Re: Macros: Scheme vs Common Lisp

Craig> At 09:33 AM 8/25/98 +0200, Michael Sperber [Mr. Preprocessor] wrote: Clemens> What is "the right thing"? Craig> Which time? Scoping rules get applied to CL macros twice, once when the Craig> macro function is executed, and again when the result of the macro is Craig> executed. (Let's remember, a macro is just a function that returns an Craig> executable form as its value.) Well, the lexical environment of the macro definition only gets applied to the *definition* of the macro. The lexical environment of the *use* gets applied to its expansion. This is contrary to how lexical scoping normally works, the body of a function (corresponding to the body, i.e. expansion of a macro) is executed in the lexical environment of its definition, not of its use. Craig> It seems that we are approaching the point where we "agree to disagree". Craig> The CL people are used to doing macros the CL way and, Craig> consequently, don't see much benefit to the Scheme method of Craig> doing macros. The Scheme people, on the other hand, are used Craig> to doing macros the Scheme way and don't see much benefit to Craig> the CL method of doing macros. We have each learned to work Craig> around the deficiencies of our preferred Lisp environments. I don't see the difference here. I definitely see the benefits of CL macros, which are very powerful and expressive. There are things you cannot do with Scheme macros because of the constraints of the pattern language. However, there are two nits I have with it: - It is not expressive enough to make macros obey standard CL scoping. - It is too expressive in the sense that even with very simple macros, you have to take special care to avoid potential name capture problems. This is especially concerning as a name capture bug can show up years after the macro definition has been written. Scheme takes the stance that there are two approaches to doing macros: a high-level approach (as standardized by R5RS), and a low-level approach. Ideally, the high-level macro system can be implemented with the low-level system. Designing a decent low-level macro system is very difficult, which is why there's no standard yet. (Even though things have boiled down to 2 or 3 alternatives by now.) CL-style macros are not powerful enough to express Scheme high-level macros. -- Cheers =8-} Chipsy Friede, Völkerverständigung und überhaupt blabla