Warning: This is the manual of the legacy Guile 2.2 series. You may want to read the manual of the current stable series instead.

6.11.6.1 Hook Usage by Example

Hook usage is shown by some examples in this section. First, we will define a hook of arity 2 — that is, the procedures stored in the hook will have to accept two arguments.

(define hook (make-hook 2))
hook
⇒ #<hook 2 40286c90>

Now we are ready to add some procedures to the newly created hook with add-hook!. In the following example, two procedures are added, which print different messages and do different things with their arguments.

(add-hook! hook (lambda (x y)
                    (display "Foo: ")
                    (display (+ x y))
                    (newline)))
(add-hook! hook (lambda (x y)
                    (display "Bar: ")
                    (display (* x y))
                    (newline)))

Once the procedures have been added, we can invoke the hook using run-hook.

(run-hook hook 3 4)
-| Bar: 12
-| Foo: 7

Note that the procedures are called in the reverse of the order with which they were added. This is because the default behaviour of add-hook! is to add its procedure to the front of the hook’s procedure list. You can force add-hook! to add its procedure to the end of the list instead by providing a third #t argument on the second call to add-hook!.

(add-hook! hook (lambda (x y)
                    (display "Foo: ")
                    (display (+ x y))
                    (newline)))
(add-hook! hook (lambda (x y)
                    (display "Bar: ")
                    (display (* x y))
                    (newline))
                    #t)             ; <- Change here!

(run-hook hook 3 4)
-| Foo: 7
-| Bar: 12