I'm looking for advice on how to implement, in Common Lisp, the
sweet-expression semantics for {backquote,comma,comma-at}+whitespace.
The "obvious" way is to re-implement these capabilities; Steele's 2nd edition
appendix C
gives public domain code to do this:
http://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node367.html
This seems a little error-prone, and it won't take advantage of whatever
simplifications exist in the underlying CL implementation. In general, CL
handles this stuff in non-portable ways, and a re-implementation won't "work
the same way".
It'd be nice to invoke the "actual" backquote or comma/comma-at, but if I do,
they will invoke "read". Yet I need to regain control of the reader, and pass
that reader the indentation level ... preferably in a thread-safe way.
Basically, historically "read" didn't care about indentation levels.. but any
indentation processing system will. One way to do that is to set a special
variable (!!) with the indent level, and use the readtable that already
redirects all chars. The readtable would redirect to a procedure that, if the
special variable indent is set, invoke the sweet-reader with that indent level.
Am I missing a simpler alternative to implement the intended semantic?
--- David A. Wheeler
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