lisp-interaction-mode binding RET

[Peter Seibel <peter@javamonkey.com>]

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In GNU Emacs 21.3.1 (i686-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2003-07-17 on xeon
Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_US.iso885915
  locale-coding-system: iso-latin-9
  default-enable-multibyte-characters: t

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

So I just installed 21.3 (after having been using Xemacs for a while).
I was immediately confused by the behavior of RET in
lisp-interaction-mode. Sometimes it is bound to newline as I'd expect;
other times it's bound to last-sexp-toggle-display. That seems
contrary to both the principle of least astonishment (to me anyway)
and the emacs coding standards which say:

  * Major modes must not define <RET> to do anything other than insert
    a newline. The command to insert a newline and then indent is
    `C-j'. Please keep this distinction uniform for all major modes.

Furthermore, the behavior of last-sexp-toggle-display is pretty
strange. For instance if I type this:

  (make-list 20 'a)

in my *scratch* buffer and eval it with C-j I get this:

  (make-list 20 'a)
  (a a a a a a a a a a a a ...)

Now if I put my cursor on the opening parentheses of the list of a's
and hit return I get an error:

  Wrong type argument: integer-or-marker-p, nil

from last-sexp-toggle-display. Then I move the cursor into the list
and l-s-t-d seems to work--it expands and contrats the list. But since
the binding of RET to l-s-t-d seems to be attached to the characters
themselves even more fun awaits. If I kill and then yank the two lines
so I have this:

  (make-list 20 'a)
  (a a a a a a a a a a a a ...)

  (make-list 20 'a)
  (a a a a a a a a a a a a ...)

And then I put the cursor at the beginning of the line containing the
second list of a's and type some stuff, giving me this:

  (make-list 20 'a)
  (a a a a a a a a a a a a ...)

  (make-list 20 'a)
  Type a bunch of stuff here(a a a a a a a a a a a a ...)


Then when I hit RET, l-s-t-d expands the list and wipes out everything
back to the original list leaving me with this:

  (make-list 20 'a)
  (a a a a a a a a a a a a ...)(a a a a a a a a a a a a a a a a a a a a)

This is all with GNU Emacs 21.3.1 running on Linux.

Bug? Feature? I'm hallucinating?

-Peter