Discussion of Tools

There is no tool support for the programmers who define the hooks in a program. The problems they are where to place the hooks in order to obtain a sufficiently open program. As already mentioned, this is a program design issue, and only skilled programmers are able to solve the problem. However, the following rules of thumbs may be helpful:

• Preserve the flavor of the program.
  It does not make much sense to open all aspects of a program, or to eliminate entire operations of it via hook attachments. If, for instance, the original program is an editor, the specialized program obtained as the result of binding open points should still be some kind of an editor.

• Be careful to pass sufficient information in hook parameters.
  In order to customize or extend a program via a hook, it is critical that the necessary information about the current state of the program is available in the attached procedures. It may be possible to obtain further information via the functional interface of the program, but the most direct way of getting access to the information is via the passed hook parameters.

• Try to make the set of hooks complete.
  Given the restrictions in the intended openness from the first rule, the set of hooks should be as complete as possible. Putting a hook into one particular operation in an interactive program, such as the personal database application, should be extended to putting hooks into all similar operations at the same level of abstraction.

The special purpose tools for open points are in some sense analogous to debuggers. A debugger lets the user examine the dynamic state of a program with the purpose of locating the cause of an error. The set of tools proposed in this paper are directed toward examination of another important aspect of a program, namely the open points. The purpose of the tools that support the hook mechanism is, as mentioned earlier, to help the programmer in the process of customizing a program. As such, the hook supporting tools are quite different from debugging tools.