[racket] using struct-copy with struct subtypes?

This version is only works with #:prefab, but don't use the string
representation so it has less problems with strange values that don't
have a good string representation:

(define (copy-prefab-struct str)
  (apply make-prefab-struct (vector->list (struct->vector str))))


And this other version works with #:prefab and #:transparent structs
(and some additional structs when you have the right inspector):

(define (copy-struct str)
  (define-values (str-struct-info _) (struct-info str))
  (define str-maker (struct-type-make-constructor str-struct-info))
  (apply str-maker (cdr (vector->list (struct->vector str)))))


On the other hand, these versions do a sallow copy. Only the outer
struct gets a new instance, it reuses the values of the fields.
David's version also makes a new instance of all the fields (that are
not "singletons" like 7, #t, ...).

Gustavo



On Mon, Nov 17, 2014 at 3:23 PM, David Vanderson
<david.vanderson at gmail.com> wrote:
> I ran into the same problem.  Since my structs are all #:prefab, I copy them
> like this:
>
> (define (copy s)
>   (read (open-input-string (with-output-to-string (lambda () (write s))))))
>
> Probably not the best, but works for me.
>
> Thanks,
> Dave
>
>
> On 11/17/2014 12:25 PM, Sam Tobin-Hochstadt wrote:
>>
>> Unfortunately, you can't really do this in a general way for structs.
>>
>> There are a couple possible solutions:
>>   - use objects/classes and implement a `clone` or a `replace` method
>>   - use generics, with a `clone` or `replace` generic function
>>
>> Sam
>>
>> On Mon, Nov 17, 2014 at 12:22 PM, Matthew Butterick <mb at mbtype.com> wrote:
>>>
>>> This seems rudimentary, but I can't figure it out. `struct-copy` requires
>>> you to specify the id of the struct type, like so:
>>>
>>> (define copied-struct (struct-copy struct-type-id instance-of-struct))
>>>
>>> The docs for struct-copy say "subtypes can be copied as if they were
>>> supertypes, but the result is an instance of the supertype". [1]
>>>
>>> OK, but suppose I want the opposite behavior: I want to make a
>>> struct-copying function that accepts instances of a supertype or subtype,
>>> but outputs a struct that's the same type as the input instance (not the
>>> supertype).
>>>
>>> Possible? Or not possible with structs, because that's what objects &
>>> classes are for?
>>>
>>>
>>> [1] http://docs.racket-lang.org/reference/struct-copy.html
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