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]