On 5/11/11 4:03 PM, Nadeem Abdul Hamid wrote:
On Wed, May 11, 2011 at 3:45 PM, David Van Horn<[email protected]> 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
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