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24.6.2 Hook Reference

When you create a hook with make-hook, you must specify the arity of the procedures which can be added to the hook. If the arity is not given explicitly as an argument to make-hook, it defaults to zero. All procedures of a given hook must have the same arity, and when the procedures are invoked using run-hook, the number of arguments passed must match the arity specified at hook creation time.

The order in which procedures are added to a hook matters. If the third parameter to add-hook! is omitted or is equal to #f, the procedure is added in front of the procedures which might already be on that hook, otherwise the procedure is added at the end. The procedures are always called from the front to the end of the list when they are invoked via run-hook.

The ordering of the list of procedures returned by hook->list matches the order in which those procedures would be called if the hook was run using run-hook.

Note that the C functions in the following entries are for handling Scheme-level hooks in C. There are also C-level hooks which have their own interface (see C Hooks).

make-hook [n_args] Scheme Procedure
scm_make_hook (n_args) C Function
Create a hook for storing procedure of arity n_args. n_args defaults to zero. The returned value is a hook object to be used with the other hook procedures.

hook? x Scheme Procedure
scm_hook_p (x) C Function
Return #t if x is a hook, #f otherwise.

hook-empty? hook Scheme Procedure
scm_hook_empty_p (hook) C Function
Return #t if hook is an empty hook, #f otherwise.

add-hook! hook proc [append_p] Scheme Procedure
scm_add_hook_x (hook, proc, append_p) C Function
Add the procedure proc to the hook hook. The procedure is added to the end if append_p is true, otherwise it is added to the front. The return value of this procedure is not specified.

remove-hook! hook proc Scheme Procedure
scm_remove_hook_x (hook, proc) C Function
Remove the procedure proc from the hook hook. The return value of this procedure is not specified.

reset-hook! hook Scheme Procedure
scm_reset_hook_x (hook) C Function
Remove all procedures from the hook hook. The return value of this procedure is not specified.

hook->list hook Scheme Procedure
scm_hook_to_list (hook) C Function
Convert the procedure list of hook to a list.

run-hook hook . args Scheme Procedure
scm_run_hook (hook, args) C Function
Apply all procedures from the hook hook to the arguments args. The order of the procedure application is first to last. The return value of this procedure is not specified.

If, in C code, you are certain that you have a hook object and well formed argument list for that hook, you can also use scm_c_run_hook, which is identical to scm_run_hook but does no type checking.

void scm_c_run_hook (SCM hook, SCM args) C Function
The same as scm_run_hook but without any type checking to confirm that hook is actually a hook object and that args is a well-formed list matching the arity of the hook.

For C code, SCM_HOOKP is a faster alternative to scm_hook_p:

int SCM_HOOKP (x) C Macro
Return 1 if x is a Scheme-level hook, 0 otherwise.

24.6.3 Handling Scheme-level hooks from C code

Here is an example of how to handle Scheme-level hooks from C code using the above functions.

if (SCM_NFALSEP (scm_hook_p (obj)))
  /* handle Scheme-level hook using C functions */
  scm_reset_hook_x (obj);
  /* do something else (obj is not a hook) */