On Wed, May 11, 2011 at 1:10 PM, David Van Horn <[email protected]> wrote: > 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 be proven wrong, but I really think you have to use OO techniques to achieve this kind of polymorphism in Racket. I wrote Land of Lisp's orc battle in a few different ways, exploring stylistic variations using Racket's class system, which I had never used before. In one version, I made all the "fields" private, e.g., with (define agility 30), and then explicitly made getter and setter methods. In another version, I specifically used the field construct, e.g., (field [agility 30]) so I wouldn't have to write the getter and setter methods. I wasn't entirely happy with either version. I felt that the code would have been much cleaner if there were some sort of analog to "protected" fields and methods, in which the subclasses could see the fields and methods but outside classes could not. The private-with-accessors version was rather verbose. On the other hand, when working up the field version, I felt the code was somewhat klunky; I was frequently tripped up by the fact that field accessors place the field name first and object second, whereas send places the object first and the method second. Furthermore, fields have their own verbosity -- overriding the initialization of a field in a subclass is a two-step process, for example, and you end up with a lot of inherit-field statements if you want to access fields in a manner that's consistent with new fields. Ultimately, I decided that the field version was the cleaner of the two, but I remain interested in seeing if there is a better way to code it using Racket's constructs. If you're curious, I can send you my code samples off-list. _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
