What are the advantages of scheme macros?

问题:

Why would anyone prefer Scheme macros over Common Lisp macros (and I genuinely want to know too, I'm not trying to be a troll)?

My experience as a Lisp newb is that Common Lisp style macros are much easier to learn than Scheme's macros. I have yet to see any advantages to Scheme's macros, but of course that doesn't mean they don't exist.

I do know that Scheme macros are "hygenic", but I'm still not convinced this is worth the additional complexity. On the other hand though, there obviously are people that are convinced that this is necessary, otherwise there wouldn't be implementations of Scheme macros in Common Lisp.

To make a long story short, can someone defend Scheme's macros to me?

回答1:

Scheme macros introduce two, essentially orthogonal, concepts: hygiene and pattern matching. Hygiene is less important in a lisp2 like Common Lisp. The pattern matching language captures many of the common macro idioms, but has the problem that it is essentially a different language from scheme. Probably the best introduction to scheme's macros, along with some of the rationale behind them is Shriram Krishnamurthi's PLAI chapters 36 and 37.

I suspect that the reason people write scheme style macro systems in common lisp is more for the pattern matching than for the hygiene.

回答2:

Because they use a different, non-Scheme language, Scheme macros are less powerful than Common Lisp macros in the almost-formal sense: you can do arbitrary compile-time computation with them, but it's hairy and convoluted. It's a lot like the argument for not using set!: less-powerful set!free languages produces less buggy code in exchange for awkward handling of state. Any time you trade power for discipline, you are betting that you will be able to build more complex systems in the long run.

That's the best argument I've seen for Scheme macros over Common Lisp ones: if you are building a complex language on top of Scheme, you are less likely to introduce subtle macro bugs if you stick with the standard macro system.

Personally, I don't build big languages using macros, so I prefer Common Lisp macros. I find them much easier for small jobs and avoiding variable capture etc isn't a big deal on a small scale.

回答3:

Scheme macros preserve referential transparency.

Quoting the "Guile Reference Manual" 6.10.2.2 Hygiene:

syntax-rules macros (..) preserve referential transparency. When you read a macro definition, any free bindings in that macro are resolved relative to the macro definition; and when you read a macro instantiation, all free bindings in that expression are resolved relative to the expression.

This property is sometimes known as hygiene, and it does aid in code cleanliness. In your macro definitions, you can feel free to introduce temporary variables, without worrying about inadvertently introducing bindings into the macro expansion.

Standard scheme offers syntax-rules and syntax-case.