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 >>> ____________________ >>> Racket Users list: >>> http://lists.racket-lang.org/users >> >> ____________________ >> Racket Users list: >> http://lists.racket-lang.org/users >> > > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
