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3. Program Structure

This section describes features of the CL package which have to do with programs as a whole: advanced argument lists for functions, and the eval-when construct.

3.1 Argument Lists  `&key', `&aux', `defun*', `defmacro*'.
3.2 Time of Evaluation  The `eval-when' construct.
3.3 Function Aliases  The `defalias' function.


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3.1 Argument Lists

Emacs Lisp's notation for argument lists of functions is a subset of the Common Lisp notation. As well as the familiar &optional and &rest markers, Common Lisp allows you to specify default values for optional arguments, and it provides the additional markers &key and &aux.

Since argument parsing is built-in to Emacs, there is no way for this package to implement Common Lisp argument lists seamlessly. Instead, this package defines alternates for several Lisp forms which you must use if you need Common Lisp argument lists.

Macro: defun* name arglist body...
This form is identical to the regular defun form, except that arglist is allowed to be a full Common Lisp argument list. Also, the function body is enclosed in an implicit block called name; see section 5.5 Blocks and Exits.

Macro: defsubst* name arglist body...
This is just like defun*, except that the function that is defined is automatically proclaimed inline, i.e., calls to it may be expanded into in-line code by the byte compiler. This is analogous to the defsubst form in Emacs 19; defsubst* uses a different method (compiler macros) which works in all version of Emacs, and also generates somewhat more efficient inline expansions. In particular, defsubst* arranges for the processing of keyword arguments, default values, etc., to be done at compile-time whenever possible.

Macro: defmacro* name arglist body...
This is identical to the regular defmacro form, except that arglist is allowed to be a full Common Lisp argument list. The &environment keyword is supported as described in Steele. The &whole keyword is supported only within destructured lists (see below); top-level &whole cannot be implemented with the current Emacs Lisp interpreter. The macro expander body is enclosed in an implicit block called name.

Macro: function* symbol-or-lambda
This is identical to the regular function form, except that if the argument is a lambda form then that form may use a full Common Lisp argument list.

Also, all forms (such as defsetf and flet) defined in this package that include arglists in their syntax allow full Common Lisp argument lists.

Note that it is not necessary to use defun* in order to have access to most CL features in your function. These features are always present; defun*'s only difference from defun is its more flexible argument lists and its implicit block.

The full form of a Common Lisp argument list is

 
(var...
 &optional (var initform svar)...
 &rest var
 &key ((keyword var) initform svar)...
 &aux (var initform)...)

Each of the five argument list sections is optional. The svar, initform, and keyword parts are optional; if they are omitted, then `(var)' may be written simply `var'.

The first section consists of zero or more required arguments. These arguments must always be specified in a call to the function; there is no difference between Emacs Lisp and Common Lisp as far as required arguments are concerned.

The second section consists of optional arguments. These arguments may be specified in the function call; if they are not, initform specifies the default value used for the argument. (No initform means to use nil as the default.) The initform is evaluated with the bindings for the preceding arguments already established; (a &optional (b (1+ a))) matches one or two arguments, with the second argument defaulting to one plus the first argument. If the svar is specified, it is an auxiliary variable which is bound to t if the optional argument was specified, or to nil if the argument was omitted. If you don't use an svar, then there will be no way for your function to tell whether it was called with no argument, or with the default value passed explicitly as an argument.

The third section consists of a single rest argument. If more arguments were passed to the function than are accounted for by the required and optional arguments, those extra arguments are collected into a list and bound to the "rest" argument variable. Common Lisp's &rest is equivalent to that of Emacs Lisp. Common Lisp accepts &body as a synonym for &rest in macro contexts; this package accepts it all the time.

The fourth section consists of keyword arguments. These are optional arguments which are specified by name rather than positionally in the argument list. For example,

 
(defun* foo (a &optional b &key c d (e 17)))

defines a function which may be called with one, two, or more arguments. The first two arguments are bound to a and b in the usual way. The remaining arguments must be pairs of the form :c, :d, or :e followed by the value to be bound to the corresponding argument variable. (Symbols whose names begin with a colon are called keywords, and they are self-quoting in the same way as nil and t.)

For example, the call (foo 1 2 :d 3 :c 4) sets the five arguments to 1, 2, 4, 3, and 17, respectively. If the same keyword appears more than once in the function call, the first occurrence takes precedence over the later ones. Note that it is not possible to specify keyword arguments without specifying the optional argument b as well, since (foo 1 :c 2) would bind b to the keyword :c, then signal an error because 2 is not a valid keyword.

If a keyword symbol is explicitly specified in the argument list as shown in the above diagram, then that keyword will be used instead of just the variable name prefixed with a colon. You can specify a keyword symbol which does not begin with a colon at all, but such symbols will not be self-quoting; you will have to quote them explicitly with an apostrophe in the function call.

Ordinarily it is an error to pass an unrecognized keyword to a function, e.g., (foo 1 2 :c 3 :goober 4). You can ask Lisp to ignore unrecognized keywords, either by adding the marker &allow-other-keys after the keyword section of the argument list, or by specifying an :allow-other-keys argument in the call whose value is non-nil. If the function uses both &rest and &key at the same time, the "rest" argument is bound to the keyword list as it appears in the call. For example:

 
(defun* find-thing (thing &rest rest &key need &allow-other-keys)
  (or (apply 'member* thing thing-list :allow-other-keys t rest)
      (if need (error "Thing not found"))))

This function takes a :need keyword argument, but also accepts other keyword arguments which are passed on to the member* function. allow-other-keys is used to keep both find-thing and member* from complaining about each others' keywords in the arguments.

As a (significant) performance optimization, this package implements the scan for keyword arguments by calling memq to search for keywords in a "rest" argument. Technically speaking, this is incorrect, since memq looks at the odd-numbered values as well as the even-numbered keywords. The net effect is that if you happen to pass a keyword symbol as the value of another keyword argument, where that keyword symbol happens to equal the name of a valid keyword argument of the same function, then the keyword parser will become confused. This minor bug can only affect you if you use keyword symbols as general-purpose data in your program; this practice is strongly discouraged in Emacs Lisp.

The fifth section of the argument list consists of auxiliary variables. These are not really arguments at all, but simply variables which are bound to nil or to the specified initforms during execution of the function. There is no difference between the following two functions, except for a matter of stylistic taste:

 
(defun* foo (a b &aux (c (+ a b)) d)
  body)

(defun* foo (a b)
  (let ((c (+ a b)) d)
    body))

Argument lists support destructuring. In Common Lisp, destructuring is only allowed with defmacro; this package allows it with defun* and other argument lists as well. In destructuring, any argument variable (var in the above diagram) can be replaced by a list of variables, or more generally, a recursive argument list. The corresponding argument value must be a list whose elements match this recursive argument list. For example:

 
(defmacro* dolist ((var listform &optional resultform)
                   &rest body)
  ...)

This says that the first argument of dolist must be a list of two or three items; if there are other arguments as well as this list, they are stored in body. All features allowed in regular argument lists are allowed in these recursive argument lists. In addition, the clause `&whole var' is allowed at the front of a recursive argument list. It binds var to the whole list being matched; thus (&whole all a b) matches a list of two things, with a bound to the first thing, b bound to the second thing, and all bound to the list itself. (Common Lisp allows &whole in top-level defmacro argument lists as well, but Emacs Lisp does not support this usage.)

One last feature of destructuring is that the argument list may be dotted, so that the argument list (a b . c) is functionally equivalent to (a b &rest c).

If the optimization quality safety is set to 0 (see section 7. Declarations), error checking for wrong number of arguments and invalid keyword arguments is disabled. By default, argument lists are rigorously checked.


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3.2 Time of Evaluation

Normally, the byte-compiler does not actually execute the forms in a file it compiles. For example, if a file contains (setq foo t), the act of compiling it will not actually set foo to t. This is true even if the setq was a top-level form (i.e., not enclosed in a defun or other form). Sometimes, though, you would like to have certain top-level forms evaluated at compile-time. For example, the compiler effectively evaluates defmacro forms at compile-time so that later parts of the file can refer to the macros that are defined.

Macro: eval-when (situations...) forms...
This form controls when the body forms are evaluated. The situations list may contain any set of the symbols compile, load, and eval (or their long-winded ANSI equivalents, :compile-toplevel, :load-toplevel, and :execute).

The eval-when form is handled differently depending on whether or not it is being compiled as a top-level form. Specifically, it gets special treatment if it is being compiled by a command such as byte-compile-file which compiles files or buffers of code, and it appears either literally at the top level of the file or inside a top-level progn.

For compiled top-level eval-whens, the body forms are executed at compile-time if compile is in the situations list, and the forms are written out to the file (to be executed at load-time) if load is in the situations list.

For non-compiled-top-level forms, only the eval situation is relevant. (This includes forms executed by the interpreter, forms compiled with byte-compile rather than byte-compile-file, and non-top-level forms.) The eval-when acts like a progn if eval is specified, and like nil (ignoring the body forms) if not.

The rules become more subtle when eval-whens are nested; consult Steele (second edition) for the gruesome details (and some gruesome examples).

Some simple examples:

 
;; Top-level forms in foo.el:
(eval-when (compile)           (setq foo1 'bar))
(eval-when (load)              (setq foo2 'bar))
(eval-when (compile load)      (setq foo3 'bar))
(eval-when (eval)              (setq foo4 'bar))
(eval-when (eval compile)      (setq foo5 'bar))
(eval-when (eval load)         (setq foo6 'bar))
(eval-when (eval compile load) (setq foo7 'bar))

When `foo.el' is compiled, these variables will be set during the compilation itself:

 
foo1  foo3  foo5  foo7      ; `compile'

When `foo.elc' is loaded, these variables will be set:

 
foo2  foo3  foo6  foo7      ; `load'

And if `foo.el' is loaded uncompiled, these variables will be set:

 
foo4  foo5  foo6  foo7      ; `eval'

If these seven eval-whens had been, say, inside a defun, then the first three would have been equivalent to nil and the last four would have been equivalent to the corresponding setqs.

Note that (eval-when (load eval) ...) is equivalent to (progn ...) in all contexts. The compiler treats certain top-level forms, like defmacro (sort-of) and require, as if they were wrapped in (eval-when (compile load eval) ...).

Emacs 19 includes two special operators related to eval-when. One of these, eval-when-compile, is not quite equivalent to any eval-when construct and is described below. This package defines a version of eval-when-compile for the benefit of Emacs 18 users.

The other form, (eval-and-compile ...), is exactly equivalent to `(eval-when (compile load eval) ...)' and so is not itself defined by this package.

Macro: eval-when-compile forms...
The forms are evaluated at compile-time; at execution time, this form acts like a quoted constant of the resulting value. Used at top-level, eval-when-compile is just like `eval-when (compile eval)'. In other contexts, eval-when-compile allows code to be evaluated once at compile-time for efficiency or other reasons.

This form is similar to the `#.' syntax of true Common Lisp.

Macro: load-time-value form
The form is evaluated at load-time; at execution time, this form acts like a quoted constant of the resulting value.

Early Common Lisp had a `#,' syntax that was similar to this, but ANSI Common Lisp replaced it with load-time-value and gave it more well-defined semantics.

In a compiled file, load-time-value arranges for form to be evaluated when the `.elc' file is loaded and then used as if it were a quoted constant. In code compiled by byte-compile rather than byte-compile-file, the effect is identical to eval-when-compile. In uncompiled code, both eval-when-compile and load-time-value act exactly like progn.

 
(defun report ()
  (insert "This function was executed on: "
          (current-time-string)
          ", compiled on: "
          (eval-when-compile (current-time-string))
          ;; or '#.(current-time-string) in real Common Lisp
          ", and loaded on: "
          (load-time-value (current-time-string))))

Byte-compiled, the above defun will result in the following code (or its compiled equivalent, of course) in the `.elc' file:

 
(setq --temp-- (current-time-string))
(defun report ()
  (insert "This function was executed on: "
          (current-time-string)
          ", compiled on: "
          '"Wed Jun 23 18:33:43 1993"
          ", and loaded on: "
          --temp--))


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3.3 Function Aliases

This section describes a feature from GNU Emacs 19 which this package makes available in other versions of Emacs.

Function: defalias symbol function
This function sets symbol's function cell to function. It is equivalent to fset, except that in GNU Emacs 19 it also records the setting in load-history so that it can be undone by a later unload-feature.

In other versions of Emacs, defalias is a synonym for fset.


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