[Chicken-janitors] #702: macro expansion time category specification filter for log5scm

[Chicken Trac]

#702: macro expansion time category specification filter for log5scm
-------------------------------+--------------------------------------------
 Reporter:  syn                |       Owner:  certainty
     Type:  enhancement        |      Status:  new      
 Priority:  not urgent at all  |   Milestone:  4.8.0    
Component:  extensions         |     Version:  4.7.x    
 Keywords:  log5scm            |  
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 The attached patch adds support for defining a category specification
 that's matched against at macro expansion time. If it matches, the {{{log-
 for}}} macro will expand to {{{(void)}}} and thus not generate any runtime
 overhead. The specification is passed through the environment variable
 {{{LOG5SCM_CAT_SPEC}}}. To implement this feature it was necessary to
 split the log5scm module into two modules (log5scm-lolevel and log5scm) so
 that the category spec matching code can be used both at macro expansion
 and run time.

 For example, consider this program:

 {{{
 (use posix log5scm)

 (define-category foo)
 (define-category bar)
 (define-category baz)

 (start-sender main
               (port-sender (current-output-port))
               (category bar))

 (log-for (foo) (begin (sleep 2) "some expensive foo bar"))
 (log-for (bar) "some bar")
 }}}

 Even though we only have a sender that's interested in log messages of the
 category bar the program will wait for 2 seconds because senders may
 change at runtime. The (sleep 2) is contrived of course, imagine building
 some elaborate data structure for debugging puproses there instead. Now
 invoking this program with {{{LOG5SCM_CAT_SPEC=bar}}} will completely
 eliminate the {{{(log-for (foo) ...)}}} call (well, replace it with
 {{{(void)}}} in fact). This can be useful for stripping debug messages
 completely from a compiled program by passing something like
 {{{LOG5SCM_CAT_SPEC='(not debug)'}}}.

-- 
Ticket URL: <https://bugs.call-cc.org/ticket/702>
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