On Sat, Jul 16, 2011 at 7:33 AM, Stephen Bloch <sbl...@adelphi.edu> wrote: > These functions would presumably maintain the invariant that each name > corresponds to at most one sprite in a world. Letting the kids manipulate > the list of sprites directly does NOT maintain that invariant, so I think I > have to do one or the other: either show the kids the list and let them do > what they will, or hide the list and provide only accessors to it.
Yes, you are right that when I said "let the kids manipulate the world directly", I meant that they manipulate the world with functions that enforce certain invariants. So yes, I was thinking that add-sprite would return an error if they try to add a sprite with the same name as one already in the list. I don't think I agree with the idea that if you let kids see the internals of the world structure, you must allow them to program to the internals of the world structure. I personally would be okay with making it all visible so that it's easier to understand, test, and debug, and then say, "But it's hard to manage this kind of complex information without relying on helper functions that maintain certain invariants." Nevertheless, I see your point, especially from a "do it on a first day perspective". With that in mind, making the world structure more opaque might be desirable. The resulting user code using the formulation with strings for sprite names looks very similar to your original formulation, with the exception of forcing the programmer to restate the sprite name in every instruction, even in the common case where you only want to manipulate one sprite. I can think of ways to make this look nicer using macros, but can't think of any way that wouldn't seem like "too much magic" for a beginner programmer. The closest idea I have is to make it so all the built-in drawing functions optionally can take one less parameter (i.e., not the sprite name), in which case the instruction applies to the first sprite in the sprite list. This would make it possible to avoid mentioning the sprite name if you only add one sprite to the world. _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
