[racket] problem with module language

From: Joop Ringelberg (joop.ringelberg at gmail.com)
Date: Fri Mar 23 04:42:16 EDT 2012


Thank you very much. I was reading up on macro's in the online 
documentation and began to understand there is a lot more to it than 
I get the general drift of your explanation without fully understanding 
it, so I will study it some more and read the docs.
Thanks again. I may come back to it.



On 03/23/2012 12:04 AM, Ryan Culpepper wrote:
> On 03/22/2012 01:54 PM, Joop Ringelberg wrote:
>> Hello all,
>> I try to write a small module language (named: biglooModuleSyntax.scm)
>> in order to be able to run some files with a bigloo module statement in
>> Racket. However, I run into a problem I do not understand at all.
>> The (Bigloo) code loads a library ssax-sxml. In Racket, this must be
>> replaced by: (require (planet "sxml.ss" ("lizorkin" "sxml.plt" 2 1))).
>> The Bigloo code also needs some extra functions not provided by sxml.ss,
>> so I require an additional small module (in missing-ssax-functions.scm).
>> To this end, biglooModuleSyntax.scm contains the following macro:
>> (define-syntax library
>>   (syntax-rules ()
>>     [(library ssax-sxml)
>>      (begin
>>        (require (planet "sxml.ss" ("lizorkin" "sxml.plt" 2 1)))
>>        (require "missing-ssax-functions.scm")
>>        )
>>      ]
>>     ))
> The problem is that 'require' is an unhygienic binding form. It 
> introduces the imported names with the lexical context of (IIRC) the 
> module path. That means that the names are imported, but they can only 
> be accessed by references that have the hygiene mark from that 
> expansion of the 'library' macro. You can see this if you put 
> something like '(display SRV:send-reply)' at the end of the 'begin' in 
> the macro definition above; it'll work (if you remove the other syntax 
> errors from the rest of the module), but (unmarked) references to 
> 'SRV:send-reply' in the module's body will still fail.
>> This, however, does not produce the desired results, as shown below. Now
>> there is another macro in biglooModuleSyntax.scm that handles (some)
>> import forms::
>> (define-syntax import
>>   (syntax-rules ()
>>       [(import (moduleName path))
>>        (require path)]
>>     ))
>> and that works just fine. [...]
> Right, because the module path comes from the macro argument, so it 
> doesn't have a mark, so the names that 'require' binds don't have a mark.
> Here's one way to fix your macro:
> (define-syntax (library stx)
>   (syntax-case stx ()
>     [(library)
>      (with-syntax ([(mod ...)
>                     (syntax-local-introduce
>                      #'((planet "sxml.ss" ("lizorkin" "sxml.plt" 2 1))
>                         "missing-ssax-functions.scm"))])
>        #'(require mod ...))]))
> 'syntax-local-introduce' cancels the mark for the currently executing 
> macro, so in the expansion of 'library' the module paths passed to 
> 'require' will be unmarked.
> There are other solutions, too. The general problem is "hygienic 
> macros that expand into unhygienic macros/forms".
> Ryan

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