5.9.6.1 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