@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
-@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998 Free Software Foundation, Inc.
+@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999
+@c Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../info/functions
@node Functions, Macros, Variables, Top
* Defining Functions:: Lisp expressions for defining functions.
* Calling Functions:: How to use an existing function.
* Mapping Functions:: Applying a function to each element of a list, etc.
-* Anonymous Functions:: Lambda expressions are functions with no names.
+* Anonymous Functions:: Lambda expressions are functions with no names.
* Function Cells:: Accessing or setting the function definition
of a symbol.
* Inline Functions:: Defining functions that the compiler will open code.
+* Function Safety:: Determining whether a function is safe to call.
* Related Topics:: Cross-references to specific Lisp primitives
that have a special bearing on how functions work.
@end menu
@dfn{built-in} functions or @dfn{subrs}. (Special forms are also
considered primitives.)
-Usually the reason we implement a function as a primitive is because it
-is fundamental, because it provides a low-level interface to operating
-system services, or because it needs to run fast. Primitives can be
-modified or added only by changing the C sources and recompiling the
-editor. See @ref{Writing Emacs Primitives}.
+Usually the reason we implement a function as a primitive is either
+because it is fundamental, because it provides a low-level interface
+to operating system services, or because it needs to run fast.
+Primitives can be modified or added only by changing the C sources and
+recompiling the editor. See @ref{Writing Emacs Primitives}.
@item lambda expression
A @dfn{lambda expression} is a function written in Lisp.
These are described in the following section.
-@ifinfo
+@ifnottex
@xref{Lambda Expressions}.
-@end ifinfo
+@end ifnottex
@item special form
A @dfn{special form} is a primitive that is like a function but does not
byte compiler. @xref{Byte-Code Type}.
@end table
-@tindex functionp
@defun functionp object
This function returns @code{t} if @var{object} is any kind of function,
or a special form or macro.
@end example
@end defun
+@defun subr-arity subr
+@tindex subr-arity
+This function provides information about the argument list of a
+primitive, @var{subr}. The returned value is a pair
+@code{(@var{min} . @var{max})}. @var{min} is the minimum number of
+args. @var{max} is the maximum number or the symbol @code{many}, for a
+function with @code{&rest} arguments, or the symbol @code{unevalled} if
+@var{subr} is a special form.
+@end defun
+
@node Lambda Expressions
@section Lambda Expressions
@cindex lambda expression
@node Lambda Components
@subsection Components of a Lambda Expression
-@ifinfo
+@ifnottex
A function written in Lisp (a ``lambda expression'') is a list that
looks like this:
[@var{interactive-declaration}]
@var{body-forms}@dots{})
@end example
-@end ifinfo
+@end ifnottex
@cindex lambda list
The first element of a lambda expression is always the symbol
There is no way to have required arguments following optional
ones---it would not make sense. To see why this must be so, suppose
that @code{c} in the example were optional and @code{d} were required.
-Suppose three actual arguments are given; which variable would the third
-argument be for? Similarly, it makes no sense to have any more
-arguments (either required or optional) after a @code{&rest} argument.
+Suppose three actual arguments are given; which variable would the
+third argument be for? Would it be used for the @var{c}, or for
+@var{d}? One can argue for both possibilities. Similarly, it makes
+no sense to have any more arguments (either required or optional)
+after a @code{&rest} argument.
Here are some examples of argument lists and proper calls:
it in several symbols using @code{fset}; then each of the symbols is
equally well a name for the same function.
- A symbol used as a function name may also be used as a variable;
-these two uses of a symbol are independent and do not conflict.
-(Some Lisp dialects, such as Scheme, do not distinguish between a
-symbol's value and its function definition; a symbol's value as a variable
-is also its function definition.)
+ A symbol used as a function name may also be used as a variable; these
+two uses of a symbol are independent and do not conflict. (Some Lisp
+dialects, such as Scheme, do not distinguish between a symbol's value
+and its function definition; a symbol's value as a variable is also its
+function definition.) If you have not given a symbol a function
+definition, you cannot use it as a function; whether the symbol has a
+value as a variable makes no difference to this.
@node Defining Functions
@section Defining Functions
records.
@end defun
+ You cannot create a new primitive function with @code{defun} or
+@code{defalias}, but you use them to change the function definition of
+any symbol, even one such as @code{car} or @code{x-popup-menu} whose
+normal definition is a primitive. However, this is risky: for
+instance, it is next to impossible to redefine @code{car} without
+breaking Lisp completely. Redefining an obscure function such as
+@code{x-popup-menu} is less dangerous, but it still may not work as
+you expect. If there are calls to the primitive from C code, they
+call the primitive's C definition directly, so changing the symbol's
+definition will have no effect on them.
+
See also @code{defsubst}, which defines a function like @code{defun}
and tells the Lisp compiler to open-code it. @xref{Inline Functions}.
write the program. Usually that's just what you want. Occasionally you
need to compute at run time which function to call. To do that, use the
function @code{funcall}. When you also need to determine at run time
-how may arguments to pass, use @code{apply}.
+how many arguments to pass, use @code{apply}.
@defun funcall function &rest arguments
@code{funcall} calls @var{function} with @var{arguments}, and returns
@end group
@end example
-Compare these example with the examples of @code{apply}.
+Compare these examples with the examples of @code{apply}.
@end defun
@defun apply function &rest arguments
list or other collection. Emacs Lisp has several such functions;
@code{mapcar} and @code{mapconcat}, which scan a list, are described
here. @xref{Creating Symbols}, for the function @code{mapatoms} which
-maps over the symbols in an obarray.
+maps over the symbols in an obarray. @xref{Hash Access}, for the
+function @code{maphash} which maps over key/value associations in a
+hash table.
These mapping functions do not allow char-tables because a char-table
is a sparse array whose nominal range of indices is very large. To map
"Apply FUNCTION to successive cars of all ARGS.
Return the list of results."
;; @r{If no list is exhausted,}
- (if (not (memq 'nil args))
+ (if (not (memq 'nil args))
;; @r{apply function to @sc{car}s.}
- (cons (apply function (mapcar 'car args))
- (apply 'mapcar* function
+ (cons (apply function (mapcar 'car args))
+ (apply 'mapcar* function
;; @r{Recurse for rest of elements.}
(mapcar 'cdr args)))))
@end group
@end smallexample
@end defun
+@defun mapc function sequence
+@tindex mapc
+@code{mapc} is like @code{mapcar} except that @var{function} is used for
+side-effects only---the values it returns are ignored, not collected
+into a list. @code{mapc} always returns @var{sequence}.
+@end defun
+
@defun mapconcat function sequence separator
@code{mapconcat} applies @var{function} to each element of
@var{sequence}: the results, which must be strings, are concatenated.
argument and return a string. The argument @var{sequence} can be any
kind of sequence except a char-table; that is, a list, a vector, a
bool-vector, or a string.
-
+
@smallexample
@group
(mapconcat 'symbol-name
the function @code{mapcar}, which applies any given function to each
element of a list.
- Here we define a function @code{change-property} which
+ Here we define a function @code{change-property} which
uses a function as its third argument:
@example
@example
@group
(defun double-property (symbol prop)
- (change-property symbol prop (function (lambda (x) (* 2 x)))))
+ (change-property symbol prop
+ (function (lambda (x) (* 2 x)))))
@end group
@end example
@noindent
The Lisp compiler cannot assume this list is a function, even though it
looks like one, since it does not know what @code{change-property} will
-do with the list. Perhaps will check whether the @sc{car} of the third
+do with the list. Perhaps it will check whether the @sc{car} of the third
element is the symbol @code{*}! Using @code{function} tells the
compiler it is safe to go ahead and compile the constant function.
+ Nowadays it is possible to omit @code{function} entirely, like this:
+
+@example
+@group
+(defun double-property (symbol prop)
+ (change-property symbol prop (lambda (x) (* 2 x))))
+@end group
+@end example
+
+@noindent
+This is because @code{lambda} itself implies @code{function}.
+
We sometimes write @code{function} instead of @code{quote} when
quoting the name of a function, but this usage is just a sort of
comment:
(function @var{symbol}) @equiv{} (quote @var{symbol}) @equiv{} '@var{symbol}
@end example
+@cindex @samp{#'} syntax
+ The read syntax @code{#'} is a short-hand for using @code{function}.
+For example,
+
+@example
+#'(lambda (x) (* x x))
+@end example
+
+@noindent
+is equivalent to
+
+@example
+(function (lambda (x) (* x x)))
+@end example
+
@defspec function function-object
@cindex function quoting
This special form returns @var{function-object} without evaluating it.
@defun fmakunbound symbol
This function makes @var{symbol}'s function cell void, so that a
subsequent attempt to access this cell will cause a @code{void-function}
-error. (See also @code{makunbound}, in @ref{Local Variables}.)
+error. (See also @code{makunbound}, in @ref{Void Variables}.)
@example
@group
Inline functions can be used and open-coded later on in the same file,
following the definition, just like macros.
-@c Emacs versions prior to 19 did not have inline functions.
+@node Function Safety
+@section Determining whether a function is safe to call
+@cindex function safety
+@cindex safety of functions
+
+Some major modes such as SES (@pxref{Top,,,ses}) call functions that
+are stored in user files. User files sometimes have poor
+pedigrees---you can get a spreadsheet from someone you've just met, or
+you can get one through email from someone you've never met. So it is
+risky to call a function whose source code is stored in a user file
+until you have determined that it is safe.
+
+@defun unsafep form &optional unsafep-vars
+Returns @code{nil} if @var{form} is a @dfn{safe} lisp expression, or
+returns a list that describes why it might be unsafe. The argument
+@var{unsafep-vars} is a list of symbols known to have temporary
+bindings at this point; it is mainly used for internal recursive
+calls. The current buffer is an implicit argument, which provides a
+list of buffer-local bindings.
+@end defun
+
+Being quick and simple, @code{unsafep} does a very light analysis and
+rejects many Lisp expressions that are actually safe. There are no
+known cases where @code{unsafep} returns @code{nil} for an unsafe
+expression. However, a ``safe'' Lisp expression can return a string
+with a @code{display} property, containing an associated Lisp
+expression to be executed after the string is inserted into a buffer.
+This associated expression can be a virus. In order to be safe, you
+must delete properties from all strings calculated by user code before
+inserting them into buffers.
+
+@ignore
+What is a safe Lisp expression? Basically, it's an expression that
+calls only built-in functions with no side effects (or only innocuous
+ones). Innocuous side effects include displaying messages and
+altering non-risky buffer-local variables (but not global variables).
+
+@table @dfn
+@item Safe expression
+@itemize
+@item
+An atom or quoted thing.
+@item
+A call to a safe function (see below), if all its arguments are
+safe expressions.
+@item
+One of the special forms @code{and}, @code{catch}, @code{cond},
+@code{if}, @code{or}, @code{prog1}, @code{prog2}, @code{progn},
+@code{while}, and @code{unwind-protect}], if all its arguments are
+safe.
+@item
+A form that creates temporary bindings (@code{condition-case},
+@code{dolist}, @code{dotimes}, @code{lambda}, @code{let}, or
+@code{let*}), if all args are safe and the symbols to be bound are not
+explicitly risky (see @pxref{File Local Variables}).
+@item
+An assignment using @code{add-to-list}, @code{setq}, @code{push}, or
+@code{pop}, if all args are safe and the symbols to be assigned are
+not explicitly risky and they already have temporary or buffer-local
+bindings.
+@item
+One of [apply, mapc, mapcar, mapconcat] if the first argument is a
+safe explicit lambda and the other args are safe expressions.
+@end itemize
+
+@item Safe function
+@itemize
+@item
+A lambda containing safe expressions.
+@item
+A symbol on the list @code{safe-functions}, so the user says it's safe.
+@item
+A symbol with a non-@code{nil} @code{side-effect-free} property.
+@item
+A symbol with a non-@code{nil} @code{safe-function} property. Value t
+indicates a function that is safe but has innocuous side effects.
+Other values will someday indicate functions with classes of side
+effects that are not always safe.
+@end itemize
+
+The @code{side-effect-free} and @code{safe-function} properties are
+provided for built-in functions and for low-level functions and macros
+defined in @file{subr.el}. You can assign these properties for the
+functions you write.
+@end table
+@end ignore
@node Related Topics
@section Other Topics Related to Functions