> > > (define default (gensym)) > > (define (default? x) (equal? x default)) > > (Using `eq?' here is a common idiom.) >
ah, of course, thanks for the reminder. > > In any case, the real point here is not the macro -- it's the > existence of a special `none' value that marks an unsupplied value. > absolutely, it's just a helper. > It's potentially a convenient thing to have (which I wish for every > time I do that `gensym' dance), but it gets tricky with the fact that > you might want to actually use that value -- same as the undefined > value (which you get from (letrec ([x x]) x)) which could be used for > the same purpose. > you mean because of shadowing? (I'm not sure I understood what you mean) Laurent
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