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- To: Moon@SCRC-STONY-BROOK.ARPA
- Subject: Reader hacks
- From: "Glenn S. Burke" <GSB@MC.LCS.MIT.EDU>
- Date: Thu, 26 Dec 85 16:45:01 EST
- Cc: common-lisp@SU-AI.ARPA, gls@AQUINAS.THINK.COM
From: David A. Moon <Moon@SCRC-STONY-BROOK.ARPA>
From: Guy Steele <gls@THINK-AQUINAS.ARPA>
I am writing code for reader macros and find I need the following
(1) A predicate that is true of whitespace characters and false
of all others. . . .
Is this something different from
(or (char-equal char #\space)
(not (graphic-char-p char))) ?
Well, i was going to say that backspace didn't fit this mold, but CLtM
says it isn't a token constituent unless quoted. Oh well. Anyway,
have we ever speculated on the relationship of predicates like
graphic-char-p or this hypothetical whitespace-p, and readtables? How
well-defined is it to transfer the syntax of one character to another?
(2) I find I can't easily write myself something akin to
READ-DELIMITED-LIST that handles dotted lists because of
the problem of recognizing all-dots tokens. PEEK-CHAR can
only peek one ahead, so I can't tell whether there is a
space after a dot before calling READ recursively.
How about a way (a special variable?) to turn off the somewhat silly
restriction that you can't have a symbol named .?
Flushing the restriction of unquoted all-dots tokens would only
ameliorate the problem a bit, because he would still have to read the
token himself because he wouldn't be able to unread-char both the dot
and the following token. Besides all the other stuff which is almost
what read-delimited-list provides. Restated, the problem is that one
might want read-delimited-list to barf about cons dots; for instance,
to implement #( syntax. Alternatively, one might want to do
variations of ordinary parentheses in which cons-dot is meaningful.
So having some sort of flag argument to read-delimited-list makes
Alternatively, how about a function that does the token-isolating
portion of READ but not the printed-representation-to-object
conversion portion of READ?
Do you think this could be done without over half the implementors
having apoplectic fits, and with no loss of life?