HOOK FUNCTIONS

If the symbol '*evalhook*' is bound to a function closure, then every call of eval will call this function. The function takes two arguements, the form to be evaluated and execution environment. During the execution of this function, *evalhook* (and *applyhook*) are dynamically bound to NIL to prevent undesirable recursion. This "hook" function returns the result of the evaluation.

If the symbol '*applyhook*' is bound to a function, then every function application within an eval will call this function (note that the function apply, and others which do not use eval, will not invoke the apply hook function). The function takes two arguments, the function closure and the argument list (which is already evaluated). During execution of this hook function, *applyhook* (and *evalhook*) are dynamically bound to NIL to prevent undesired recursion. This function is to return the result of the function application.

Note that the hook functions cannot reset *evalhook* or *applyhook* to NIL, because upon exit these values will be reset. An excape mechanism is provided -- execution of 'top-level', or any error that causes return to the top level, will unhook the functions. Applications should bind these values either via 'progv', 'evalhook', or 'applyhook'.

The functions 'evalhook' and 'applyhook' allowed for controlled application of the hook functions. The form supplied as an argument to 'evalhook', or the function application given to 'applyhook', are not hooked themselves, but any subsidiary forms and applications are. In addition, by supplying NIL values for the hook functions, 'evalhook' can be used to execute a form within a specific environment passed as an argument.

An additional hook function exists for the garbage collector. If the symbol '*gc-hook*' is bound to a function, then this function is called after every garbage collection. The function has two arguments. The first is the total number of nodes, and the second is the number of nodes free. The return value is ignored. During the execution of the function, *gc-hook* is dynamically bound to NIL to prevent undesirable recursion.