@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2001,
-@c 2002, 2003, 2004, 2005, 2006, 2007 Free Software Foundation, Inc.
+@c 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../../info/functions
@node Functions, Macros, Variables, Top
@defun functionp object
This function returns @code{t} if @var{object} is any kind of
-function, or a special form, or, recursively, a symbol whose function
-definition is a function or special form. (This does not include
-macros.)
+function, i.e.@: can be passed to @code{funcall}. Note that
+@code{functionp} returns @code{nil} for special forms (@pxref{Special
+Forms}).
@end defun
Unlike @code{functionp}, the next three functions do @emph{not}
of mapcar}.
@end defun
+@cindex partial application of functions
+@cindex currying
+ Sometimes it is useful to fix some of the function's arguments at
+certain values, and leave the rest of arguments for when the function
+is actually called. The act of fixing some of the function's
+arguments is called @dfn{partial application} of the function@footnote{
+This is related to, but different from @dfn{currying}, which
+transforms a function that takes multiple arguments in such a way that
+it can be called as a chain of functions, each one with a single
+argument.}.
+The result is a new function that accepts the rest of
+arguments and calls the original function with all the arguments
+combined.
+
+ Here's how to do partial application in Emacs Lisp:
+
+@defun apply-partially func &rest args
+This function returns a new function which, when called, will call
+@var{func} with the list of arguments composed from @var{args} and
+additional arguments specified at the time of the call. If @var{func}
+accepts @var{n} arguments, then a call to @code{apply-partially} with
+@w{@code{@var{m} < @var{n}}} arguments will produce a new function of
+@w{@code{@var{n} - @var{m}}} arguments.
+
+Here's how we could define the built-in function @code{1+}, if it
+didn't exist, using @code{apply-partially} and @code{+}, another
+built-in function:
+
+@example
+@group
+(defalias '1+ (apply-partially '+ 1)
+ "Increment argument by one.")
+@end group
+@group
+(1+ 10)
+ @result{} 11
+@end group
+@end example
+@end defun
+
@cindex functionals
It is common for Lisp functions to accept functions as arguments or
find them in data structures (especially in hook variables and property
@defmac declare-function function file &optional arglist fileonly
Tell the byte compiler to assume that @var{function} is defined, with
-arguments @var{arglist}, and that the definition should come from
-the file @var{file}. @var{fileonly} non-nil means only check that
+arguments @var{arglist}, and that the definition should come from the
+file @var{file}. @var{fileonly} non-@code{nil} means only check that
@var{file} exists, not that it actually defines @var{function}.
@end defmac
@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.
+A symbol with a non-@code{nil} @code{safe-function} property. The
+value @code{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