Thanks for the quick reply! On Tue, Oct 1, 2013 at 2:13 PM, Sam Tobin-Hochstadt <sa...@cs.indiana.edu> wrote: > On Tue, Oct 1, 2013 at 2:01 PM, Greg Hendershott > <greghendersh...@gmail.com> wrote: >> >> tl;dr: Can anyone suggest how I could improve my >> understanding of this? Enough that I could improve Fear of >> Macros and help others, too? Thanks in advance. > > If you've read the resources you described, then you're definitely > prepared to read [Macros that work together], which lays out all the > underpinnings of the Racket macro system (and comes with a redex > model).
Oh, I've read that, too. I should have thought to to re-read that one, as well. Thanks. (I'm not sure a redex model will increase its explanatory value for _me_, but...) > For this specific macro, here's the answer. In your situation, we can > look at all of the syntax objects you tried, and where they're from > (note that the macro stepper would help you here -- look at the > foreground colors of the identifiers): Ah. I hadn't grokked the meaning of the colors before. > `stx`: that's the whole thing, so you're taking the lexical context of > the () around `inner` in the definition of `outer`. Not what you > want. > `k`: that's the occurrence of `inner` in the definition of `outer`. > Still not what we want. > `a`: that's the occurrence of `get` in the definition of `outer`. > Still not what we want. > > `body0`: that's `"hi"`, which is in the _input_ to `outer`. So now we > have lexical context from the use site, which is what we want. Yay! > > However, that's not really the right solution. What if we abstracted > over `outer`: > > (define-syntax-rule (define-as-hi id) (outer id "hi")) > > If we use that, it still won't work, because `"hi"` is now in the > definition of `define-as-hi`, which isn't in the input when you use > the macro. So the right choice for the lexical context is probably > `b`. Well, two things about that: 1. For me, using #'body0 _does_ still work in this example. 2. Plus it seems like it would be really unfortunate if carefully choosing one of #'a or #'b were required -- because that would mean `inner` couldn't be written to be usable by other macros independent of knowing the origin of what they give to `inner`, right? I mean, without knowing if a comes from `outer` or from the usage of `outer`, and likewise for b, and therefore which of a or b to choose carefully. Anyway I'll re-read the paper. Thanks again! ____________________ Racket Users list: http://lists.racket-lang.org/users
