Issue GET-MACRO-CHARACTER-READTABLE Writeup

Status: Passed, Jan 89 X3J13

Forum: Cleanup

Issue: 	GET-MACRO-CHARACTER-READTABLE


References:  CLtL p.361: COPY-READTABLE, SET-SYNTAX-FROM-CHAR, and

                         GET-MACRO-CHARACTER

             CLtL p.364: GET-DISPATCH-MACRO-CHARACTER, 

             CLtL p.378: Example in middle of page


Category:    CLARIFICATION/CHANGE


Edit history:  Version 1, 16-Nov-88, by JonL

               Version 2,  8-Dec-88, by Masinter (fix typo)

		Version 3, 11-Feb-89, as amended Jan 89 X3J13



Problem description:


The 'readtable' argument to GET-DISPATCH-MACRO-CHARACTER and to

GET-MACRO-CHARACTER must be of type READTABLE, without mention of

what happens when NIL is supplied. This may have been simply

an oversight, since it makes more sense for it to refer to values from

the standard readtable.  Both COPY-READTABLE and SET-SYNTAX-FROM-CHAR

explicitly say that a NIL in the 'from-readtable' argument refers to the

standard readtable.  Also, an example in the middle of the page, CLtL

p.378, supplies a NIL to GET-MACRO-CHARACTER, and is clearly intending

to access the standard readtable values.



Proposal (GET-MACRO-CHARACTER-READTABLE:NIL-STANDARD)


Specify that a NIL readtable argument to GET-DISPATCH-MACRO-CHARACTER

and to GET-MACRO-CHARACTER mean the same thing it does for COPY-READTABLE,

and SET-SYNTAX-FROM-CHAR; namely a reference to the standard readtable.  

Thus (GET-MACRO-CHARACTER <char> NIL) would be equivalent to 

(GET-MACRO-CHARACTER <char> (COPY-READTABLE)), but without consing.


Rationale:


Probably was the original intent; somebody wants it; also see "Esthetics".


Current practice:


Symbolics and Xerox have already implemented the proposal; Lucid, VAXLISP,

and KCL stuck to the more rigid interpretation.



Cost to Implementors:


Trivial.


Cost to Users:


None.


Cost of non-adoption:


Minor worry about porting between implementations that support the

generalization and those that don't; minor worry about consing when

calling (COPY-READTABLE) to get at standard readtable semantics.


Performance impact:


See "Cost of non-adoption".


Benefits:


See "Cost of non-adoption".


Esthetics:


Increases parallel design among similar readtable functions.


Discussion:


This question was raised in the Common Lisp mailing last summer:

    Date: 19 Jul 88 13:35

    Subject: Question about readtable null arguments

    From: quiroz%cs.rochester:EDU:Xerox

    To: common-lisp%sail.stanford:EDU:Xerox

 

This issue passed at the Jan 89 X3J13 meeting. The only

difference between version 2 and 3 was to remove the

test case because of problems of "function equivalence":


If "same-function-p" compared functions:


(let ((standard-rt (copy-readtable))

      (chars '(#\* #\= #\| #\A #\  #\( #\# #\1)))

  ;; Test Case 1

  (dolist (char chars)

    (assert (same-function-p (get-macro-character char nil)

                (get-macro-character char standard-rt))

            () "Lose on character ~C" char))

  ;; Test Case 2

  (dolist (char chars)

    (assert (same-function-p (get-dispatch-macro-character #\# char nil)

                (get-dispatch-macro-character #\# char standard-rt))

            () "Lose on #\# dispatch character ~C" char))

  ;; Test Case 3

  (assert (same-function-p (get-dispatch-macro-character #\# #\A nil)

              (get-dispatch-macro-character #\# #\a nil))

          () "Lose on #\# dispatch character ~C" char)

 )