black hole
    n 1: a region of space resulting from the collapse of a star;
         extremely high gravitational field

black hole \black" hole`\
   A dungeon or dark cell in a prison; a military lock-up or
   guardroom; -- now commonly with allusion to the cell (the
   Black Hole) in a fort at Calcutta (called the Black Hole of
   Calcutta), into which 146 English prisoners were thrust by
   the nabob Suraja Dowla on the night of June 20, 1765, and in
   which 123 of the prisoners died before morning from lack of
   air.
   [1913 Webster]

         A discipline of unlimited autocracy, upheld by rods,
         and ferules, and the black hole.         --H. Spencer.
   [1913 Webster]

   2. (Physics, Astron.) An astronomical object whose mass is so
      condensed that the gravitational force does not allow
      anything, even light, to escape from its outer limit (the
      event horizon). The existence of such objects was first
      proposed from theoretical considerations. Because light
      cannot escape from such objects, they have not yet been
      detected with certainty (1998), but several "candidates"
      have been observed whose properties strongly suggest that
      they are black holes. Some theorists suggest that the
      centers of many galaxies may have large black holes at
      their cores. See also escape velocity.
      [PJC]

   3. [from the astronomical black hole.] a place into which
      things may enter, but can never emerge. [Fig., Jocose] "He
      was so disorganized his office was a black hole."
      [PJC]

black hole
 n.,vt.

    [common] What data (a piece of email or netnews, or a stream of TCP/IP
    packets) has fallen into if it disappears mysteriously between its origin
    and destination sites (that is, without returning a bounce message). ?I
    think there's a black hole at foovax!? conveys suspicion that site foovax
    has been dropping a lot of stuff on the floor lately (see drop on the
    floor). The implied metaphor of email as interstellar travel is
    interesting in itself. Readily verbed as blackhole: ?That router is
    blackholing IDP packets.? Compare bit bucket and see RBL.

black hole

   1. An expression which depends on its own value or a technique
   to detect such expressions.  In graph reduction, when the
   reduction of an expression is begun, the root of the
   expression can be overwritten with a black hole.  If the
   expression depends on its own value, e.g.

   	x = x + 1

   then it will try to evaluate the black hole which will usually
   print an error message and abort the program.  A secondary
   effect is that, once the root of the expression has been
   black-holed, parts of the expression which are no longer
   required may be freed for garbage collection.

   Without black holes the usual result of attempting to evaluate
   an expression which depends on itself would be a stack
   overflow.  If the expression is evaluated successfully then
   the black hole will be updated with the value.

   Expressions such as

   	ones = 1 : ones

   are not black holes because the list constructor, : is lazy so
   the reference to ones is not evaluated when evaluating ones to
   WHNF.

   2. Where an electronic mail message or news aritcle has
   gone if it disappears mysteriously between its origin and
   destination sites without returning a bounce message.
   Compare bit bucket.

   [Jargon File]