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Maciej Stachowiak <email@example.com> writes: > This differs most markedly from code I posted a while back (with a > similar interface but rather different implementation) in that setter > is a macro, and setf! attempts to determine the setter of the passed > procedure at macro-expansion time, not run-time. I think this is a bad > idea because it _appears_ to be dynamic, by being based on the > procedure object rather than the symbol, unlike Common Lisp, but it > actually isn't because of the macro-expansion property described > above. This mechanism wasn't based on your code, but was, what was supposed to be, an improvement on the mechanism in STk which also is Common Lispish. > For instance, code like this: > > (define (mutate-cons-cell accessor cell new-value) > (setf! (accessor cell) new-value)) > > (define my-cell (cons 3 4)) > (define my-cell (cons 3 4)) > > (mutate-cons-cell car my-cell 'x) > > (mutate-cons-cell cdr my-cell 'y) > > Will silently do the shockingly unexpected wrong thing and leave > my-cell with a value of (y . 4), whereas a fully Common Lisp-like setf > would have complained, and my code would have done the right thing. I > think the current code is thus the worst of all possible worlds, > semantics-wise. (I predicted this on reading the code, but testing > confirms). I agree. I think it should complain. I made a thought error (Christian's code was OK, but I added the strange stuff) when implementing this. I must have thought about dynamic properties one second and static in the other. Sorry. :( > I also don't think it is useful to allow getters and setters to be > macros (and obviously that can't be done with the non-memoizing > version where setter is a procedure since code written to use that > with macros would be utterly uncompilable). Is there a particular > purpose that was envisioned for? It was supposed to be an implementation of Common Lisp's generalized locations, where getters and setters can be special forms. On the other hand I don't see a strong need to have dynamic behaviour. There is a reason why `set' wasn't included in RnRS. E.g., in your example above I think it would be more natural to write (mutate-cons-cell set-car! my-cell 'x) or (mutate-cons-cell (setter car) my-cell 'x) and (define (mutate-cons-cell setter cell new-value) (setter cell new-value)) (I'm aware that you wrote the code above to demonstrate my bug, which it excellently did, not to show a typical usage of dynamic setf!.) In fact, the point of doing a thing like (set! (car x) 3) is to get better syntax. It's supposed to be "syntactic sugar". If you let the cadar be a variable, it seems to defeat the purpose to yield clearer code. I'll change the implementation to a purely static one. /mdj