Commit 80f85d7c authored by Eli Zaretskii's avatar Eli Zaretskii

(Calling Functions): Document `apply-partially'.

parent d24ae8f0
2008-10-19 Eli Zaretskii <eliz@gnu.org>
* functions.texi (Calling Functions): Document `apply-partially'.
* hooks.texi (Standard Hooks): Mention
`before-hack-local-variables-hook' and `hack-local-variables-hook'.
......
......@@ -725,6 +725,41 @@ For an interesting example of using @code{apply}, see @ref{Definition
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. Emacs provides a function for partial evaluation:
@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 an example of using @code{apply-partially} to produce a
function @code{incr}, that will increment its argument by one, based
on the Emacs Lisp primitive @code{+}:
@example
(fset 'incr (apply-partially '+ 1))
@group
(incr 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
......
......@@ -1581,6 +1581,7 @@ times the default column width.
*** `format-seconds' converts a number of seconds into a readable
string of days, hours, etc.
+++
*** `apply-partially' performs a "curried" application of a function.
*** `read-shell-command' does what its name says, with completion. It
......
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