Setting a trap is done in two parts. First the trap is defined by creating an instance of the appropriate trap class, with slot values specifying the condition under which the trap will fire and the action to take when it fires. Secondly the trap object thus created must be installed.
To make this immediately concrete, here is an example that sets a trap
to fire on the next application of the facti procedure, and to
handle the trap by entering the command line debugger.
(install-trap (make <procedure-trap>
#:procedure facti
#:single-shot #t
#:behaviour debug-trap))
Briefly, the elements of this incantation are as follows. (All of these are described more fully in the following subsubsections.)
<procedure-trap> is the trap class for trapping on invocation
of a specific procedure.
#:procedure facti says that the specific procedure to trap on for this
trap object is facti.
#:single-shot #t says that this trap should only fire on the
next invocation of facti, not on all future invocations
(which is the default if the #:single-shot option is not
specified).
#:behaviour debug-trap says that the trap infrastructure should
call the procedure debug-trap when this trap fires.
install-trap call installs the trap immediately.
It is of course possible for the user to define more convenient shorthands for setting common kinds of traps. See Trap Shorthands, for some examples.
The ability to install, uninstall and reinstall a trap without losing its definition is Guile's equivalent of the disable/enable commands provided by debuggers like GDB.