[racket] Macros and literal-id

What is the reason for using syntax-rules instead of syntax-parse? (To maintain RnRs compatibility?)

RAC

On Aug 9, 2011, at 8:15 AM, Neil Van Dyke <neil at neilvandyke.org> wrote:

> BTW, for future readers... in this example:
> 
> (define-syntax if*
> (syntax-rules (then else)
>   ((_ ?test (then ?consequent) (else ?alternate))
>    (if ?test ?consequent ?alternate))))
> 
> The "?" part of the pattern variables is just a naming convention of individual programmers.  The "?" is the first character of the pattern variable identifier, not special syntax.
> 
> I've tried various naming conventions here, including "?".  Once my macro clauses got large, with a mix of a number of pattern variables as well as Racket identifiers to be captured, I settled on making the pattern variable identifiers all-upppercase:
> 
> (define-syntax if*
> (syntax-rules (then else)
>   ((_ TEST (then CONSEQUENT) (else ALTERNATE))
>    (if TEST CONSEQUENT ALTERNATE))))
> 
> This example is too small too appreciate the difference, with "if" being the only Racket identifier.  Imagine you had a "syntax-rules" clause with block of code with 20 Racket identifiers in it, and 3 pattern variables.  You'd want the pattern variables scattered throughout this to really stand out.  At least on my screen, all-caps does that better than "?", and all-caps also takes up less horizontal space, which can get to be an issue with patterns that you're trying to keep to a single line for readability symmetries.  Anyone else looking at the code will instantly see which convention you're using, so you can use whatever you want.
> 
> I have a few more useful conventions for "syntax-rules", from which people can pick&choose depending on the situation and preferences, but no time today to type them.  Typing them up might have to wait for pounding out a chapter of my tentative practical Racket book.  (Not trying to sell anything; book will be free in Scribble format, probably as a PLaneT package that installs the book into your searchable local copies of Racket documentation.)
> 
> -- 
> http://www.neilvandyke.org/
> 
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