Re: Question named-readtables

   From: Luis Oliveira <[email protected]>
   
   I have a quick question about Allegro's named-readtable facility. What
   is the rationale for restricting the names to keywords? It seems to me
   that using symbols would help avoid naming conflicts between similarly
   named readtables from multiple codebases.

I'm the one who implemented Allegro's named-readtable facility back
around 1991.  I can't necessarily remember all the thinking behind it,
but I though that I copied the basic concept from Symbolics.  However,
yesterday when I was in the office I checked a full set of Genera
documentation and could find nothing at all documenting named
readtables.  Can anyone suggest where the idea might have come from?

Anyway, you are correct that allowing arbitrary symbols to be used as
readtable names would reduce the possibility of collision, but no
application ever uses more than a few readtables so the probability of
collision in practice is essentially zero.  I believe at the time I
wanted to make sure there would be no package problems with the tools
like the Emacs-Lisp interface and the Allegro IDE (which will
communicate the readtable name when evaluating/compiling from a source
buffer) trying to communicate a readtable name with a package that
doesn't yet exist.

The package of a readtable name not yet existing would be a different
error circumstance than the named readtable not yet being defined, so
it simplifies these tool interfaces to restrict the names to the
keyword package.  Also, coercing to keywords means that an Emacs
variables line can look like this

  ;; -*- mode: common-lisp; package: user; readtable: xml -*-

instead of like this

  ;; -*- mode: common-lisp; package: user; readtable: :xml -*-

Do you really think that the collision possibility is an important
issue here?  The mode line examples above show that changing the
package handling would not easily be back compatible.