Re: can gh_run_hook return value??

[Neil Jerram]

>>>>> "Martin" == Martin Grabmueller <address@hidden> writes:

    >> From: "Dale P. Smith" <address@hidden> Date: Tue, 03 Jul
    >> 2001 22:08:18 -0400
    >> 
    >> "M.P.Anand Babu" wrote:
    >> > If I run a gh_list_p on hook_return, it returns false !!
    >> 
    >> The return value of scm_run_hook is always SCM_UNPECIFIED.

    Martin> I have added information about the return values of the
    Martin> hook procedures to the GRM.

The thing with hooks, though, is that on the one hand, they are a
jolly good mechanism for allowing multiple callers (that perhaps don't
"know" about each other) to register for a certain event; but on the
other hand, it does seem desirable for some uses of hooks that the
hook procedures should be able to return a value.

I've been thinking about this recently (in the context of the Guile
debugger) and came up with two possibilities.

The first is `hook-fold', which doesn't exist in Guile yet, but would
work like this: given a hook with three procedures added to it,

(hook-fold hook init . args)

would return

(proc3 (proc2 (proc1 init . args) . args) . args)

The problem with this, though, is that all the hook procedures have to
be designed to understand this behaviour.

The second is:

(catch 'hook-return
       (lambda () (run-hook hook args) #f)
       (lambda (key value) value))

With this one, as soon as a hook procedure wants to return a value, it
does

(throw 'hook-return value)

subsequent hooks won't be run, of course, but that may in fact be what
you want.

Well, there's my rambling; just in case it helped...

        Neil