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From: Kaz Kylheku <480-992-1380@kylheku.com>
Newsgroups: comp.compilers
Subject: Re: Who first invented dotted-item notation?
Date: Sun, 22 May 2022 18:29:01 -0000 (UTC)
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On 2022-05-22, Christopher F Clark <christopher.f.clark@compiler-resources.com> wrote:
> I know the notation from LR(0) machine construction, but also know
> that Gluskhov used it in his solution to NFA construction.  Earley
> also used the notation to describe his method if I understand right.
> I'm presuming that there is some first use of the notation.  Do we
> know who invented it?

In Lisp, we can write (1 2 3 4 5 6) in any of these ways:

(1 . (2 3 4 5 6))
(1 2 . (3 4.5 6))
(1 2 3 . (4 5 6))
(1 2 3 4 . (5 6))
(1 2 3 4 5 . (6))
(1 2 3 4 5 6 . NIL)

As well as:

(1 . (2 . (3 4 5 6)))

The dot doesn't actually exist; it isn't represented anywhere in the
data structure, but in a hand-written expression it could be used to
convey (purely as a comment) some semanticaly interesting division in
the list.

E.g. suppose we have some data structure which holds a list of symbols,
and an integer indicating a position among those symbols.

A custom printing routine for that structure could take advantage
of this, rendering it as, say:

  #S(sym-structure
    :dot-position 2
    :syms (a b . (c d)))

where the custom printer for sym-structure looks at dot-position
and renders the syms slot accordingly, to provide a visual aid.

The following is not possible in any mainstream Lisp I know of: (. (a b c))
as a way of denoting (a b c).  The custom printer could omit the
dot notation.

(I know of a dialect in which it that leading dot notation is allowed,
with that semantics, but starting in a git commit made in 2014, making
it irrelevant here.)

--
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal