altho it is
>>> more like a new feature since the code below still doesn't work--- you
>>> have to explicitly name the struct that had the field when it isn't
>>> the first argument to struct-copy.
>>>
>>> Robby
>>>
>>> On Fri, Mar 25, 2011 at 7:52 AM, Pierpaolo Bernardi <olopierpa at gmail.com> wrote:
>>>> On Wed, Mar 23, 2011 at 00:49, Nadeem Abdul Hamid <nadeem at acm.org> wrote:
>>>>> Is there anything like struct-copy that works to functionally update a
>>>>> field value in a structure, where the field happens to be defined in
>>>>> the super-type? In the example below, I want to "functionally update"
>>>>> a field of the sub structure inherited from the base definition...
>>>>>
>>>>>
>>>>> (struct base (a b) #:transparent)
>>>>> (struct sub base (c) #:transparent)
>>>>>
>>>>> (define S (sub 1 2 3))
>>>>>
>>>>> (struct-copy sub S [c 4]) ; works
>>>>> ; doesn't work: (struct-copy sub S [b 4])
>>>>> (struct-copy base S [b 4]) ; produces a base, not sub
>>>>
>>>> No answers?
>>>>
>>>> it looks to me that
>>>>
>>>> (struct-copy sub S [b 4])
>>>>
>>>> should work, but it doesn't.  Isn't this a bug?
>>>>
>>>> If it is not considered a bug, maybe the docs need a clarification?
>>>>
>>>> Cheers
>>>> P.
>>>> _________________________________________________
>>>>  For list-related administrative tasks:
>>>>  http://lists.racket-lang.org/listinfo/users
>>>>
>>>
>>
>