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Looking through the August 1979 issue of Byte magazine, it discusses a dialect of Lisp in which arithmetic operations are denoted by words like PLUS and TIMES.
Later dialects like Common Lisp and Scheme use the symbols common to other languages like + and *.
When did Lisp generally switch from one convention to the other?
All the various LISP 1.5 systems (on the IBM 7090 and
otherwise) appears always to have used only PLUS, DIFFERENCE,
MINUS (unary), etc. (§4.2 p.25) Its small derivative PDP-1
LISP (1964) also did as well (§2 p.3 Table 1, though I don't
know what happened to DIFFERENCE.)
LISP 2, discussed extensively in the early '60s but never
implemented, did use symbols for arithmetic in the ALGOL-like "source
language," but I think you're not talking about this. The S-expression
"internal language" still used PLUS etc. (See example of both in
§5.2 on p.13.)
The BBN descendent, INTERLISP (also started on the PDP-1, but
eventually moving to the PDP-10 amongst many other machines) was still
using plus etc. in 1974, and had added iplus and friends to do
faster integer arithmetic that avoided boxing the values where
possible. (§13.1 p.13.2) This continued on through at least
1983. (§2.9.4 p.2.44)
The other direct descendant, Maclisp, through at least 1970 was the same, as far as names of numerical functions go. (No list of functions in this one, but see code on pp. 14, 20, 25, and 30.)
However, by 1973, as well as using 'A as a shorter form
of (QUOTE A) (which I think was first introduced in CONNIVER in 1972),
Maclisp started using symbols for arithmetic functions. (§7.1.4). (This seems
to have been part of the changes made during Moon's project to
reimplement Maclisp on the Honeywell 6180 running MULTICS.) However,
these were used for type-specific versions of the
functions; plus etc. was still used for the generic versions. Here I
quote from the published manual of 1974 because it's much
easier to read.
Section 7 "Functions on Numbers" covers the numerical operations. From §7.2 "Comparison":
=SUBR 2 args(= x y) is
tif x and y are numerical equal. x and y must be both fixnums or both flonums....
greaterpLSUBR 2 or more args
greaterpcompares its arguments, which must be numbers, from left to right. If any argument is not greater than the next,greaterpreturnsnil. But if the arguments togreaterpare strictly decreasing, the reault ist.
>SUBR 2 args(> x y) is
tif x is strictly greater than y andnilotherwise. x and y must be both fixnums or both flonums.
And from §7.4 "Arithmetic":
plusLSUBR 0 or more args
plusreturns the sum of its arguments, which may be any kind of numbers. Conversions to flonum or bignum representation are done as needed. Flonum representation will be used if any of the arguments are flonums; otherwise fixnum representation will be used if the results can fit in fixnum form. If it cannot, bugnum representation will be used.
+LSUBR 0 or more args
+returns the sum of its arguments. The arguments must be fixnums, and the result is always a fixnum. Overflow is ignored.
The original paper on Scheme, AIM-349 "Scheme: An Interpreter for the
Extended Lambda Calculus" (Sussman and Steele, December 1975)¹,
uses symbols such as = and + for the EQUAL and PLUS functions,
as you can immediately see from the first example in section 2, and
there's no a mention of the word versions. This seems to be the first
time that these were used for generic, rather than integer
type-specific, functions and replaced, rather than augmenting, the
word functions. This is not formally defined, however, nor
is it in AIM-452 R¹RS. It's not until 1985, with the publication of
AIM-848 R²RS (§II.6 p.39) that the numerical operations are
specifically defined to be +, -, etc. and by that time of course
Common LISP had already done this too.
If you want to dig around yourself for more information or try to find earlier examples, a good source of documentation for many, many versions of LISP is the Software Preservation Group's History of LISP page.
¹This is the first "Lambda Paper", and might also retroactively be called "R⁰RS."
Lisp is not a single language, but a whole ecosystem of different languages. Moreover, there's no standard covering all Lisps, like with C or Fortran, so for this reason, + and plus are equally "valid".
When Lisp 1 (March 1960) was written, the primitive operations defined were car, cdr, cons, and, or, cond, etc. The arithmetic operations were not primitives at that time, so the programmers chose their own names.
At least Lisp 1.5 (early 60s) had both.
But this Lisp from 1970 had PLUS and MINUS but no + nor -.
If you consider Scheme (1975) to be a Lisp, then it is a specimen having both + and &+ (the latter is an optimisation for two arguments only).
And Common Lisp (1984) has + but not plus as you have noted.
So I posit that we gradually settled on +-style symbols starting in the 70s, and the situation was a state of flux before then, for the reason that arithmetic operations were not even primitive operations to begin with.
