Ken Tilton <firstname.lastname@example.org> wrote:
| Matthias Buelow wrote:
| > Ken Tilton <email@example.com> wrote:
| >>The use case described by the OP is a good one for special variables.
| >>Those are not as unclean as classic globals.
| > Special variables are a poor and obscure alternative to functional
| > arguments and shouldn't be used in place of them.
| I meant to ask, when /should/ they be used?
For me, the two classic use cases are:
1. Things that work more-or-less like the ANSI-defined specials
such as *READ-BASE*, that is, large numbers of miscellaneous
global parameters that you *usually* want to just leave alone
but sometimes want to rebind for the dynamic extent of one
subtree of function calls.
Thanks *goodness* for specials for this case!! Passing the ~20 (!)
or so specials that affect the Lisp printer [see CLHS 18.104.22.168]
around in every function call just to enforce some kind of
functional "purity" would simply be insane.
2. Things that hold "this request" (or things *about* "this request")
for request/response protocols, especially when using some kind
of threading so that there are multiple "this" objects live at
any one instant. E.g., my personal web infrastructure has a special
named *HTTP-REQUEST*, a structure of type HTTP-REQUEST (duh!) that
holds all the gunk related to one -- freshly bound by the thread
that gets spawned upon each new ACCEPT on port 80. As the request
goes through the steps of its processing, various additional bits
of gunk get added to (or deleted from) the HTTP-REQUEST object.
After responding, the thread dies, and on its way out *HTTP-REQUEST*
gets unbound and the object gets GC'd, eventually. [No, I don't try
to re-use them, though I do (sometimes) re-cycle the connections
they had open to PostgreSQL.]
Another similar, though non-threaded, case is a discrete
event simulator, where specials with names like *CURRENT-TIME*,
*CURRENT-PROCESS*, *CURRENT-EVENT*, *CURRENT-NET*, *CURRENT-
DRIVER*, etc., may get bound & re-bound by the simulator's
main event loop [possibly a *very* large number of times
for one tick of *CURRENT-TIME*!], so that any given instance
of an object being simulated has a lexically-constant but
dynamically-varying environment. [Yes, Kenny, such things
should probably all be re-written in Cells. Humor me.]
Rob Warnock <firstname.lastname@example.org>
627 26th Avenue <URL:http://rpw3.org/>
San Mateo, CA 94403 (650)572-2607