Structure sharing

[David A. Moon <Moon@STONY-BROOK.SCRC.Symbolics.COM>]

    Date: 18 Jun 1986 23:08-EDT
    From: NGALL@G.BBN.COM

    KCL recently surprised me with the following behavior:

    (setf foo 'filename) => FILENAME

    (setf bar (namestring foo)) => "FILENAME"

    (setf bar (nstring-downcase bar)) => "filename"

    foo => {causes error}

    It seems that in KCL. namestring of a symbol merely returns a
    simple-string whose characters are SHARED with the print-name of the
    symbol FILENAME!

    Date: Wed, 18 Jun 1986  23:35 EDT
    From: "Scott E. Fahlman" <Fahlman@C.CS.CMU.EDU>

    Page 168:
    "It is an extremely bad idea to modify a string being used as the print
    name of a symbol.  Such a modification may tremendously confuse the
    function READ and the package system."

Does this tell us whether the behavior of NAMESTRING in KCL is valid
or invalid?  I think it's valid, but nothing in the manual directly
supports that claim.

    From: NGALL@G.BBN.COM again

    I think it would be a good idea for CLtL to explicity state that
    unless otherwise noted, a given function may return an object that
    shares structure with another object.  And it would help to give a few
    examples like parse-namestring and namestring to point out the
    non-obvious consequences.

It sounds like the agenda of language clarifications ought to include
such an item.