[racket] hygienic read-macros (was: module-level definitions vs. local definitions and how binding works)

Mon, 28 Jul 2014 13:51:12 -0400 от "Alexander D. Knauth" <alexander at knauth.org>:
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>On Jul 28, 2014, at 11:41 AM, Roman Klochkov < kalimehtar at mail.ru > wrote:
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>> I tried to make it with module .....
>
>> (require 'm)
>> 
>> Works just fine
>
>Yes because GENSYM was provided by ‘m, but if it’s a #lang then that doesn’t matter unless you specifically require experiment-with-hygenic-reader-extensions/lang/reader.  And as I say below, I shouldn’t have to do that, and I shouldn’t have to provide GENSYM either.  Read macros return source code. So there are no other way.
>
>
>> I'm not very proficient with lang, but you should require module with `make-reader' to you last module.
>> 
>> I installed your package via `raco pkg install git://github.com/AlexKnauth/experiment-with-hygenic-reader-extensions', but
>> 
>> $ /usr/racket/bin/racket try-it.rkt
>> default-load-handler: cannot open module file
>>  module path: experiment-with-hygenic-reader-extensions/lang/reader
>>  path: /home/monk/.racket/6.0.1/pkgs/experiment-with-hygenic-reader-extensions/experiment-with-hygenic-reader-extensions/lang/reader.rkt
>>  system error: No such file or directory; errno=2
>> 
>> Maybe something wrong.
>
>Hmm, it doesn't do that for me. 
>But I’m just wondering, when you look at …/pkgs/experiment-with-hygenic-reader-extensions, does it have …extensions/langreader.rkt instead of …extensions/lang/reader.rkt ?  Yes, …extensions/langreader.rkt
>
>
>> I think you should add (require (submod experiment-with-hygenic-reader-extensions/lang/reader reader)) to try-it. Or make another module with make-reader, which will be required by lang and result program.
>
>Well another good thing about hygienic macros and syntax-objects is encapsulation so that a macro can use GENSYM without having to provide GENSYM, so that GENSYM does not have visible to the user of the macro.  So you shouldn’t have to add a (require experiment-…/lang/reader) to try-it.rkt. Yes. Because macros in Scheme/Racket don't build code, they build syntax. But read-macros build source code (in the meaning of "program text"). Don't know why, maybe because syntax context, returned by read-syntax should be current context, not the context of language definition.
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>But for some reason that’s not always working for reader extensions. 
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>It seems to work when the binding is from racket/base and there’s no local binding conflicting with it (but module-level bindings conflicting with it are ok), but not when it’s from a different module other than racket/base?  It always work, when the binding is from current module (if it doesn't require racket/base, then its bindings won't work also).
>
>
>I don’t know. 
>
>> 
>> Mon, 28 Jul 2014 11:08:20 -0400 от "Alexander D. Knauth" < alexander at knauth.org >:
>>> Well that works fine when it’s in the same module (or namespace), but when I put it in a #lang, it breaks and I get this error:
>>> experiment-with-hygenic-reader-extensions/try-it.rkt:
>>> #lang experiment-with-hygenic-reader-extensions
>>> $
>>> ; g15667: unbound identifier in module in: g15667
>>> 
>>> That’s one problem with unhygienic reader extensions. 
>>> Since you used (datum->syntax #f ‘(GENSYM …)), it eventually took on the lexical context of try-it.rkt, where the GENSYM name isn’t there.
>>> 
>>> Though strangely, if I put some lexical context on GENSYM by using (datum->syntax #f `(,#'GENSYM …)), then it gives me this error:
>>> experiment-with-hygenic-reader-extensions-2/try-it.rkt:
>>> #lang experiment-with-hygenic-reader-extensions-2
>>> $
>>> ; require: namespace mismatch;
>>> ; reference to a module that is not available
>>> ;  reference phase: 0
>>> ;  referenced module: “…/experiment-with-hygenic-reader-extensions-2/lang/reader.rkt"
>>> ;  referenced phase level: 0 in: g24
>>> 
>>>  https://github.com/AlexKnauth/experiment-with-hygenic-reader-extensions
>>> 
>
>> 
>> 
>> -- 
>> Roman Klochkov
>


-- 
Roman Klochkov
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