type: function (subr) location: built-in source file: xlbfun.c and xleval.c Common LISP compatible: related supported on: all machines
(evalhook <expr> <eval-expr> <apply-expr> [ <env> ] ) <expr> - an expression to evaluate <eval-expr> - an expression for the evaluation routine <apply-expr> - an expression for APPLY - not used <env> - an environment expression - default is NIL
EVALHOOK is a function that performs evaluation. The routine specified by <eval-expr> is called with the <expr> and <env> parameters. If <eval-expr> is NIL, then the normal system evaluator is called. The <apply-hook> is a dummy parameter that is not used in the current XLISP system. The <expr> contains the expression to be evaluated. If the <env> argument to EVALHOOK is not specified, NIL is used, which specifies to use the current global environment. The <env>, if specified, is a structure composed of dotted pairs constructed of the symbol and its value which have the form ((( (<sym1> . <val1> ) (<sym2> . <val2> ) ... ))).
(setq a 100) (setq b 200) ; set up global values (evalhook '(+ a b) NIL NIL) ; returns 300 - no <env> (evalhook '(+ a b) NIL NIL ; eval with a=1 and b=2 '((((a . 1)(b . 2))))) ; returns 3 (defun myeval (exp env) ; define MYEVAL routine (princ "exp: ") (print exp) ; (princ "env: ") (print env) ; (evalhook exp #'myeval NIL env)) ; (defun foo (a) (+ a a)) ; create simple function (setq *evalhook* #'myeval) ; and install MYEVAL as hook (foo 1) ; prints ; exp: (FOO 1) env:NIL ; exp: 1 env:NIL ; exp: (+ A A) env:((((A . 1)))) ; exp: A env:((((A . 1)))) ; exp: A env:((((A . 1)))) ; returns 2 (top-level) ; to clean up *evalhook*)
NOTE: The EVALHOOK function and *EVALHOOK* system variable are very useful in the construction of debugging facilities within XLISP. The TRACE and UNTRACE functions use EVALHOOK and *EVALHOOK* to implement their functionality. The other useful aspect of EVALHOOK and *EVALHOOK* is to help in understanding how XLISP works to see the expressions, their environment and how they are evaluated.
CAUTION: Be careful when using *EVALHOOK* and EVALHOOK. If you put in a 'bad' definition into *EVALHOOK*, you might not be able to do anything and will need to exit XLISP.
UNUSUAL BEHAVIOUR: The EVALHOOK function and *EVALHOOK* system variable, by their nature, cause some unusual things to happen. After you have set *EVALHOOK* to some non-NIL value, your function will be called. However, when you are all done and set *EVALHOOK* to NIL or some other new routine, it will never be set. This is because the XEVALHOOK function (in the xlbfun.c source file) saves the old value of *EVALHOOK* before calling your routine, and then restores it after the evaluation. The mechanism to reset *EVALHOOK* is to execute the TOP-LEVEL function, which sets *EVALHOOK* to NIL.