On Wed, Nov 24, 2010 at 11:05 AM, Robby Findler <ro...@eecs.northwestern.edu> wrote: > On Wed, Nov 24, 2010 at 12:55 PM, Joe Marshall <jmarsh...@alum.mit.edu> wrote: >>> On Nov 20, 2010, at 6:58 PM, Matthias Felleisen wrote: >>> >>>> Hygiene is a technical term. The idea is roughly that >>>> the __macro system__ (as a whole) should respect the >>>> lexical structure of your program. >> >> It is somewhat unfortunate that the name `hygiene' has caught >> on here. It really ought to be called `lexical scoping' (with the >> understanding that macros have no special permission to violate >> lexical scope any more than lambda bindings do). > > You know about Oleg's macro called, bind-x-to-5 that has one > subexpression does exactly its name claims, but in a hygenic macro > system?
Yes. And I have to admit that I've used this trick myself to write some questionable (but useful) macros. > That example suggests to me that what is called hygiene should > probably not be called lexical scope. I'm not sure. Certainly the r5rs hygienic macro system has the ability to mimic non hygienic use. But consider these statements: `Any sufficiently powerful macro system can appear to break lexical scope.' `Any macro system weak enough to preserve lexical scope is too weak to do X' (where X is something one would normally expect to be able to do with a macro) I have not seen a claim or proof of either of these or a denial of either. In other words, perhaps there is a macro system that is both sufficiently powerful and general that we'd all agree it qualifies as a macro system, yet is weak enough that we all agree it cannot violate what we consider `lexical scope'. Alternatively, perhaps one could argue that our understanding of `lexical scope' is too narrow and primitive, and that Oleg's bind-x-to-5 macro is simply a surprising, but consequential artifact of the true meaning of `lexical scope'. I rather prefer the former alternative. The current macro system is an amazing piece of insight and engineering, but it does have a few dark corners: Oleg's bind-x-to-5 is one, accidental export of internal bindings is another. -- ~jrm _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
