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Re: A macro and an unwanted containing list in the resulting form


From: Pascal Bourguignon
Subject: Re: A macro and an unwanted containing list in the resulting form
Date: Thu, 24 May 2007 01:48:12 +0200
User-agent: Gnus/5.1008 (Gnus v5.10.8) Emacs/22.0.99 (gnu/linux)

Sebastian Tennant <address@hidden> writes:

>> And finally, it would be better if we could have several forms in each
>> branches, and if we lost the dot:
>
> "lost the dot" = "do without cons cells" -----+
>                                               |
>                                      +--------
>                                      |
>                                     \/
>>   (defmacro string-case (strexpr &rest alist)
>>     (let ((var-name (gensym)))
>>        `(let ((,var-name ,strexpr))
>>           (cond ,@(mapcar (lambda (each)
>>                          `((equal ,var-name ,(car each)) ,@(cdr each)))
>>                      alist)))))                           ^
>                                                             |
> I see.  ----------------------------------------------------+--+
>                                                                |
>> (macroexpand '(string-case (aref my-strings (incf i))        |
>>                  ("hello"    (message "hi") (message "ho"))  |
>>                  ("goodbye"  (message "bye"))))              |
>                                                                |
> Unique variable name created by (gensym) ---+                  |
>                                             |                  |
>            ---------------------------------+                  |
>            |                                                   |
>           \/                                                   |
>> (let ((G42298 (aref my-strings (incf i))))                   |
>>   (cond                                                      |
>>     ((equal G42298 "hello")                                  |
>>      (message "hi") (message "ho")) <--+---------------------+
>>     ((equal G42298 "goodbye")          |
>>      (message "bye"))))                |
>                                          |
> "several forms in each branch" ----------+
>
> Well Pascal, thank you for an excellent crash-course in macro
> expansion!
>
> I hope my annotations are correct, and not out of place, although
> people not reading this in a fixed-with font will no doubt wonder what
> on earth is going on, not to mention the other nasty things
> line-wrapping news-manglers are inclined to do!

They're correct.


-- 
__Pascal Bourguignon__                     http://www.informatimago.com/

NOTE: The most fundamental particles in this product are held
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