hook

programming A software or hardware feature included in

order to simplify later additions or changes by a user.

For example, a simple program that prints numbers might always

print them in base 10, but a more flexible version would let a

variable determine what base to use; setting the variable to 5

would make the program print numbers in base 5. The variable

is a simple hook. An even more flexible program might examine

the variable and treat a value of 16 or less as the base to

use, but treat any other number as the address of a

user-supplied routine for printing a number. This is a

hairy but powerful hook; one can then write a routine to

print numbers as Roman numerals, say, or as Hebrew characters,

and plug it into the program through the hook.

Often the difference between a good program and a superb one

is that the latter has useful hooks in judiciously chosen

places. Both may do the original job about equally well, but

the one with the hooks is much more flexible for future

expansion of capabilities.

Emacs, for example, is *all* hooks.

The term "user exit" is synonymous but much more formal and

less hackish.

(1997-06-25)