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This package is meant to be used as an extension to Emacs Lisp, not as an Emacs implementation of true Common Lisp. Some of the remaining differences between Emacs Lisp and Common Lisp make it difficult to port large Common Lisp applications to Emacs. For one, some of the features in this package are not fully compliant with ANSI or Steele; see section Common Lisp Compatibility. But there are also quite a few features that this package does not provide at all. Here are some major omissions that you will want watch out for when bringing Common Lisp code into Emacs.
fooin one place and
FOOin another. Emacs Lisp will treat these as three distinct symbols.
Some Common Lisp code is written in all upper-case. While Emacs
is happy to let the program’s own functions and variables use
this convention, calls to Lisp builtins like
defun will have to be changed to lower-case.
letbindings apply only to references physically within their bodies (or within macro expansions in their bodies). Emacs Lisp, by contrast, uses dynamic scoping wherein a binding to a variable is visible even inside functions called from the body.
Variables in Common Lisp can be made dynamically scoped by
special or using
defvar. In Emacs
Lisp it is as if all variables were declared
Often you can use code that was written for lexical scoping even in a dynamically scoped Lisp, but not always. Here is an example of a Common Lisp code fragment that would fail in Emacs Lisp:
(defun map-odd-elements (func list) (loop for x in list for flag = t then (not flag) collect (if flag x (funcall func x)))) (defun add-odd-elements (list x) (map-odd-elements (function (lambda (a) (+ a x))) list))
In Common Lisp, the two functions’ usages of
x are completely
independent. In Emacs Lisp, the binding to
x made by
add-odd-elements will have been hidden by the binding
map-odd-elements by the time the
(+ a x) function
(This package avoids such problems in its own mapping functions
by using names like
cl-x instead of
as long as you don’t use the
cl- prefix for your own
variables no collision can occur.)
See section Lexical Bindings, for a description of the
form which establishes a Common Lisp-style lexical binding, and some
examples of how it differs from Emacs’ regular
#'xto stand for
(function x), just as
(quote x). In Common Lisp, one traditionally uses
#'notation when referring to the name of a function. In Emacs Lisp, it works just as well to use a regular quote:
(loop for x in y by #'cddr collect (mapcar #'plusp x)) ; Common Lisp (loop for x in y by 'cddr collect (mapcar 'plusp x)) ; Emacs Lisp
#' introduces a
lambda form, it is best to
(function ...) longhand in Emacs Lisp. You can
use a regular quote, but then the byte-compiler won’t know that
lambda expression is code that can be compiled.
(mapcar #'(lambda (x) (* x 2)) list) ; Common Lisp (mapcar (function (lambda (x) (* x 2))) list) ; Emacs Lisp
#' notation starting with version 19.8.
', whereas Emacs Lisp’s parser just treats quote as a special case. Some Lisp packages use reader macros to create special syntaxes for themselves, which the Emacs parser is incapable of reading.
#that the Emacs Lisp parser won’t understand. For example, ‘#| ... |#’ is an alternate comment notation, and ‘#+lucid (foo)’ tells the parser to ignore the
(foo)except in Lucid Common Lisp.
The number prefixes ‘#b’, ‘#o’, and ‘#x’, however, are supported by the Emacs Lisp parser to represent numbers in binary, octal, and hexadecimal notation (or radix), just like in Common Lisp.
Emacs Lisp has a single namespace for all interned symbols, and
then uses a naming convention of putting a prefix like
in front of the name. Some Emacs packages adopt the Common Lisp-like
convention of using
cl:: as the prefix.
However, the Emacs parser does not understand colons and just
treats them as part of the symbol name. Thus, while
lisp:mapcar may refer to the same symbol in Common
Lisp, they are totally distinct in Emacs Lisp. Common Lisp
programs which refer to a symbol by the full name sometimes
and the short name other times will not port cleanly to Emacs.
Emacs Lisp does have a concept of “obarrays,” which are package-like collections of symbols, but this feature is not strong enough to be used as a true package mechanism.
:test-notin Common Lisp really is a shorthand for
keyword:test-not; keywords are just symbols in a built-in
keywordpackage with the special property that all its symbols are automatically self-evaluating. Common Lisp programs often use keywords liberally to avoid having to use quotes.
In Emacs Lisp a keyword is just a symbol whose name begins with
a colon; since the Emacs parser does not treat them specially,
they have to be explicitly made self-evaluating by a statement
(setq :test-not ':test-not). This package arranges
to execute such a statement whenever
defun* or some
other form sees a keyword being used as an argument. Common
Lisp code that assumes that a symbol
:mumble will be
self-evaluating even though it was never introduced by a
defun* will have to be fixed.
formatfunction is quite different between Common Lisp and Emacs Lisp. It takes an additional “destination” argument before the format string. A destination of
nilmeans to format to a string as in Emacs Lisp; a destination of
tmeans to write to the terminal (similar to
messagein Emacs). Also, format control strings are utterly different;
~is used instead of
%to introduce format codes, and the set of available codes is much richer. There are no notations like
\nfor string literals; instead,
formatis used with the “newline” format code,
~%. More advanced formatting codes provide such features as paragraph filling, case conversion, and even loops and conditionals.
While it would have been possible to implement most of Common
format in this package (under the name
of course), it was not deemed worthwhile. It would have required
a huge amount of code to implement even a decent subset of
format*, yet the functionality it would provide over
format would rarely be useful.
#(a b c)notation in Common Lisp. To further complicate matters, Emacs 19 introduces its own
#(notation for something entirely different—strings with properties.
string-equalare synonyms in Emacs Lisp whereas the latter is case-insensitive in Common Lisp.
defconstantwhere Emacs Lisp uses
make-listtakes its arguments in different ways in the two Lisps but does exactly the same thing, so this package has not bothered to implement a Common Lisp-style
(defun sum-list (list) (if list (+ (car list) (sum-list (cdr list))) 0))
where a more iteratively-minded programmer might write one of these forms:
(let ((total 0)) (dolist (x my-list) (incf total x)) total) (loop for x in my-list sum x)
While this would be mainly a stylistic choice in most Common Lisps, in Emacs Lisp you should be aware that the iterative forms are much faster than recursion. Also, Lisp programmers will want to note that the current Emacs Lisp compiler does not optimize tail recursion.
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