Re: Lisp history: IF, etc.

* Alan Bawden <86h6gjpq3i.fsf_-_@williamsburg.bawden.org> :
Wrote on Tue, 02 Apr 2024 14:38:25 -0400:

I don't think that McCarthy invented IF as an abbreviation for COND, but
I could be wrong.  He certainly _could_ have invented it, but if he did,
it was forgotten by the mid 1970s.  Defining IF as a _personal_ macro was
common practice for 1970s Lisp programmers, and I don't think IF was
provided as standard by _any_ major Lisp implementation at that time.
(Maybe InterLisp did?  I don't recall, and I no longer have an InterLisp
manual on my bookshelf.)  Scheme had IF _instead_ of COND, but nobody
was using Scheme for anything serious yet.  And I don't think the T
project had started at that point.
>
In fact, when the Lisp Machine group decided to introduce a standard IF
macro into Lisp Machine Lisp, we were a bit worried that another
significant body of Lisp code was already using a slightly different
syntax for IF than the one we wanted to use.  Multics Emacs (written in
Multics MacLisp) had popularized an IF macro that used an ELSE keyword
somehow.  We negotiated a deal with the Multics guys (Bernie Greenberg)
where they agreed to change their IF to guarantee that an IF with no
ELSE keyword would work exactly the same way ours did.
>
This was perhaps the very beginning of the road that eventually led us
to Common Lisp.  A bunch of Lisp implementations (Lisp Machine Lisp, NIL
and both branches of MacLisp) all introduced a bunch of new syntax (IF,
LET, DEFSTRUCT, SETF, backquote, sharpsign, etc.) into their standard
environment so that individual programmers and programs no longer had to
define them themselves.
>
But I'd love to see evidence that McCarthy arrived at ternary IF before
anyone else.  He certainly invented COND, where the interesting thing
about COND was that it was an _expression_ rather than a statement, so
we often say that "McCarthy invented the conditional expression".  And
sometimes that gets shortened to "McCarthy invented the if-expression".
But I don't think that he ever literally wrote "(IF ...)" rather than
"(COND ...)".

I've forgotten the distinction between a statement and an expression in
lisp.  Maybe someone can remind me.  is it that IF is a special operator
but COND is a macro?  From the point of view of surface syntax, aren't
they the same sort of beast?  Both evaluate to a result which can be
uses elsewhere, so they can't be "statements" in the sense of other
programming languages.

I'm not even sure that McCarthy would have seen the advantage of IF over
COND given that what he originally wanted us all to write was actually
"[expr -> expr; expr]"!

CLHS has

 (if test-form then-form else-form)
 ==  (cond (test-form then-form) (t else-form))

so by inventing COND he had already invented IF, and probably felt
secure enough and not compelled publish another result