[racket] Again on bindings visibility in eval

This is an excellent explanation. Thanks. 

I'd like to add a little-understood that Lisp-style macros as found 
in Common Lisp and Clojure are LESS expressive than Racket macros. 

In essence, Racket (and Scheme) chose to use a data structure called
SYNTAX instead of S-EXPRESSION but one can argue that the former is 
'just' a generalization of the latter. (Plus we made it an 'abstract'
type instead of a transparent one so that the expander can preserve
a large number of invariants -- which programmers usually lose track
off.) 

-- Matthias




On Jul 14, 2011, at 7:35 AM, Thomas Chust wrote:

> Markku Rontu wrote:
>> [...]
>> On the topic of macros, I find it a bit confusing that to metaprogram in
>> Racket, I must use such a different language of macros (with
>> syntax-rules et al.), instead of being able to manipulate common data
>> structures with plain old Racket functions.
>> [...]
> 
> Hello,
> 
> this is not really true. The domain specific languages of syntax-rules
> and syntax-case macro transformers exist purely for your convenience.
> You can always manipulate syntax objects directly, all the necessary
> procedures are available and documented in section 11 of the Racket
> reference manual.
> 
>> [...]
>> The approach in Clojure,
>> that the code is just a common data structure, easily manipulatable with
>> Clojure code, is intriguing.
>> [...]
> 
> The approach taken by Clojure and Common Lisp looks simple but lacks
> support for hygienic macros and is therefore error prone and actually
> not at all simple, if not impossible, to use correctly.
> 
> While Clojure *mitigates* the problems of non-hygienic macros using
> namespaces and a shorthand syntax for freshly generated identifiers, it
> doesn't *solve* the problems. Racket's macro system, on the other hand,
> does solve the problems and since that involves some heavy lifting, it
> may seem more complicated at first glance.
> 
> Ciao,
> Thomas
> 
> 
> -- 
> When C++ is your hammer, every problem looks like your thumb.
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