Can I just add this: First, as Andy already hinted, it's not how a data type is implemented but the operations in its API which defines it. A list does not map directly to a hook. A hook can be implemented as a list, but that is not important.
An example of a hook is before-print-hook which is used in (ice-9 history) and which can be used, for example, to support a communication protocol when running Guile in an IDE-like environment in Emacs. If we had a naive use of lists as hooks, the correct way to extend it would be something like: (set! before-print-hook (cons my-action before-print-hook)) but one could be tempted to do: (set! before-print-hook (list my-action)) which would erase anything else added there or, even worse, (define before-print-hook ...) which would not affect the real before-print-hook since a new binding in the local module would be created. (add-hook! before-print-hook my-action) doesn't tempt you that way. Also, with the current hook API, hooks are first-class citizens, which they would not be with a naive list implementation. Not sure that matters much, though. On Mon, Jan 9, 2017 at 12:03 AM, Andy Wingo <wi...@pobox.com> wrote: > On Thu 15 Dec 2016 11:48, Jan Synáček <jan.syna...@gmail.com> writes: > > > I've read about hooks in the manual recently and I don't understand > > why they are special. What is the difference between a hook and a > > plain list of procedures? Why do hooks have their own API? > > Historical reasons I think. Early Emacs inspired a number of Guile > extension points, and hooks are a thing there. (Hooks are not just a > list of procedures -- they're a settable place as well and a way of > running all of the procedures.) Anyway I agree, nothing to shout about, > and probably something the manual should mention less prominently. > > Andy > >
