David Rush scripsit: > Which is all well and good, but what does any of this have to do with > macros? Early Lisps treated macros as syntactic abstractions and
Rather, early Lisps treated them as *procedural* abstractions and ... > therefore incorporated their reduction into the APPLY function. > This led us to FEXPRs, FEXPRs were there from the Lisp 1.5 days; macros came much later. > However, it seems that macros could also be > viewed as syntactic *extensions* and as such be more properly > considered as part of EVAL's functional remit. That is in fact the realization that led to the abolition of user-written FEXPRs from Common Lisp and Scheme: that macros do everything FEXPRs do that's worthwhile, and are far more tractable. (You still need some primitive FEXPRs, now renamed "syntax" in Scheme and "special forms" elsewhere.) > From this view it seems that we ultimately derive hygienic macros - > as their reduction can be accomplished entirely in terms of textual > manipulation with only minimal (actually zero I think) reference to > anything outside of the syntactic reduction machinery in the MCI. Well, it depends. Certainly true for syntax-rules, but low-level macro systems (hygienic or not) depend on the whole Lisp system, since they provide access to the entire language to construct the macro. (ISLisp is an exception; only a subset of ISLisp is available at macro expansion time; roughly speaking, nothing that can side-effect the environment in which the macro expander is executing.) > Now it is interesting to note that hygienic macros float the syntactic > extension functionality outside of the entire EVAL/APPLY loop and into > a conceptually separate pass. That's true of any macros whatsoever; it is, in some sense, the point of macros. It's possible to intertwine that pass with ordinary evaluation, as SCM shows, but at a high price in conceptual machinery. > This makes environmental manipulations completely impossible to the > macro processor. Again, not so. That's a matter of level. C macros can't do environmental manipulation because they are high-level, not because they are hygienic (they aren't). It's very important not to conflate hygiene with level -- they are utterly orthogonal. -- Yes, chili in the eye is bad, but so is your John Cowan ear. However, I would suggest you wash your co...@ccil.org hands thoroughly before going to the toilet. http://www.ccil.org/~cowan --gadicath _______________________________________________ r6rs-discuss mailing list r6rs-discuss@lists.r6rs.org http://lists.r6rs.org/cgi-bin/mailman/listinfo/r6rs-discuss
