I like 'copy-prefab-struct' better than my write/read hack. Thanks!
On 11/17/2014 02:06 PM, Gustavo Massaccesi wrote:
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
<[email protected]> 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 <[email protected]> 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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