Ya, vanilla structs can't do that. It is possible for there to be no syntactic connection between a parent's definition (and uses of it) and an arbitrary child definition, so we can't even use gross expand phase state to communicate.
I'm a big believer in prototype object systems as a foundation of functional OO like this. Here's a pretty good implementation I made a while back: https://github.com/jeapostrophe/exp/blob/master/foo/test.rkt Jay 2011/5/11 David Van Horn <dvanh...@ccs.neu.edu>: > On 5/11/11 4:03 PM, Nadeem Abdul Hamid wrote: >> >> On Wed, May 11, 2011 at 3:45 PM, David Van Horn<dvanh...@ccs.neu.edu> >> wrote: >>> >>> Is there a good way to do a functional struct updates with subtypes of a >>> structure? >> >> There was some discussion about it a while ago in reference to >> struct-copy. The documentation has been updated with some useful >> examples, including usage of the new #parent parameter: >> http://docs.racket-lang.org/reference/struct-copy.html > > I actually fat-fingered sending that email, so let me give some more > context. The #:parent parameter doesn't really help me. > > Here are some struct definitions and a function, based on Land of Lisp: > > (struct monster (id [hit #:mutable]) #:transparent) > (struct orc monster (club) #:transparent) > (struct hydra monster () #:transparent) > (struct slime monster (sliminess) #:transparent) > (struct brigand monster () #:transparent) > > (define (damage! m n) > (set-monster-hit! m (max 0 (- (monster-hit m) n)))) > > I'd like to write a functional version of damage!, but it looks like I > can't. The best I can do is write an ugly, not-really-generic version like > this: > > (define (damage m n) > (let ((h (max 0 (- (monster-hit m) n)))) > (cond [(orc? m) (struct-copy orc m [hit #:parent monster h])] > [(hydra? m) (struct-copy hydra m [hit #:parent monster h])] > [(slime? m) (struct-copy slime m [hit #:parent monster h])] > [(brigand? m) (struct-copy brigand m [hit #:parent monster h])]))) > > David > _________________________________________________ > For list-related administrative tasks: > http://lists.racket-lang.org/listinfo/users > -- Jay McCarthy <j...@cs.byu.edu> Assistant Professor / Brigham Young University http://faculty.cs.byu.edu/~jay "The glory of God is Intelligence" - D&C 93 _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
