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# Porting Common Lisp

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.

- Case-insensitivity. Symbols in Common Lisp are case-insensitive by default. Some programs refer to a function or variable as `foo` in one place and `Foo` or `FOO` in 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 `if` and `defun` will have to be changed to lower-case.
- Lexical scoping. In Common Lisp, function arguments and `let` bindings 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 declaring them `special` or using `defvar`. In Emacs Lisp it is as if all variables were declared `special`. 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 in `map-odd-elements` by the time the `(+ a x)` function is called. (This package avoids such problems in its own mapping functions by using names like `cl-x` instead of `x` internally; 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 `lexical-let` form which establishes a Common Lisp-style lexical binding, and some examples of how it differs from Emacs' regular `let`.
- Common Lisp allows the shorthand `#'x` to stand for `(function x)`, just as `'x` stands for `(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

  When `#'` introduces a `lambda` form, it is best to write out `(function ...)` longhand in Emacs Lisp. You can use a regular quote, but then the byte-compiler won't know that the `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

  Lucid Emacs supports `#'` notation starting with version 19.8.
- The "backquote" feature uses a different syntax in Emacs Lisp.

      (defmacro foo (v &rest body) `(let ((,v 0)) @,body))  ; Common Lisp
      (defmacro foo (v &rest body) (` (let (((, v) 0)) (@, body)))  ; Emacs

- Reader macros. Common Lisp includes a second type of macro that works at the level of individual characters. For example, Common Lisp implements the quote notation by a reader macro called `'`, 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. The lack of reader macros, incidentally, is the reason behind Emacs Lisp's unusual backquote syntax. Since backquotes are implemented as a Lisp package and not built-in to the Emacs parser, they are forced to use a regular macro named ``` which is used with the standard function/macro call notation.
- Other syntactic features. Common Lisp provides a number of notations beginning with `#` 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.
- Packages. In Common Lisp, symbols are divided into packages. Symbols that are Lisp built-ins are typically stored in one package; symbols that are vendor extensions are put in another, and each application program would have a package for its own symbols. Certain symbols are "exported" by a package and others are internal; certain packages "use" or import the exported symbols of other packages. To access symbols that would not normally be visible due to this importing and exporting, Common Lisp provides a syntax like `package:symbol` or `package::symbol`. Emacs Lisp has a single namespace for all interned symbols, and then uses a naming convention of putting a prefix like `cl-` in front of the name. Some Emacs packages adopt the Common Lisp-like convention of using `cl:` or `cl::` as the prefix. However, the Emacs parser does not understand colons and just treats them as part of the symbol name. Thus, while `mapcar` and `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.
- Keywords. The notation `:test-not` in Common Lisp really is a shorthand for `keyword:test-not`; keywords are just symbols in a built-in `keyword` package 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 like `(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.
- The `format` function is quite different between Common Lisp and Emacs Lisp. It takes an additional "destination" argument before the format string. A destination of `nil` means to format to a string as in Emacs Lisp; a destination of `t` means to write to the terminal (similar to `message` in 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 `\n` for string literals; instead, `format` is 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 Lisp `format` in this package (under the name `format*`, 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 Emacs Lisp's `format` would rarely be useful.
- Vector constants use square brackets in Emacs Lisp, but `#(a b c)` notation in Common Lisp. To further complicate matters, Emacs 19 introduces its own `#(` notation for something entirely different--strings with properties.
- Characters are distinct from integers in Common Lisp. The notation for character constants is also different: `#\A` instead of `?A`. Also, `string=` and `string-equal` are synonyms in Emacs Lisp whereas the latter is case-insensitive in Common Lisp.
- Data types. Some Common Lisp data types do not exist in Emacs Lisp. Rational numbers and complex numbers are not present, nor are large integers (all integers are "fixnums"). All arrays are one-dimensional. There are no readtables or pathnames; streams are a set of existing data types rather than a new data type of their own. Hash tables, random-states, structures, and packages (obarrays) are built from Lisp vectors or lists rather than being distinct types.
- The Common Lisp Object System (CLOS) is not implemented, nor is the Common Lisp Condition System.
- Common Lisp features that are completely redundant with Emacs Lisp features of a different name generally have not been implemented. For example, Common Lisp writes `defconstant` where Emacs Lisp uses `defconst`. Similarly, `make-list` takes 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 `make-list`.
- A few more notable Common Lisp features not included in this package: `compiler-let`, `tagbody`, `prog`, `ldb/dpb`, `parse-integer`, `cerror`.
- Recursion. While recursion works in Emacs Lisp just like it does in Common Lisp, various details of the Emacs Lisp system and compiler make recursion much less efficient than it is in most Lisps. Some schools of thought prefer to use recursion in Lisp over other techniques; they would sum a list of numbers using something like

      (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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