[racket] Another Canonical Use of Macros?

Say you want a three-state machine where the states are labeled 'go, 'stop, 'warning and you use a stream of signals to switch between states say 'button-down 'x 'y, you can eliminate the quotes from the surface syntax with macros that place the quotes on the right pieces. Instead of symbols you can think string and/or nests of these. 




On Nov 25, 2013, at 9:23 AM, Ben Duan <yfefyf at gmail.com> wrote:

> > - changing evaluation order,
> > - implementing a data sublanguage, and
> > - creating new binding forms.
> 
> Thanks for the insights. But what does "data sublanguage" mean?
> 
> 
> On Sat, Nov 23, 2013 at 2:46 PM, John Clements <clements at brinckerhoff.org> wrote:
> I'm preparing a 10-minute lightning talk on hygienic macros in rust (preview: I'm barely going to *mention* hygiene), and in the process, I've been surveying some of the Rust macros, and roughly categorizing them in terms of the "three canonical categories" that Matthias described--apologies if I'm misrepresenting him/you:
> - changing evaluation order,
> - implementing a data sublanguage, and
> - creating new binding forms.
> 
> Some of the Rust macros seem to fall into a fourth category, which arises from the fact that certain things are not expressions:
> 
> - abstracting over things that are not expressions.
> 
> For instance:
> 
> cmp_impl!(impl Eq, eq, ne)
> cmp_impl!(impl TotalEq, equals)
> cmp_impl!(impl Ord, lt, gt, le, ge)
> cmp_impl!(impl TotalOrd, cmp -> cmp::Ordering)
> 
> Each of these expands into a top-level "impl" declaration, extending implementations of, e.g., Ord, from type T to type Ratio<T>.
> 
> More generally, it seems to me that every time you constrain first-class-ness by making things not-first-class (e.g. module-level stuff in Racket), you will be required to use macros to abstract over these things.
> 
> Thoughts?
> 
> Back to writing my talk...
> 
> John
> 
> 
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