|1.||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 |
|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 |
|3.||a place into which things may enter, but can never emerge.|
|Noun||1.||black hole - a region of space resulting from the collapse of a star; extremely high gravitational field|
|1.||black hole - 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.||black hole - 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.|