Backquote history

[Kent M Pitman <KMP @ MIT-MC>]

    Date: 11 MAR 84 11:38 PST
    From: MASINTER.PA at PARC-MAXC.ARPA
    To:   LISP-FORUM at MIT-MC.ARPA
    Re:   Backquote history

    Anybody remember where backquote came from? I.e., who invented it, 
    when it got added to MacLisp, etc?

Seems like everyone and his brother had some sort of backquote.
It may be hard to come up with a single originator. The LispM
version predates the Maclisp version, so its presence in Maclisp
was probably most directly a compatibility measure; though at the
same time, obviously, a good idea in its own right. But the LispM
crowd probably didn't originate it either. It goes back farther
than I can remember (which would be 77 or so) anyway.

What is perhaps more interesting than who created it was the amount 
of variety in these things. Here are some of the design decisions
that became involved ...

 * There was a lot of variety in how , and ,@ things were marked. These
   questions included what chars we could do without. eg, I was one of the
   people making a lot of noise about "," not being whitespace any more,
   so you couldn't write (5,3) to mean ordered pairs. In retrospect, this
   didn't turn out to be much of a loss. Also, there was a question of
   whether two char markers like ",@" and ",." were a good idea since 
   for example, (LAMBDA (X @X) `(FOO ,X ,@X ,@ X , @X)) could be visually
   confusing. But it was a choice between two char markers and losing another
   readmacro char, and no one wanted to give up yet another char.

 * There was question about where copying should be allowed. ie,
   if you did (DEFUN F (X) `(,X Y)) could you do (SETF (CADR (F ...)) ...)
   on the result without affecting F's future behavior. Early versions
   of backquote guaranteed copying, so `(A B C) was like (COPYTREE '(A B C)).
   This was eventually decided to be a bad idea. The current backquote does
   not define the sharing behavior; it tends to share structure where possible
   but is not defined to do so.

 * There was question about how nesting worked. In particular, some people
   claimed that ,, type things should be disallowed. Others claimed that 
   you should do outside-in counting rather than inside-out. The current 
   behavior is inside-out. There was a long argument between Bawden, McDermott,
   Steele, Rees, myself and a few others. If you're interested in that,
   MC: LSPMAI; BACKQ XMAIL contains this discussion.

 * There was also question about whether quoting or non-quoting should be
   the default. Dick Waters had a variant where quoting was not the default.
   Without seeing it, you might not guess the advantage, but the answer is
   that you didn't have to write LIST, CONS, etc. eg, [X Y Z] was like
   (LIST X Y Z), [X ! Y] was like (CONS X Y). There were advantages to this,
   but basically it loses out when you start to look at large expressions
   with little variability, since `(LAMBDA (,X Y) (+ ,X Y)) is almost the
   same "shape" as the result it will produce, which is not true of
   ['LAMBDA [X] ['+ X 'Y]] which is not as visually similar to its result.

 * I'm sure, though I can't recall specifically, there must have been heavy
   controversy about whether destructive operations like ,. should be
   allowed. I certainly don't like this one.

 * There was also a question of what the backquote macro should read as.
   It could either read directly as calls to LIST, CONS, etc. or it could
   read as macros which would later expand to calls to LIST, etc. when
   evaluted or compiled. According to the initial release notes, the first
   version expanded at readtime, which of course meant that GRINDEF called
   on such code gave a very verbose display. A switch, BACKQUOTE-EXPAND-WHEN,
   was later added so that you could select EVAL or READ time expansion.
   The default, EVAL, now allows GRINDEF to win on backquoted code.

 * There were also some bogus arguments made like that we didn't need `
   and ' to be different chars and that we should just let , be put in any
   ' expression. Such were beaten down by appeals to expressions like `',X
   which are quite common.

On Sunday, 17-Sep-78, HIC and JONL announced backquote as an official
autoloading part of Lisp (version 1742).

On Thurday, 7-Jun-79, JONL announced the introduction of ",.", the 
BACKQUOTE-EXPAND-WHEN feature, and the ability of GRINDEF to win on this
stuff.

I doubt the names of who announced these features are related to who came
up with them. Alan and I just tried to think of places where we
had seen backquote-like ideas; we came up with early versions of the
sources and/or sources to Scheme, LispM, PLASMA and Macsyma.
It was all a product of much negotiation among many people with similar 
but subtly differing ideas.