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Clarify list terminology

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* doc/lispintro/emacs-lisp-intro.texi (Lists diagrammed):
Mention "cons cell".  Add index entries.
(car & cdr): Simplify etymology of `car' and `cdr'.
Explain why for some purposes they are better than `first' and `rest'.
Mention cons cells.
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Richard M. Stallman 2023-06-24 19:19:53 -04:00 • committed by Richard Stallman
parent e85ebb3d82
commit 188c90c7c1
1 changed files with 35 additions and 20 deletions
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doc/lispintro/emacs-lisp-intro.texi
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@ -6761,16 +6761,13 @@ The name of the @code{cons} function is not unreasonable: it is an
abbreviation of the word ``construct''. The origins of the names for
@code{car} and @code{cdr}, on the other hand, are esoteric: @code{car}
is an acronym from the phrase ``Contents of the Address part of the
Register''; and @code{cdr} (pronounced ``could-er'') is an acronym from
the phrase ``Contents of the Decrement part of the Register''. These
phrases refer to specific pieces of hardware on the very early
computer on which the original Lisp was developed. Besides being
obsolete, the phrases have been completely irrelevant for more than 25
years to anyone thinking about Lisp. Nonetheless, although a few
brave scholars have begun to use more reasonable names for these
functions, the old terms are still in use. In particular, since the
terms are used in the Emacs Lisp source code, we will use them in this
introduction.
Register''; and @code{cdr} (pronounced ``could-er'') is an acronym
from the phrase ``Contents of the Decrement part of the Register''.
These phrases refer to the IBM 704 computer on which the original Lisp
was developed.
The IBM 704 is a footnote in history, but these names are now beloved
traditions of Lisp.
@node car & cdr
@section @code{car} and @code{cdr}
@ -6791,9 +6788,6 @@ evaluating the following:
After evaluating the expression, @code{rose} will appear in the echo
area.
Clearly, a more reasonable name for the @code{car} function would be
@code{first} and this is often suggested.
@code{car} does not remove the first item from the list; it only reports
what it is. After @code{car} has been applied to a list, the list is
still the same as it was. In the jargon, @code{car} is
@ -6825,6 +6819,22 @@ Incidentally, in the example, the list of flowers is quoted. If it were
not, the Lisp interpreter would try to evaluate the list by calling
@code{rose} as a function. In this example, we do not want to do that.
For operating on lists, the names @code{first} and @code{rest} would
make more sense than the names @code{car} and @code{cdr}. Indeed,
some programmers define @code{first} and @code{rest} as aliases for
@code{car} and @code{cdr}, then write @code{first} and @code{rest} in
their code.
However, lists in Lisp are built using a lower-level structure known
as ``cons cells'' (@pxref{List Implementation}), in which there is no
such thing as ``first'' or ``rest,''and the @sc{car} and the @sc{cdr}
are symmetrical. Lisp does not try to hide the existence of cons
cells, and programs do use them for things other than lists. For this
reason, the names are helpful for reminding programmers that
@code{car} and @code{cdr} are in fact symmetrical, despite the
asymmetrical way they are used in lists.
@ignore
Clearly, a more reasonable name for @code{cdr} would be @code{rest}.
(There is a lesson here: when you name new functions, consider very
@ -6834,6 +6844,7 @@ these names is that the Emacs Lisp source code uses them, and if I did
not use them, you would have a hard time reading the code; but do,
please, try to avoid using these terms yourself. The people who come
after you will be grateful to you.)
@end ignore
When @code{car} and @code{cdr} are applied to a list made up of symbols,
such as the list @code{(pine fir oak maple)}, the element of the list
@ -9429,13 +9440,15 @@ pointed to. Hence, a list is kept as a series of electronic addresses.
@unnumberedsec Lists diagrammed
@end ifnottex
For example, the list @code{(rose violet buttercup)} has three elements,
@samp{rose}, @samp{violet}, and @samp{buttercup}. In the computer, the
electronic address of @samp{rose} is recorded in a segment of computer
memory along with the address that gives the electronic address of where
the atom @samp{violet} is located; and that address (the one that tells
where @samp{violet} is located) is kept along with an address that tells
where the address for the atom @samp{buttercup} is located.
For example, the list @code{(rose violet buttercup)} has three
elements, @samp{rose}, @samp{violet}, and @samp{buttercup}. In the
computer, the electronic address of @samp{rose} is recorded in a
segment of computer memory called a @dfn{cons cell} (because it's what
the function @code{cons} actually creates). That cons cell also holds
the address of a second cons cell, whose @sc{car} is the atom
@samp{violet}; and that address (the one that tells where to find
@samp{violet}) is kept along with the address of a third cons cell
which holds the address for the atom @samp{buttercup}.
@need 1200
This sounds more complicated than it is and is easier seen in a diagram:
@ -9652,6 +9665,8 @@ to say, the symbol @code{flowers} holds the address of the pair of
address-boxes, the first of which holds the address of @code{violet},
and the second of which holds the address of @code{buttercup}.
@cindex dotted pair
@cindex cons cell
A pair of address-boxes is called a @dfn{cons cell} or @dfn{dotted
pair}. @xref{Cons Cell Type, , Cons Cell and List Types, elisp, The GNU Emacs Lisp
Reference Manual}, and @ref{Dotted Pair Notation, , Dotted Pair