<mathematics, programming> When a {function} (or {procedure})
	calls itself.  Such a function is called "recursive".  If the
	call is via one or more other functions then this group of
	functions are called "mutually recursive".

	If a function will always call itself, however it is called,
	then it will never terminate.  Usually however, it first
	performs some test on its arguments to check for a "base case"
	- a condition under which it can return a value without
	calling itself.

	The {canonical} example of a recursive function is
	{factorial}:

		factorial 0 = 1
		factorial n = n * factorial (n-1)

	{Functional programming languages} rely heavily on recursion,
	using it where a {procedural language} would use {iteration}.

	See also {recursion}, {recursive definition}, {tail recursion}.

	[{Jargon File}]

	(1996-05-11)

After-sale handholding; something many software vendors
	promise but few deliver.  To hackers, most support people are
	useless - because by the time a hacker calls support he or she
	will usually know the software and the relevant manuals better
	than the support people (sadly, this is *not* a joke or
	exaggeration).  A hacker's idea of "support" is a tte--tte
	or exchange of {electronic mail} with the software's designer.

	[{Jargon File}]

	(1995-02-15)

<mathematics> 1. Bigger than any {natural number}.  There are
	various formal set definitions in {set theory}: a set X is
	infinite if

	(i) There is a bijection between X and a proper subset of X.

	(ii) There is an injection from the set N of natural numbers
	to X.

	(iii) There is an injection from each natural number n to X.

	These definitions are not necessarily equivalent unless we
	accept the {Axiom of Choice}.

	2. The length of a line extended indefinitely.

	See also {infinite loop}, {infinite set}.

	[{Jargon File}]

	(1995-03-29)

<data> A data structure holding many values, possibly of
	different types, which is usually accessed sequentially,
	working from the head to the end of the tail - an "ordered
	list".  This contrasts with a (one-dimensional) {array}, any
	element of which can be accessed equally quickly.

	Lists are often stored using a cell and pointer arrangement
	where each value is stored in a cell along with an associated
	pointer to the next cell.  A special pointer, e.g. zero, marks
	the end of the list.  This is known as a (singly) "linked
	list".  A doubly linked list has pointers from each cell to
	both next and previous cells.

	An unordered list is a {set}.

	(1998-11-12)

<programming> (See below for synonyms) A data structure for
	storing items which are to be accessed in last-in first-out
	order.

	The operations on a stack are to create a new stack, to "push"
	a new item onto the top of a stack and to "pop" the top item
	off.  Error conditions are raised by attempts to pop an empty
	stack or to push an item onto a stack which has no room for
	further items (because of its implementation).

	Most processors include support for stacks in their
	{instruction set architectures}.  Perhaps the most common use
	of stacks is to store {subroutine} arguments and return
	addresses.  This is usually supported at the {machine code}
	level either directly by "jump to subroutine" and "return from
	subroutine" instructions or by {auto-increment} and
	auto-decrement {addressing modes}, or both.  These allow a
	contiguous area of memory to be set aside for use as a stack
	and use either a special-purpose {register} or a general
	purpose register, chosen by the user, as a {stack pointer}.

	The use of a stack allows subroutines to be {recursive} since
	each call can have its own calling context, represented by a
	stack frame or {activation record}.  There are many other
	uses.  The programming language {Forth} uses a data stack in
	place of variables when possible.

	Although a stack may be considered an {object} by users,
	implementations of the object and its access details differ.
	For example, a stack may be either ascending (top of stack is
	at highest address) or descending.  It may also be "full" (the
	stack pointer points at the top of stack) or "empty" (the
	stack pointer points just past the top of stack, where the
	next element would be pushed).  The full/empty terminology is
	used in the {Acorn Risc Machine} and possibly elsewhere.

	In a list-based or {functional language}, a stack might be
	implemented as a {linked list} where a new stack is an empty
	list, push adds a new element to the head of the list and pop
	splits the list into its head (the popped element) and tail
	(the stack in its modified form).

	At {MIT}, {pdl} used to be a more common synonym for stack,
	and this may still be true.  {Knuth} ("The Art of Computer
	Programming", second edition, vol. 1, p. 236) says:

	  Many people who realised the importance of stacks and queues
	  independently have given other names to these structures:
	  stacks have been called push-down lists, reversion storages,
	  cellars, dumps, nesting stores, piles, last-in first-out
	  ("LIFO") lists, and even yo-yo lists!

	[{Jargon File}]

	(1995-04-10)

<language, text> (XML) An initiative from the {W3C} defining
	an "extremely simple" dialect of {SGML} suitable for use on
	the {World-Wide Web}.

	{(http://www.w3.org/XML/)}.

	[Relationship to the {XSL} forthcoming subset of {DSSSL}?]

	(1997-11-20)

<language> An {algorithm} or program to determine the
	syntactic structure of a sentence or string of symbols in some
	language.  A parser normally takes as input a sequence of
	{tokens} output by a {lexical analyser}.  It may produce some
	kind of {abstract syntax tree} as output.  One of the best
	known {parser generators} is {yacc}.

	(1997-07-21)

(From Zen Buddhist koans of the form "Does an X have the
	Buddha-nature?") Common hacker construction for "is an X",
	used for humorous emphasis.  "Anyone who can't even use a
	program with on-screen help embedded in it truly has the
	{loser} nature!"  See also {the X that can be Y is not the
	true X}.

	[{Jargon File}]

	(1995-01-11)

<language> A language compatible with the language
	{Dylan}(TM).  Thomas is NOT {Dylan}(TM).

	The first public release of a translator to {Scheme} by Matt
	Birkholz, Jim Miller, and Ron Weiss, written at {Digital
	Equipment Corporation}'s {Cambridge Research Laboratory} runs
	(slowly) on {MIT}'s {CScheme}, DEC's {Scheme->C}, Marc
	Feeley's {Gambi}, {Macintosh}, {PC}, {Vax}, {MIPS}, {Alpha},
	{680x0}.

	{(ftp://gatekeeper.pa.dec.com/pub/DEC/Thomas)}.

	Mailing list: <info-thomas@crl.dec.com>.

	["Dylan(TM) an object-oriented dynamic language", {Apple
	Computer}, Eastern Research and Technology, April 1992].

	(1992-09-11)

<networking> Statement ritually appended to many {Usenet}
	postings (sometimes automatically, by the posting software)
	reiterating the fact (which should be obvious, but is easily
	forgotten) that the article reflects its author's opinions and
	not necessarily those of the organisation running the computer
	through which the article entered the network.

	[{Jargon File}]

	(1995-07-30)